SOLIDS ACCUMULATION RATES FOR ONSITE SEWAGE TREATMENT AND
DISPOSAL SYSTEMS: A FOCUS ON CHARLOTTE COUNTY, FLORIDA
By
TRISHA LURTZ HOWARD
A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ENGINEERING
UNIVERSITY OF FLORIDA
2003
Copyright 2003
by
Trisha Lurtz Howard
This document is dedicated to the Lord Jesus Christ, my Lord and Savior, who brought me through my weakness with his perfect love. And this document is also dedicated to my loving husband.
iv
ACKNOWLEDGMENTS
I would like to thank to the Charlotte County Department of Health, the Florida
Department of Environmental Protection, and the United States Environmental Protection
Agency for the opportunity to work on this project. I would also like to thank Dr.
Alexandre Trinidad in the Department of Statistics and Dr. Yong-Song Joo from IFAS
Statistics for their willingness to volunteer their time, and Dr. Gary Stevens from the
BCL for running the statistical analysis of the data. I am grateful to Mr. Paul Booher
from the FDOH for the information on tank manufacturers and schematic information.
This study would not have been possible without the willingness and cooperation
of the residents in Charlotte County to allow us to study their systems and provide their
household practices information. Finally, I wish to give thanks to Mr. Robert Vincent,
Mr. Bud Wimer, and Mr. Jeffrey Tompkins for their vigilance and guidance throughout
this project.
Most of all I would like to thank my family and friends. I would like to give a
special thank you to my brother-in-law, Thomas, and my dear friend, Liz, who helped me
through the trials of writing. And, of course, I would like to thank my parents for
preparing me for college and my husband for loving me through this research.
v
TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................................................................................. iv
LIST OF TABLES........................................................................................................... viii
LIST OF FIGURES .............................................................................................................x
ABSTRACT...................................................................................................................... xii
CHAPTER 1 INTRODUCTION ........................................................................................................1
A Case for Standardized Management of OSTDSs......................................................2 Evidence of Failure................................................................................................2 Failure Reasons .....................................................................................................4 System Failure Impacts on Surrounding Area.......................................................5
Charlotte County, Florida .............................................................................................6 Research Scope.............................................................................................................9
2 REVIEW OF LITERATURE.....................................................................................11
OSTDS Design and Operation....................................................................................11 Septic Tank Design and Operation......................................................................11
Septic tank design.........................................................................................11 Septic tank operation....................................................................................14
Drainfield Design and Operation.........................................................................16 Drainfield design ..........................................................................................17 Drainfield operation .....................................................................................18
OSTDS Maintenance ..................................................................................................20 Annual Inspection................................................................................................21 Maintenance Pumping .........................................................................................21 EPA Management Guidelines .............................................................................22
Previous Solids Accumulation Rate Research............................................................23 Discussion of Prior Studies .................................................................................23
United States Public Health Service.............................................................23 Bounds Study ...............................................................................................26 Moore Study.................................................................................................28
Comparison of Pump Out Frequencies................................................................29
vi
3 METHODS.................................................................................................................32
Objectives ...................................................................................................................32 Task 1: Identification of Relevant Variables ..............................................................33
Wastewater Characteristics .................................................................................33 System Design .....................................................................................................35 Surrounding Environment ...................................................................................35
Task 2: Selection and Recruitment of Participants.....................................................36 Subtask 2A: Participant Selection .......................................................................37
Location of systems in Charlotte County .....................................................37 Identification of OSTDS density by soil type ..............................................37 Random selection of potential participants ..................................................37
Subtask 2B: Recruiting of Participants................................................................38 Newspaper article .........................................................................................38 Mailing campaign.........................................................................................38 County fair....................................................................................................40
Subtask 2C: Division of Recruits into Phase 1 and Phase 2 of the Study ...........40 Subtask 2D: Data Collection and Databasing .....................................................41
Initial survey.................................................................................................41 Follow-up survey..........................................................................................42 Permit information .......................................................................................42 Inspection information .................................................................................42 Height measurements ...................................................................................43 Databasing of all variables collected............................................................43
Task 3: Calculation of Solids Accumulation ..............................................................43 Subtask 3A: Determination of Tank Dimensions................................................44 Subtask 3B: Calculation of Solids Volumes in Each Tank .................................46 Subtask 3C: Calculation of Accumulation Rates ................................................48 Subtask 3D: Assessment of Trends in Accumulation .........................................48
Task 4: Interpretation of Phase I and Phase II Model Results....................................49 Subtask 4A: Assess Sample Size.........................................................................49 Subtask 4B: Assessment of Correlations.............................................................50 Subtask 4C: Assessment of Model Results .........................................................50
4 RESULTS AND DISCUSSION.................................................................................52
Task 1: Identification of Relevant Variables ..............................................................52 Task 2: Selection and Recruitment of Participants ....................................................55
Subtask 2A: Selection of Potential Participants ..................................................55 Subtask 2B: Recruit Participants .........................................................................56 Subtask 2C: Separation of Recruits into Phase 1 and 2.......................................59 Subtask 2D: Data Collection and Databasing .....................................................60
Task 3: Calculation of Solids Accumulation ..............................................................62 Subtask 3A: Calculation of Tank Dimensions ....................................................62 Subtask 3B: Calculation of Solid Volumes Septic Tank.....................................62 Subtask 3C: Calculation of Accumulation Rates ................................................63
Accumulation rate per system ......................................................................64
vii
Accumulation rate per person ......................................................................65 Subtask 3D: Trends in Accumulation..................................................................66
Accumulation trends during the first year....................................................66 Accumulation trends in relation to people and septic tank volume .............67
Task 4: Interpretation of Phase 1 and Phase 2 Model Results....................................71 Subtask 4A: Assess Sample Size.........................................................................71 Subtask 4B: Development of Correlations ..........................................................74
Sludge accumulation rate correlations .........................................................74 Scum accumulation rate correlations ...........................................................75
Subtask 4C: Development of Model Results.......................................................77 5 SUMMARY AND CONCLUSIONS.........................................................................80
6 RECOMMENDATIONS FOR FUTURE WORK .....................................................84
Identification of Relevant Variables...........................................................................84 Selection, Recruitment, and Monitoring of Participants ............................................85
Participant Selection............................................................................................85 Recruiting Participants ........................................................................................85 Division of Recruits for Two Groups..................................................................86
Data Collection ...........................................................................................................86 Calculation of Solids Accumulation and Model Development ..................................87
APPENDIX
A CONTENTS OF PACKAGE INITIALLY MAILED TO PARTICIPANTS ............88
B INTIAL AND FOLLOWUP SURVEYS ...................................................................92
C DESCRIPTION OF METHOD USED TO DIVIDE PARTICIPANTS INTO PHASE 1 AND 2 GROUPING ..................................................................................98
D TANK DIMENSIONS..............................................................................................102
Manufactured Tank Dimensions...............................................................................102 Method 1 Tank Dimension Calculation....................................................................102 Method 2 Tank Dimension Calculations ..................................................................103
LIST OF REFERENCES.................................................................................................105
BIOGRAPHICAL SKETCH ...........................................................................................110
viii
LIST OF TABLES
Table page 2-1 Residential Estimated Sewage Flow* ......................................................................14
2-2 Minimum Effective Septic Tank Capacities ............................................................14
2-3 USPHS Septic Tank Years of Service (Weibel et al., 1949)....................................24
2-4 USPHS Accumulation Means and Medians (Weibel et al., 1949)...........................24
2-5 Total per Capita Sludge and Scum Accumulation (Weibel et al., 1949) .................26
2-6 Moore Study (2002) Distribution of Years of Service .............................................28
2-7 Moore Study (2002) Scum and Sludge Accumulation Trends ................................29
2-8 Summaries of Prior Studies......................................................................................30
4-1 Potential Performance Factors Describing Wastewater Quality and Hydraulics.....54
4-2 Potential Performance Factors Describing System Design and Condition ..............55
4-3 Potential Performance Factors Describing Surrounding Environment ....................55
4-4 “Most Populated” Soils (with Largest Number of OSTDSs) in Charlotte County..56
4-5 Recruitment Results .................................................................................................58
4-6 Distribution for Quantitative of Household Characteristic for All Recruits ............60
4-7 Distribution for Qualitative of Household Characteristics for All Recruits.............60
4-8 Frequencies for Each Level of Household Characteristics for Each Phase...............61
4-9 Scum Volume 6 and 12 Months After Pump out .....................................................63
4-10 Sludge Volume 6 and 12 Months After Pump out ...................................................63
4-11 Total Solids Volume 6 and 12 Months After Pump out...........................................63
4-12 Scum Accumulation Rates .......................................................................................64
ix
4-13 Sludge Accumulation Rates .....................................................................................64
4-14 Total Solids Accumulation Rates .............................................................................64
4-15 Scum and Sludge Accumulation Rate Data Determined Using Linear Fits ............65
4-16 Scum per Person Accumulation Rate.......................................................................65
4-17 Sludge per Person Accumulation Rate.....................................................................65
4-18 Total Solids per Person Accumulation Rate ............................................................65
4-19 Accumulation Rates of Solids per Person Determined Using Linear Fits ...............66
4-20 Solids Volume Change from 6 months to 12 months ..............................................66
4-21 Solids Accumulation Rate Change from 6 months to 12 months ............................67
4-22 Accumulation Rates Based on the Number of People in the Home.........................68
4-23 Accumulation Rate Based on Septic Tank Volume .................................................69
4-24 Accumulation Rate based on Septic Tank Capacity ................................................71
4-25 Significant Sample Size Values ...............................................................................73
C-1 Quantitative Data Description................................................................................100
C-2 Variable Phase Distribution ...................................................................................101
D-1 Manufactured Tank Dimensions ............................................................................102
D-2 Method 1 Dimension Calculations..........................................................................103
D-3 Method 2 Ratios and Dimensions ..........................................................................103
D-4 Method 2 Calculated Dimensions ..........................................................................104
x
LIST OF FIGURES
Figure page 1-1 OSTDS Locations in Charlotte County......................................................................7
1-2 Average Number of New Systems Installed from 1971 to 2001 ...............................8
2-1 A Multiple Chamber Septic Tank Schematic...........................................................13
2-2 Septic Tank Width Profiles (a) V-bottom (b) Taper ................................................13
2-3 PHS Rate of Scum & Sludge Accumulation. ...........................................................25
2-4 Bounds Study Volume of Total Solids For Eight Years ..........................................27
3-1 Selected Systems in Mid-County 2 Basin per Soil Type .........................................39
3-2 V-bottom Septic Tank Schematic with Dimensions ................................................45
3-3 Taper-Side Septic Tank Schematic with Dimensions ..............................................45
3-4 Height Dimensions for Liquid Height (HL), Scum Height (hsc), and Sludge ..........47
4-1 Divisions of Performance Variables into the Three Characteristic Groups .............53
4-2 Selected Homes having OSTDSs within Mid-County-2..........................................57
4-3 Accumulation Rate Trends Based on Number of People in the Home ....................69
4-4 Accumulation Rate Trends Based on Septic Tank Volume.....................................70
4-5 Accumulation Rate Trends Based on Septic Tank Capacity....................................72
4-6 Scatter Plots for Sludge Accumulation Rates at 6 Months ......................................78
4-7 Scatter Plots for Scum Accumulation Rates at 12 Months ......................................79
A-1 Flier (a) front (b) back..............................................................................................89
A-2 Consent Form Page 1 ...............................................................................................90
A-3 Consent Form Page 2 ...............................................................................................91
xi
B-1 Initial Survey Page 1 ................................................................................................93
B-2 Initial Survey Page 2 ................................................................................................94
B-3 Initial Survey Page 3 ................................................................................................95
B-4 Follow-up Survey Page 1 .........................................................................................96
B-5 Follow-up Survey Page 2 .........................................................................................97
C-1 Number of Occupants per Home Frequency Chart ..................................................98
C-2 Occupancy in Home during the Year Frequency Chart ...........................................99
C-3 Washing Machine Loads per Week Frequency Chart..............................................99
C-4 Dishwasher loads per Week Frequency Chart .......................................................100
xii
Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Engineering
SOLIDS ACCUMULATION RATES FOR ONSITE SEWAGE TREATMENT AND DISPOSAL SYSTEMS: A FOCUS ON CHARLOTTE COUNTY, FLORIDA
By
Trisha Lurtz Howard
December 2003
Chair: Angela S. Lindner Major Department: Environmental Engineering Sciences
In 1999, 24.1% of American households and more than 60 million people used
onsite sewage treatment and disposal systems (OSTDSs) for domestic wastewater
treatment, with Florida consistently containing the largest number of systems than any
other state. For example, in 1990, Florida was reported as having1.56 million OSTDSs.
In Charlotte County, Florida, approximately 24,000 systems were installed before 1983,
and, as of 1998, nearly 40,000 systems were in place, with 82% of these in residential
areas.
As these systems age, they may eventually fail if not maintained. A survey
conducted by the U.S. Census Bureau (1997) estimated that 403,000 homes experienced
system failure within the 3-month survey period, and 31,000 of these homes were
reported to have four or more breakdowns during this period. Studies reviewed by the
U.S. Environmental Protection Agency (U.S. EPA) cite failure rates ranging from 10-
xiii
20%. System failure surveys typically do not include systems that might be
contaminating their surrounding environment, as these failures are not always obvious.
The primary goal of this study was to better understand the underlying factors
(social, geographical, system design, etc.) that impact system performance. The ultimate
goal of this study was to provide CCDOH with a list of factors that potentially hamper
the ability of the region’s OSTDSs to perform effectively. The primary methods to
determine these factors involved a combination of recruiting for participants, data
gathering using surveys, inspections, and permits, and multiple linear regression (MLR)
analysis. Two hundred twenty-one Charlotte County residents were recruited for
participation in this study. MLR analysis related solids accumulation rates (as dependent
variables) with the performance variables collected in the data gathering phases.
Those performance variables that were identified as having the greatest impact on
solids accumulation were related to wastewater generating activities in the household and
flooding frequencies. The scum accumulation rate significantly related to the use of
garbage disposals, the disposal of food particles while rinsing dishes, the washing
machine capacity, and the dishwasher detergent. The sample size was limited in yielding
significant results for the sludge accumulation rate.
Despite the inability of constructing statistically robust multiple linear correlations,
this study yielded a greater understanding of the importance of adopting better household
practices that result in less potential for OSTDS failure. By establishing community
outreach programs for OSTDS owners, CCDOH can prevent risk to the environment and
public health by ensuring that Charlotte County’s systems do not fail and intrude on
surrounding surface waters.
1
CHAPTER 1 INTRODUCTION
Results from the U.S. Census in the past 20 years indicate that use of onsite sewage
treatment and disposal systems (OSTDSs) is a common method of managing household
wastewater in the United States. In 1999, approximately twenty-three percent
(26,051,000) of all of the nation’s homes used OSTDSs (U.S. Census Bureau, 2003),
including one-third of newly constructed homes and one-half of all mobile homes. Over
50% of OSTDSs in the United States were found to exceed 30 years of service,
surpassing the intended 20-30 years of useful life of these systems. Florida possesses the
largest number of OSTDSs than any other state in nation with 1.56 million in 1999 (U.S.
Census Bureau, 2003). In 1987, Florida exceeded 60,000 permits for new systems
annually (Ayres Associates, 1987). As the population in Florida increases along with the
number of households using OSTDSs for wastewater management, there is an urgent
need to address factors that affect the performance of these systems in order to minimize
and ultimately prevent negative public and environmental health impacts.
This report focuses on the results of a study, funded by the Charlotte County
Department of Health (CCDOH), that began in July 2001. The overall goal of this work
was to identify the most significant human and environmental factors that have potential
to cause an OSTDS to fail, and, with these results, to allow CCDOH to recommend best
management practices to its residents. Chapter 1 of this report provides an overview of
this project with a description of Charlotte County and the research scope. Chapter 2
presents a thorough literature review of all research to date concerning OSTDSs,
2
including detailed descriptions of their design, normal maintenance, and wastewater
characteristics. Chapter 3 presents the methods used in this study, and Chapter 4 presents
the results of this work. Chapter 5 presents the conclusion of this study, and Chapter 6
presents recommendations for best practices in OSTDS management and for extending
this study.
A Case for Standardized Management of OSTDSs
Homeowners typically have the sole responsibility for managing and maintaining
OSTDSs (U.S. EPA, 2002; Laak and Crates, 1978). During a 3-month period in 1995,
1.57% of the total systems in the nation experienced system failures, with 31,000 of those
homes reporting four or more OSTDS failures during that same period (U.S. Census
Bureau, 1997). During a 3-month period in 1999, 1.44% of the total systems in the
nation experienced system failures, with 27,000 of those homes reporting four or more
OSTDS failures during that same period (U.S. EPA, 2002). Despite an increase of
systems, the number of reported failures decreased from 1995 to 1999, possibly
indicating improved management of the systems. The OSTDS failure rate throughout the
U.S. ranges from 10-20% of all systems (U.S. EPA, 2000a).
Evidence of Failure
An OSTDS is considered to have failed when it no longer treats wastewater to
dischargeable conditions. Monitoring for system failure is often difficult because both
wastewater treatment and discharge occur underground. A wide variety of criteria exist
to signal system failure, and a brief description of these failure characteristics is presented
below.
Class I. The first type of failure symptoms, known as Class I, is the reentry of raw
sewage back into the house, commonly termed a “sewage backup” and immediately
3
noticed by the homeowner (Brown, 1998). The raw sewage returns through the drains or
toilets (Vogel and Rupp, 1999; Brown, 1998) from either blockage in the pipes
connecting to the septic tank or the septic tank approaching full capacity for solid storage.
A slow flushing toilet or emptying drain can be the first signs of a blockage in the pipes
or a near-full septic tank (Vogel and Rupp, 1999).
Class II. The second group of failure symptoms, called Class II, is raw or partially
treated sewage surfacing in the yard (Brown, 1998). In this class of failure symptoms,
the toilet and other water-using facilities usually function properly, but untreated or
poorly treated sewage surfaces in the yard or seeps along the surface of the ground near
the system (Vogel and Rupp, 1999; Brown, 1998). Evidence of a system failure for class
of symptoms is apparent when lush green grass flourishes over the system despite no
sewage surfacing. Excessive growth of plant materials indicates an excessive amount of
nutrient-rich liquid moving up through the soil (Vogel and Rupp, 1999).
Class III. The third type of failure characteristics, called Class III, is the decline in
water quality while the household plumbing and drainfield seem to be working perfectly
without evident odors or excess wetness around the drainfield (Brown, 1998). Failures of
this type result in a decline in the quality of groundwater and/or surface water; however,
because there is no other obvious evidence, homeowners may be skeptical that this is a
result of their systems (Brown, 1998). The build up of aquatic weeds or algae in lakes or
ponds adjacent to a homeowner’s system is indication of nutrient overload from failing
OSTDSs (Vogel and Rupp, 1999).
Class IV. The final evidence of failures is a long-term, gradual environmental
degradation (Brown, 1998). There is little, if any, scientific evidence that ground water
4
or surface water is being degraded at a rate likely to be a problem for the current or next
generation of residents. However, modeling and long-term monitoring indicate that very
gradual environmental degradation may result from improper septic system practices at a
particular home site, in a neighborhood, or in a region. Of all the classes of system
failure, this one is the most difficult to prove (Brown, 1998).
Failure Reasons
Failing systems are often left unnoticed until Class I-IV evidence of failure (as
previously described) becomes bothersome to the homeowner or apparent to the
regulatory agency. System failures are typically the consequence of several factors, but
the most common reason is system mismanagement. OSTDS failures can be grouped
into three possible causes: the system location, the system design and condition, or
management of the system by the homeowner.
The system location can jeopardize the performance of an OSTDS depending on
the OSTDS density in the region, the presence of effective drainage, and elevation of the
seasonal high groundwater table. A high density of OSTDSs decrease the soil’s natural
capacity to purify wastewater (Yates, 1985), while effective drainage (via permeable
subsoils) is critical to prevent liquids from entering the septic tank from a flooded
drainfield or leaky tank (Vogel and Rupp, 1999; Brown, 1998; Bounds, 1996; Bell,
1977). High groundwater elevation results in an insufficient unsaturated soil depth under
the drainfield, which must be maintained when the groundwater elevation is at its highest
elevation (Brownand Bicki, 1987).
System deterioration and under-design can also prevent proper functioning.
System failures can be minimized through informed planning and installation of the
system (Brown, 1998). Deterioration of a system is caused by root growth in pipes,
5
crushed drainfield pipes, poor original design, or poor installation. Septic tanks, suffering
from corrosion or root infiltration, allow water outside of the system to enter, and the
resulting hydraulic overload of the tank causes sewage to return to the home or to
surfaces in the nearby yard (U.S. EPA, 1980).
The daily habits of the occupants, such as water use and disposal activities, can
cause a system with proper siting and excellent condition to fail. Management of an
OSTDS, including use and maintenance practices, is the sole responsibility of the owner.
Therefore, for an owner to avoid the most commonly reported failure, hydraulic
overloading, he/she must routinely perform tasks, such as monitoring tank residual
buildup, scheduling pumping, ensuring proper flow distribution, checking pumps and
float switches, inspecting filtration media for clogging, and performing monitoring and
maintenance tasks (U.S. EPA, 2002). Unfortunately, most homeowners have a limited
knowledge of OSTDS management practices, thus increasing the risk of their system to
reach overcapacity and, ultimately, leading to clogged drainfields (U.S. EPA, 2000a;
Vogel and Rupp, 1999; Bell, 1977).
System Failure Impacts on Surrounding Area
OSTDS failures can result in endangering public and environmental health, not to
mention high costs for system repairs. Improper maintenance can yield undesirable
odors, mosquito breeding areas, repair or replacement, condemnation of residence,
groundwater contamination, and the spreads of disease associated with sewage (Bicki,
1989). Failed systems, mostly caused by improper maintenance, are the third most
common source of groundwater contamination (Robertson et al., 1991; Canter and Knox,
1985; Yates, 1985), and sewage has been attributed to being the source of nearly one
third of the miles of contamination observed along the U.S. ocean shoreline (U.S. EPA,
6
1996a; LaPointe and Clark, 1992; Weiskel and Howes, 1992). In 1996, the Clean Water
Needs Survey revealed that more than 500 communities in the nation have public health
problems caused by failed systems (U.S. EPA, 1996b). Over half of the waterborne
disease outbreaks in the nation are from consuming contaminated groundwater. The
contaminated groundwater is most frequently contaminated with OSTDS effluents
(Yates, 1985).
Charlotte County, Florida
Charlotte County was the subject of this study. Located approximately 100 miles
south of Tampa on the southwest coast of Florida, Charlotte County is 894 square miles in
area with the Gulf of Mexico serving as its west border. 705 square miles of Charlotte
County is land and the rest is water. In the 1960’s and 1970’s, 68 square miles were
subdivided into several hundred thousand ¼-acre lots. The maximum elevation of the land
is 35 feet above sea level, approximately 13 miles inland from the Charlotte Harbor, the
second largest estuarine harbor in Florida (also in Charlotte County). The county has over
300 miles of navigable and drainage canals with nearly 300,000 waterfront lots. The build-
able areas, developed and undeveloped, have a seasonal high groundwater table within
one inch of the ground surface. Figure 1-1 illustrates the location of homes served by
OSTDSs and sewer within Charlotte County.
As of 1998, just fewer than 40,000 OSTDSs existed in Charlotte County with 82%
residential and 18% commercial or institutional and in industrial zones. Approximately
24,000 systems were installed before 1983. The median age of systems in 1998 was 12
years. Charlotte County experienced rapid growth between the years of 1977 and 1990
with the peak of installations in 1978.
7
Figure 1-1 OSTDS Locations in Charlotte County
.
.
8
Figure 1-2 Average Number of New Systems Installed from 1971 to 2001 in Charlotte County (Florida Department of Health, 2003)
Charlotte County has eleven major municipal wastewater treatment plants
(MWWTP), all permitted at or above 50,000 GPD and 2 larger MWWTP permitted
above 2 MGD. The county also has 36 smaller MWWTP with a loading capacity of
50,000 to 75,000 GPD. The high count of OSTDSs used in the county is because of a
lack of existing infrastructure (Vincent, 1998).
In the mid-1990s, the Charlotte County Commission was prepared to begin a utility
expansion to connect most of Port Charlotte to sewer. The residents opposed the
expansion mainly because of the anticipated cost and the lack of evidence that the
OSTDSs were a source of pollution in the county. The cost of the expansion is $8,000
per property expected to be paid by the property owner, not including the monthly water
bill. With one-third of the population over 65 and one of the highest median ages of 58,
the community of Charlotte County attracts citizens for its excellent retirement lifestyle.
In 1999, there were 140,000 year-round residents and 60,000 winter residents visiting 4-6
0
500
1000
1500
2000
2500
1970 1975 1980 1985 1990 1995 2000 2005
Year
Num
ber o
f New
OST
DS
Inst
alla
tions
9
months a year. The median income is $42,000 a year, and the median home price is
$74,000.
Research Scope
This project sought a novel approach to the study of OSTDSs in Charlotte County –
by better understanding the underlying factors (social, geographical, system design, etc.)
that impact system performance, a predictive model could be developed to enable
prevention of problems leading to system failures. The objective of this research by the
University of Florida was to determine the underlying factors that correlate with sludge
and scum accumulation and to develop, if possible, an accumulation model of sludge,
scum, and total solids using data measured during one year of service. The specific tasks
of this project are the following:
1. Identify relevant variables that may impact OSTDS performance.
2. Collect specific information concerning relevant performance variables from a selected population of Charlotte County OSTDS owners.
3. Create a model to predict accumulation that will aid in the creation of a pump out schedule specific to the individual OSTDS.
4. Interpret the model results that will, in turn, allow CCDOH to establish recommended best management practices for OSTDSs.
The United States Environmental Protection Agency (U.S. EPA), Florida
Department of Environmental Protection (FDEP), and the Charlotte County Department
of Health (CCDOH) sponsored this research. More specifically, this project was funded
in part by a Section 319 Nonpoint Source Management Program Implementation grant
from the U.S. EPA through an agreement/contract with the Nonpoint Source
Management Section of FDEP.
10
By completion of these tasks, performance factors that influence solids
accumulation during the first year of service of an OSTDS were identified and best
management practices determined, thus aiding in preventing further environmental
deterioration caused by these systems and adding to the continuously growing body of
knowledge on trends in solids accumulation and required pumping frequencies.
11
CHAPTER 2 REVIEW OF LITERATURE
OSTDS Design and Operation
OSTDSs provide effective wastewater treatment when properly sited, designed, and
managed. A conventional OSTDS consists of a septic tank and a drainfield. An OSTDS
has many advantages over a wastewater treatment plant. OSTDSs require minimum
energy and technology, while also being low in cost. Unlike a wastewater treatment
plant, an OSTDS requires no operators, no moving parts, little scheduled maintenance,
and produces less sludge (Laak and Crates, 1978). However, wastewater treatment plants
have better management and maintenance when compared with OSTDSs.
Septic Tank Design and Operation
Quality septic tank design ensures proper operation and treatment of wastewater.
Septic tanks must be designed to promote the separation of solids from the wastewater
effluent. The septic tank must also be at adequate capacity to guarantee retention of
effluent to allow settling and storage of the settled solids. The stored solids are then
subjected to partial treatment by degradation.
Septic tank design
Septic tanks are designed to prevent failure and increase longevity of a drainfield
by preventing solids from exiting. A septic tank has the following components: entrance
and exit baffles, a manhole, and possibly an interior wall (Figure 2-1). The entrance and
exit baffles are designed to allow wastewater to enter the septic tank and clarified effluent
to leave without stored solids exiting. Like a settling chamber, the entrance and exit
12
baffles are located on the walls of furthest distance to allow adequate retention time and
settling of solids. The manhole allows access to monitor the retained solids, to pump
septage when solids reach capacity, and to inspect septic tank conditions. An interior
wall is used to divide the septic tank into two separate chambers. Multiple chambers
reduce the amount of solids that can exit when the tank is hydraulically overloaded. The
Florida Department of Health regulates the design of septic tanks through the Florida
Administrative Code of Standards. Major criteria for septic tank design by the Florida
Department of Health are as follows:
• must be watertight (64E-6.002(49)),
• can have a single or double compartment (64E-6.013(2)(a)),
• tanks with a double compartment: the first chamber must be 2/3 the total volume and the second chamber must be 1/3 of the total volume (64E-6.013(2)(a)),
• liquid depth must be at least 42 inches (64E-6.013(2)(b)),
• airspace must be 15% of the effective capacity (64E-6.013(2)(c)),
• the inlet device must enter one to three inches above the liquid level with a diameter greater than 4 inches and must not extend greater than 33% of the liquid depth below the water surface (64E-6.013(2)(d)),
• the outlet device must have a diameter greater than 4 inches and extend down below the water surface between 33% to 44% of the liquid depth (64E-6.013(2)(e)),
• the inlet and outlet devices must be on opposite ends of the septic tank (64E-6.013(2)(f)), and
• constructed of concrete, fiberglass, corrosion-resistant steel, or other equally durable material ((64E-6.010(2)(a)(1)).
Concrete septic tanks are the most common constructed septic tanks while
fiberglass septic tanks are used in areas where the heavy concrete septic tanks cannot be
transported. Concrete septic tanks are typically rectangular in shape with a 2:1 length-to-
width ratio (Canter and Knox, 1985). The width of a concrete septic tank generally
13
tapers approximately two inches from the top to the bottom. The width profile is usually
a taper or a V-bottom profile (Figure 2-2). Although concrete tanks are commonly used,
they have difficulty remaining watertight from corrosion or at the seal of the tank and the
tank lid. A fiberglass septic tank is commonly cylindrical and easily remains watertight
while in service.
Sludge
Scum
Effluent Inlet Tee Outlet
Tee
Figure 2-1 A Multiple Chamber Septic Tank Schematic
Figure 2-2 Septic Tank Width Profiles (a) V-bottom (b) Taper
Septic tank capacity must be adequate to allow sufficient retention of wastewater
while storing the accumulated solids. The estimated wastewater flow to an OSTDS is 50
gallons per person per day. The effective volume of a septic tank is the volume required
for twenty-four hour retention of entering wastewater and is located between the sludge
14
and scum layer (Khan et al., 2000). The effective capacity is the volume below the liquid
in the septic tank (64E-6.002(20)). The required minimum effective capacity for a home
is based on the regulated estimated flow of two occupants per bedroom (Table 2-1) and is
outlined by the Florida Administrative Codes of Standards. Minimum effective septic
tank capacities for different average sewage flow are presented in Table 2-2.
Table 2-1 Residential Estimated Sewage Flow* Single and Multiple family per dwelling Estimated Sewage Flow (Gallons/Day)1 bedroom with 750 sq. ft. 100 2 bedrooms with 751-1200 sq. ft. 200 3 bedrooms with 1201-2250 sq. ft. 300 4 bedrooms with 2251-3300 sq. ft. 400 *For each additional bedroom or each additional 750 sq. ft., the estimated sewage flow increases by 100 gallons per dwelling unit.
Table 2-2 Minimum Effective Septic Tank Capacities Average Sewage Flow (Gallons/day) Minimum Effective Capacity (Gallons)
0 – 200 900 201 – 300 900 301 – 400 1050 401 – 500 1200 501 – 600 1350 601 – 700 1500
Septic tank operation
Primary treatment of wastewater is performed within the septic tank operation,
consisting of separation of settable solids from wastewater, digest organic matter, store
settled solids, and discharge of clarified liquid for further treatment and disposal in the
drainfield. The septic tank separates solids and grease from liquid through gravity
settling or floatation. The wastewater flow is reduced once it enters the tank, allowing
solids to sink to the bottom while fats and grease float to the top (U.S. Department of
Health, Education, Welfare, 1967). The solids that rise to the top of the liquid level
15
consisting of fat and grease are termed “scum”, and the solids that sink to the bottom of
the tank are termed “sludge”. The effluent within the septic tank containing scum and
sludge is termed septage. The scum and sludge combined represent the total solids in the
septic tank. Wastewater clarification is promoted by three main design parameters: tank
depth, retention time, and chambers (Canter and Knox, 1985; U.S. EPA, 1980; Barshied
and El-Baroudi, 1974). A large tank depth increases the available depth for storage and
settling (Barshied and El-Baroudi, 1978).
Retention time is the average time that the wastewater spends in the septic tank
from entry to exit and is a function of the effective volume, daily household wastewater
flow, and the volume of stored solids (Kahn et al., 2000). As the sludge depth increases,
the liquid volume and detention time in the septic tank will decrease (U.S. EPA, 1980).
At maximum sludge depth, the septic tank should be able to retain the wastewater for a
minimum of 24 hours (Kahn et al., 2000; U.S. EPA, 1980). However, under ordinary
conditions when the storage is not at capacity, wastewater may be retained in the septic
tank for up to three days (Kahn et al., 2000).
Multiple chambers prevent solids from exiting the septic tank when mixing is
induced by hydraulically overloading the tank. Multiple chambers also decrease mixing
and increase the removal of biochemical oxygen demand (BOD) and suspended solids
(U.S. EPA, 1980). The multiple chambers allow mixing of solids with liquid in the first
chamber, while the second compartment receives wastewater at a lowered velocity
allowing the remaining solids to settle from the wastewater again before effluent exits the
tank (Canter and Knox, 1985). A tank with multiple chambers will lead to longer service
time between pump outs (Canter and Knox, 1985).
16
Solids storage uses the remaining septic tank volume that is not required for the
effective volume to settle solids. Scum storage is a layer at the top of the liquid level
with two-thirds of the layer above the liquid and one-third of the layer submerged (U.S.
EPA, 1980). Forty percent of the septic tank volume is typically designed for sludge and
scum storage (U.S. EPA, 1980). Maintenance must be performed when the scum or the
sludge layer is endangered of exiting the tank.
Stored solids are partially degraded within the septic tank. The septic tank is an
anaerobic environment, thus the term “septic” (U.S. EPA, 1980; Laak and Crates, 1978;
U.S. Department of Health, Education, Welfare, 1967). The available anaerobic
microbial cultures partially decompose and liquefy solids (U.S. EPA, 1980; Laak and
Crates, 1978; U.S. Department of Health, Education, Welfare, 1967). The
microorganisms in the septic tank destroy pathogens, mineralize organic matter, and
oxidize material (Canter and Knox, 1985), while reducing sludge and scum to finer
particles, liquid, and gas during decomposition and digestion (U.S. Department of Health,
Education, Welfare, 1967; Amerisep Inc., 1999). The major chemicals in the retained
solids are carbon, hydrogen, nitrogen and sulfur that are transformed during digestion and
decomposition to methane, carbon dioxide, water, ammonia, and hydrogen sulfide
(Baumann, 1978, Wilhelm et al., 1994).
Drainfield Design and Operation
The drainfield treats and disposes clarified effluent by distributing it into the soil
for percolation (U.S. Dept. of Agriculture, 1982). The soil type and appropriate sizing
determine treatment performance of clarified effluent in the drainfield. The treatment in
the drainfield is final treatment of the effluent before disposal and is achieved by
percolation and biological degradation.
17
Drainfield design
The drainfield is designed to discharge clarified effluent into the soil below for
final treatment and disposal (Kahn et al., 2000). The drainfield uses either an adsorption
bed or a drain trench with a mineral aggregate for disposing clarified effluent to the soil
(64E-6.014(5)). The drainfield size depends on of the soil type and estimated
wastewater flow (64E-6.008(5); U.S. Department of Agriculture, 1982). Soil types that
are slightly to moderately limitation are satisfactory for standard subsurface systems that
are determined by a percolation test. A limited soil type is defined as by the percolation
rate with a slightly limited soil having a percolation of less than 2 minutes per inch and a
moderately limited soil having 5 to 10 minutes per inch. Severely limited soil types have
a percolation rate of greater than 30 minutes per inch or less than one minute per inch and
a water table less than 4 feet below the drainfield and are unsatisfactory for standard
subsurface systems because the soil type will not promote the minimum 2 foot
unsaturated soil condition needed for proper treatment (64E-6.008(5)).
A drain trench is the preferred method of subsurface drainfield systems (64E-
6.014(b)). The trench system consists of a perforated pipe discharging clarified effluent
within its own trench (Brown and Peart, 1996). The trench cannot be greater than 36
inches wide (64E-6.014(5)(a)). A trench width less than 12 inches must have a minimum
of 12 inches separating the sidewalls of adjacent trenches (64E-6.014(5)(a)), while a
trench width greater than 12 inches must have a minimum of 24 inches separating the
sidewalls of adjacent trenches (64E-6.014(5)(a)).
An absorption bed system must have the entire earth content of the absorption area
removed and replaced with aggregate and distribution pipes (64E-6.014(5)(b)). The
perforated pipes are all in the same trench, being the single bed. The bottom surface of
18
the absorption bed must not exceed 1500 square feet (64E-6.014(5)(b)), and the
dimensions of the absorption bed must achieve maximum length-to-width ratio practical
(64E-6.014(5)(b)).
The maximum depth of the drainfield from bottom to finished surface must not
exceed 30 inches with a minimum earth cover of 6 inches (64E-6.014(5)(e)). A polyester
bonded layer is used to prevent earth backfill from infiltrating and is placed only on top
of the drainfield (64E-6.014(5)(e); Brown and Peart, 1996). The mineral aggregate
material that is used within the trench or bed is either limestone, slag, quartz rock,
granite, river gravel, recycled crushed concrete, lightweight aggregate, or another equally
durable material (64E-6.014(5)(c)1). The total depth of aggregate must be at least 12
inches throughout the bed or trench with a minimum of 6 inches below the pipe, but not
exceeding 10 inches when total depth is at the minimum (64E-6.014(5)(c)1). A profile of
a drainfield is shown in Figure 2-3.
Backfill
Mineral aggregate
Original soil
Polyester bonded layer
Perforated pipe 6”
12”
Figure 2-3 Drainfield Profile
Drainfield operation
Septic tank effluent is discharged into the drainfield for further treatment through
biological processes, adsorption, and infiltration into underlying soil (U.S. EPA, 2002).
19
The clarified effluent is discharged from perforated pipes through aggregate to evenly
distribute effluent to the soil. Remaining solids (suspended) in the effluent will form a
clogging mat above the soil layer, lowering hydraulic conductivity that prevents soil
saturation (Ayres Associates, 1993; U.S. EPA, 1980). The clarified effluent receives
primary treatment as it percolates through the soil and is disposed of to the water table.
During percolation, clarified effluent is treated by physically entrapping the
remaining particulate matter in the clarified effluent and is responsible for the majority of
wastewater treatment (U.S. EPA, 1980). The clarified effluent percolates through
unsaturated soil to provide adequate removal of pathogenic organisms and other
pollutants from the effluent before reaching the groundwater (Bicki, 1989; U.S. EPA,
1980). The drainfield is responsible for treating organics, inorganics, and pathogens in
the clarified wastewater. Affective treatment is accomplished by a complex arrangement
of primary minerals and organic particles that differ in composition, size, shape, and
arrangement in the soil (U.S. EPA, 1980).
The quality of wastewater treatment depends on how the effluent moves into and
through the soil. Treatment in the soil depends on the soil’s physical properties: size,
shape, and continuity of pore space (U.S. EPA, 1980) and is dependent on the wastewater
nature, soil type, and degree of saturation (Brown, 1998). Unsaturated soil forces
clarified effluent to flow through smaller pores, maximizing treatment compared to
saturated soil where smaller pores are occupied by water (U.S. EPA, 1980). The clarified
effluent travels more slowly in unsaturated soil than in the same soil when saturated. The
slower the velocity of flow, the longer the residence time of the wastewater in the
20
unsaturated soil and the greater the opportunity to treat the wastewater as it travels
through the soil (Brown, 1998).
The two most important soil attributes are soil permeability and seasonal high
ground water table (Brown, 1998). Soil permeability refers to the ability of a soil to
transmit water or air (U.S. Department of Agriculture, 1984). Permeability is based on
the soil characteristics observed in the field, particularly structure, porosity, and texture
(U.S. Department of Agriculture, 1984). Low permeability, such as in clayey soils,
would prohibit vertical flow and would result in horizontal flow with little or no
treatment. A high permeability soil, such as sandy soil, has rapid vertical flow with little
residence time in the unsaturated soil, resulting in limited treatment.
Final disposal occurs when the treated clarified effluent has percolated through
the unsaturated soil and reaches the water table. In areas limited in unsaturated soil
depth, the soil must be built up (mounded) to meet this requirement. Once the effluent
makes it to the water table, its concentration will dilute and eventually make its way to
surface water or a lower aquifer.
OSTDS Maintenance
Inspection ensures that the system is functioning as designed. Pumping the septic
tank should be done when accumulated solids have reached the storage capacity. A
combination of visual, physical, bacteriological, chemical, and remote monitoring can be
used to access system performance (U.S. EPA, 2002). A recurring weakness of many
existing OSTDS management programs has been the failure to ensure proper operation
and maintenance of installed systems (U.S. EPA, 2002).
21
Annual Inspection
The type of inspection and frequency should be determined by the size of the area,
site conditions, resource sensitivity, the complexity of the system, and number of systems
(U.S. EPA, 2002). Mandatory inspections are an effective method for identifying system
failures or systems in need of corrective action (U.S. EPA, 2002). The inspection would
ensure the system is functioning properly and identify any obvious problems such as the
septic tank structure and accumulated solids level (Crites and Tchobanoglous, 1998; U.S.
Department of Health, Education, Welfare, 1967). Scheduled inspections during seasonal
rises in ground water levels facilitate monitoring of performance during “worst case”
conditions (U.S. EPA, 2002). The OSTDS should be inspected for the following:
1. watertight tank condition, 2. clogs in the entrance and exit baffles, and 3. accumulated sludge and scum levels. Maintenance Pumping
Tank pumping or other routine maintenance tasks are seldom required or even
tracked by regulatory authorities of management for information purposes (U.S. EPA,
2002). Septic tanks must be pumped when solid layers are above designed storage
capacity to prevent solids from entering the drainfield and ensure proper operation (U.S.
EPA, 1980; U.S. Department of Health, Education, Welfare, 1967). The septic tank
reaches capacity when the retention time is less than twenty-four hours or the sludge
layer is within three inches of the bottom of the exit baffle (U.S. EPA, 1999; U.S. EPA
1980; U.S. Department of Health, Education, Welfare, 1967).
The pumping frequency is dependent on the accumulation rate of solids that vary
from system to system and is a factor of septic tank design, wastewater characteristics,
and drainfield performance (U.S. EPA, 2000b; Vogel and Rupp, 1999; Bounds, 1996;
22
Mancl, 1984). Recommended pumping frequencies range from 2 to 12 years (Benton
County Environmental Health, 1998; Jones and Yahner, 1992; Loudon, 2002). In some
counties such as Brown County, WI, the pumping frequency is regulated. In this
particular case, tanks are required to be pumped out every three years unless the solids
are less than one-third of liquid capacity (County Codes 11.073(c)(5)(a)). Pumping a
system too often increases the volume of septage that must be unnecessarily treated.
EPA Management Guidelines
The importance of OSTDS management became a national issue when it was found
that improperly operated systems were becoming contributors to major water quality
problems. The U.S. EPA created OSTDS Management Guidelines to raise the level of
performance of OSTDSs through improved management programs (U.S. EPA, 2000).
The management guidelines present five separate model programs as a progressive
series where management requirements become more rigorous as the system technologies
become more complex or when environmental sensitivity increases. The management
guidelines are voluntary and apply to all OSTDSs. The models are structured to allow
communities to pick and choose or adopt complete sets of management criteria that will
provide the necessary level of protection while balancing costs and other institutional
factors. The model programs provided by the U.S. EPA are intended to be benchmarks
that aid communities in identifying a management objective, in evaluating whether
current programs are adequate, and ultimately, in determining an appropriate
management program and the necessary program enhancements to achieve management
objectives and public health and environmental goals.
23
Previous Solids Accumulation Rate Research
Discussion of Prior Studies
Three previous studies examined solids generation in a septic tank. A study by the
Public Health Service (1949) was the first extensive study, and the results reported in this
study are still consulted today (Bounds, 1992). A second study by Terry Bounds in 1986
was used to validate the results of the Public Health Service. A third study by J. D.
Moores in Novia Scotia was performed in 2002 to examine accumulation rates in
OSTDSs.
United States Public Health Service
The United States Public Health Service (USPHS) presented a study on the
accumulation rate of solids in a septic tank in 1949. The USPHS study used 300 single
family homes with septic systems in 9 areas - Dallas, TX; Kansas City, MO; Cook
County, IL; St. Joseph, IN; Cincinnati, OH; Kalamazoo, MI; Warwick County, VA;
Cuyahoga County, OR; Montgomery County, AL; and Dade County, FL. An inspection
was made of each home that had available history on its system. The criteria for
participation was that it
1. must be a single family home, 2. have reasonably accurate history with known last pump out date of the tank, 3. the tank could not be full, and 4. the outlet device must be intact. The inspection documented the number of people occupying the home, the number of
bedrooms, water usage from the water meter, the discharge units (bathroom, laundry,
kitchen and/or water closet), and the measured solids in the septic tank.
A total of 205 tanks were used in their analysis, with years of service ranging
from 0.5 to 39 years (Table 2-3). The years of service is the time since either the tank
24
was pumped-out or installed. The mean and median accumulation rates in gallons per
person per year for the participants who had their systems in service for 2 years or less
are provided in Table 2-4. Table 2-4 shows that the accumulation rate in these tanks was
high in the first half year of service, after which it quickly decreased.
Table 2-3 USPHS Septic Tank Years of Service (Weibel et al., 1949) Years of Service Number of Participants
0.5 4 0.75 4
1 13 1.5 2 2 13 3 24 4 14 5 14 6 30 7 18 8 23 9 19 10 4 11 6 12 5 13 4 16 1 20 3 21 1 32 2 39 1
Table 2-4 USPHS Accumulation Means and Medians (Weibel et al., 1949) Years of Service
Number of Reports
Mean Scum
Mean Sludge
Mean SC* & SL*
Median Scum
Median Sludge
Median SC* & SL*
0.5 4 17.35 71.44 88.79 19.60 63.06 89.61 0.75 4 2.84 31.34 31.94 0 32.39 34.41
1 13 5.16 18.25 23.41 2.02 16.23 22.59 1.5 2 13.09 8.53 21.62 13.09 8.53 21.62 2 24 4.04 10.17 14.21 2.84 9.35 15.41 * SC = scum; SL = sludge.
25
From the results of the Weibel et al. study, the USPHS concluded that sludge and
scum accumulated more quickly per person in the first 6 years of operation. Sludge
accumulation became constant at the 7-year point, while scum accumulation became
constant at the 3-year point.
Septic tanks with capacities less than 125 – 175 gallons per person retained fewer
solids than tanks greater than 175 gallons per person. The lower capacity tanks could
result in a lower retention than the larger was because of a lower retention time that
allowed settling of the solids. The increased depth of the liquid level was found to
increase the solid removal from the wastewater that also increases retention time,
increasing solids accumulation. The average accumulation of scum, sludge, and total
solids is presented in Figure 2-3 with values in Table 2-5.
0
5
10
15
20
25
30
35
0 5 10 15 20 25
Years of Service
Solid
s A
ccum
ulat
ion
Gal
lons
/per
son/
year
Sludge & Scum
Sludge
Scum
Figure 2-3 PHS Rate of Scum & Sludge Accumulation. The points marked on the figure
are from the mean accumulation rate of each year and the trend line marks the average accumulation over all. (Weibel et al., 1949)
26
Table 2-5 Total per Capita Sludge and Scum Accumulation (Weibel et al., 1949) Gallons/ Capita/Year Total Gallons/Capita No. Years
Scum Sludge Scum &Sludge
Scum Sludge Scum & Sludge
1 6.21 18.33 24.54 6.21 18.33 24.54 2 3.52 12.19 15.71 9.72 30.52 40.25 3 3.22 9.58 12.79 12.94 40.10 53.04 4 3.22 8.15 11.37 16.16 48.25 64.41 5 3.22 7.48 10.70 19.37 55.73 75.10 6 3.22 7.11 10.32 22.59 62.84 85.43 7 3.22 6.88 10.10 25.81 69.72 95.53 8 3.22 6.88 10.10 29.02 76.60 105.62 9 3.22 6.88 10.10 32.24 83.48 115.72 10 3.22 6.88 10.10 35.46 90.36 125.82 11 3.22 6.88 10.10 38.67 97.25 135.92 12 3.14 6.88 10.02 41.82 104.13 145.94 13 3.14 6.88 10.02 44.96 111.01 155.97 14 3.14 6.88 10.02 48.10 117.89 165.99 15 3.14 6.88 10.02 51.24 124.78 176.02 16 3.14 6.88 10.02 54.38 131.66 186.04 17 3.14 6.88 10.02 57.53 138.54 196.06 18 3.14 6.88 10.02 60.67 145.42 206.09 19 3.14 6.88 10.02 63.81 152.30 216.11 20 3.14 6.88 10.02 66.95 159.19 226.14
Bounds Study
The Bounds study (1992, 1996) was centered in Glide County, Oregon where the
largest septic tank effluent pump (STEP) system (in the nation) was in place. A STEP
system uses septic tanks (for initial wastewater treatment) in conjunction with a sewer
system (receives clarified effluent from the septic tanks). A pump is in place in the septic
tank that pumps the clarified effluent into the sewer lines. The use of a STEP system
within a septic tank can cause turbulence and washout of solids. The STEP system in
Glide County had 468 septic tanks and over 20 miles of collection lines. A step system,
like OSTDS systems, still needs to be pumped out as solids fill the tank, and the septic
tanks in Glide County were pumped when sludge was within 6 inches and scum was
within 3 inches of the outlet (Bounds, 1992).
27
In his study, Bounds used 450 tanks in Glide County. Residents were interviewed
and information on the number of people, their ages, and their habits that would impact
performance was gathered (Wilcox, 1992). A total of three measurements were taken in
this study at 2.8 years, 5 years, and 8 years. The first measurement of solids was 2.8
years after initial pump out, and 19 of the tanks were randomly selected for solids
measurement 5 years after pump out. On the third measurement at 8 years after pump
out, solids were measured in all the tanks again. Figure 2-4 illustrates the average
accumulation of total solids in Glide County.
0
20
40
60
80
100
120
0 2 4 6 8 10
Time (Years)
Tota
l Sol
ids
Volu
me
per C
apita
(G
allo
ns)
Figure 2-4 Bounds Study Volume of Total Solids For Eight Years
As shown in Figure 2-4, after 8 years, the solids levels were at half of the storage
capacity. Bounds (1992) also noted that microorganisms required up to 2 years to reach
solid decomposition activity levels that are high enough to impact accumulation rates.
The results of this study also showed that the infiltration of groundwater resulted in solids
accumulation decreasing and that scum accumulation increased with increased garbage
disposal use. The Bounds Study is the first and only study that observed the
28
accumulation of solids over an extended period of time, unlike the single inspection made
by the Public Health Service study.
Moore Study
The Moore study (2002) centered in Nova Scotia and presented a literature review
and results of a field analysis of pumping frequencies and maintenance procedures.
Measurements were collected from 40 tanks that had accurate records of the most recent
pump out. The OSTDSs used in this study, ranged in operation time from 0.75 to 12
years from the last pumping and ranged in the occupants it served from 1 to 60
occupants. The mean and median numbers of years of service were 3.64 years and 3
years, respectively, whereas the mean and median occupants in the home were 6.4 people
and 3 people, respectively. The average sludge accumulation rate from those tanks was
10.93 gallons per person per year, and the scum accumulation rate was 5.86 gallons per
person per year.
Table 2-6 Moore Study (2002) Distribution of Years of Service Years of Service Number of Systems
0.75 1 1 3
1.5 3 2 3
2.5 2 3 11 4 6
4.5 1 5 6 6 2 11 1 12 1
29
Table 2-7 Moore Study (2002) Scum and Sludge Accumulation Trends Years
of Service
Number of
Reports
Mean Scum
Mean Sludge
Mean SC* &
SL*
Median Scum
Median Sludge
Median SC* & SL*
0.75 1 20.36 0 20.36 20.36 0 20.36 1 3 12.09 0.85 12.94 9.16 0 9.16
1.5 3 17.41 7.54 24.94 16.07 9.04 28.12 2 3 17.73 6.53 24.26 22.91 3.05 17.82
*SC = Scum; SL = Sludge.
The results of this study in Novia Scotia were used to educate homeowners in best
management practices. While Moore did not determine significant influences on
accumulation, he but did discuss the effects of system additives and water softener brine.
System tank additives, in some cases, resulted in limitations in solid separation from
wastewater, and systems receiving water softener brine showed a decrease in scum
accumulation.
Comparison of Pump Out Frequencies
The prior studies illustrate the current documented knowledge in the accumulation
rate of solids in the septic tank. A summary of their results is presented in Table 2-8,
which lists the number of participants, the accumulation of scum and sludge in gallons
per capita for the first year, the model equation for total solids, and the solids
measurement method. The accumulations in the first year for the PHS study (1949) and
the Moore study (2002) are similar. Also, the Bounds study (1992) is considered to be in
agreement with the PHS study in terms of accumulation rates, even though the 1-year
sludge and scum accumulation volumes are not available.
30
Table 2-8 Summaries of Prior Studies
Study Participant
Count
1 year Sludge Accumulation Gallons/Capita
1 year Scum Accumulation Gallons/Capita
Total Solids Model*
Gallons/Capita Method of
Measurement PHS, 1949 205 18.33 6.21 8.6t + 14.96 Single
Inspection Bound, 1992 450 N/A N/A 23.4t0.7
Multiple inspections during 8 years
Moore, 2002 40 16.94 5.91 22.85t Single
Inspection * t = time, years.
The volume of total solids accumulated during the first year is within less than 25
gallons per capita for all 3 studies. The PHS and Bounds studies result that the
accumulation rate during the first year is greater than the accumulation rate in subsequent
years. The PHS and Bounds studies also used over 200 participants, with the Bounds
study using twice the number of participants than with the PHS study. Unlike the PHS
and Bounds study, the Moore study had less than 50 participants and had a constant
accumulation rate from the first to the last year (8th).
The shortcomings of these studies were the lack of statistical information and the
method of measurement. Unfortunately, the studies did not present the standard
deviations for the data that was collected. The standard deviation was calculated for the
Moore study with the mean and standard deviation for scum, 23.32 gallons per year and
28.74 gallons per year, respectively, and for sludge, 40.46 gallons per year and 47.78
gallons per year, respectively. The single inspection made during the PHS and Moore
studies assumed that accumulation trends in the OSTDSs are similar. More than one
measurement from a system determines accumulation trends that result in identifying the
accumulation rate peak for each solid. In the Bounds study two measurements were
31
made, and the peak volume is assumed to be between 2.8 years and 8 years. The Bounds
study was the first to monitor an OSTDS over an extended time period after pump out.
The objective of this study was to determine the accumulation rate for scum and
sludge during the first year of service. The accumulation rate was determined from
multiple measurements of over 200 participants. Performance variables were determined
from wastewater, system design, and surrounding environmental characteristics that
could possibly affect the accumulation and those variables that do affect the system were
to be included in a model to predict solids accumulation. The overall outcome of this
study is an attempt to produce best management practices to control solids accumulation,
along with the expected solids accumulation rate when considering an individual system
in its entirety.
32
CHAPTER 3 METHODS
Objectives
The objectives of this study were to identify the relevant variables, collecting them
from a select population, develop a model with the collected data and assess the relative
impact of variables to the OSTDS performance. The result of this study is aimed to also
develop best management practices that homeowners can implement. The goals used to
address these objectives was:
• Identify potential performance variables from household activity, system factors, and surrounding environment characteristics,
• Create a survey to collect identified performance variables from OSTDS-using homeowners,
• Create a database of the systems to store contact information, collected performance variables, and inspection measurements,
• Determine expected accumulation rates for scum and sludge, • Determine what performance factors that impact OSTDS performance, and • Create a model using performance factors that impact OSTDS performance.
In this study, the OSTDS performance and management practices were monitored
over a sample population of 221 Charlotte County residents to determine the most
significant underlying factors that influence accumulation of solids. Potential
performance-influencing variables were identified and databased by UF using a
combination of surveys, permits, and inspections. After pumping out the participating
homeowners’ OSTDS during an initial inspection by CCDOH, the depths of solids were
measured at approximately 6- and 12-month time intervals, and this accumulation
information, transferred to UF, served as the response variable in multiple linear
33
regression (MLR) models. The Biostatistics Consulting Laboratory in the Department of
Statistics at UF performed all analysis, and the resulting trends showing which variables
influenced solids accumulation were identified.
Task 1: Identification of Relevant Variables
As a first step in better understanding the problem of OSTDS failure and its causes,
all factors influencing accumulation were determined using an exhaustive literature
search of OSTDS care and maintenance fact sheets, reports from previous research, and
manuals for system management. The factors identified in the literature as having
potential impact on OSTDS performance were categorized into three major groupings
describing 1) wastewater characteristics, 2) system design and condition, and 3)
surrounding environment. The wastewater characteristics, consisting of hydraulic
loading and wastewater quality, are factors that describe the wastewater that enters into
the system from the home. The OSTDS system design and condition consists of specific
information on the septic tank and the drainfield. The surrounding environment consists
of the site elevation and groundwater elevation. A more thorough description of each
grouping of factors is provided below.
Wastewater Characteristics
The hydraulic loading and quality of wastewater entering the system from the
home can be best predicted by household practices, such as occupancy, home activity,
home characteristics, and water consumption (U.S. EPA, 2000; Crites and
Tchobanogloos, 1998; Anderson and Siegrist, 1989; Siegrist, 1983). Home activity
varies with occupants’ practices and includes water-using appliances and disposal
practices (U.S. EPA, 2002; U.S. EPA, 2000a). Home description factors typically stay
constant despite change in occupancy and include water-conserving plumbing fixtures,
34
water-using appliances, water source, and home characteristics (U.S. EPA, 2002;
Hammer and Hammer, 2001; Kahn et al., 2000).
Occupancy factors potentially impact water characteristics by the number and age
of residents, annual time of occupancy, and ownership period. The number of residents
significantly alters water consumption and wastewater production (U.S. EPA, 2000). The
annual occupancy duration differentiates seasonal residences that accumulate solids for a
short period from year-round residences, which accumulate solids all year. The seasonal
residences allow the system to rest, resulting in less accumulation; however, a longer
length of residency result in a more acclimated microbial system for decomposing
organic particles, thus decreasing accumulation as well.
Water-using appliances and homeowner practices alter the hydraulic loading and
wastewater quality. Wastewater-creating appliances are typically washing machines,
automatic dishwashers, and garbage disposals, which drain to the septic tank (Kuhner et
al., 1979). Washing machine and dishwasher descriptions alter the hydraulic loading
depending on load capacity and number of loads per week. Water-conserving washing
machines, typically front-loading, use significantly less water compared to traditional
top-loading machines. Detergent type and use, liquid fabric softener, and bleach also
alter wastewater quality.
Kitchen sink activity influences wastewater. For example, removal of solids from
dishes to a solid waste container before cleaning in a sink or dishwasher reduces the
amount of particles exiting through the drain (Kuhner et al., 1979). Garbage disposal use,
therefore, increases solid accumulation by introducing food particles into the system
(U.S. EPA, 2002; Hammer and Hammer, 2001; Bounds, 1996). Also, frying foods
35
compared to baking introduces a higher amount of oil and fats if the grease from frying is
disposed down the sink drain (U.S. EPA, 2002; Kahn et al., 2000).
Direct disposal of solids other than toilet tissue and non-water liquids increases
solids accumulation. Typical liquids other than water that are disposed in the drain
include paint thinners, pesticides, salad dressing oil, sour milk, disinfectants, bleach, and
food grease. Typical solids that are improperly introduced in the wastewater system are
feminine products, disposable diapers, plastic items, nylon products, medications, rubber
products, cigarette filters, paper towels, and any type of food item (U.S. EPA, 2002;
Hammer and Hammer, 2001; Lesikar, 1999a).
System Design
System design components are functions of the septic tank and drainfield (Lesikar,
1999b; Laak, 1980). The septic tank’s age, dimensions, tank volume, number of
chambers, and number of tanks along with the drainfield configuration, area, and
elevation are included in this study. System age is used to note an impact of
performance, especially with the many systems in the nation over 30 years old. The
drainfield description is used to determine if the drainfield impairs or promotes the
performance of solid accumulation. When a drainfield is not operating correctly and
drainage is not occurring, water can reenter into the septic tank allowing solids to exit.
Bounds (1996) noticed this effect from high groundwater levels on accumulation in the
septic tank. The elevation of the drainfield is in relation to the elevation of the center of
the paved frontage road of the residence.
Surrounding Environment
The surrounding environment factors that potentially hinder system performance
are site elevation and the contact surface of the tank or drainfields with the groundwater.
36
The elevation of a system determines its ability to drain and the possibility of water
infiltration. Systems in contact with groundwater can cause infiltration in leaky tanks
(Bounds, 1996), while storm and floodwaters that flood drainfields can cause infiltration
to both leaky and non-leaky tanks. The estimated water table is the water level that is
expected to be in a year of normal rainfall during the wet season characterized by zero
pore pressure or simply described as the water level in an unlined, augered hole at the end
of the wet season in a year of normal rainfall.
As previously mentioned, the purpose of the literature review phase of this project
was to construct a thorough list of all potential factors that may impact OSTDS
performance. The potential performance variables that were included on this list are
presented in the results chapter to follow. Values of each these variables were
subsequently obtained by a combination of surveys of selected participants, permits, and
inspections.
Task 2: Selection and Recruitment of Participants
Three criteria were used in choosing participants. First, a participant should live in
a region with dominant soil type containing 2% or more of the total population of
OSTDSs in Charlotte County. Using systems within these “most populated” soils would
ensure representation of a majority of Charlotte County’s OSTDS population. However,
because of the unanticipated difficulty that CCDOH would have in visiting the large
number of widely distributed OSTDSs, a second criterion was followed that participants
would live in only one largely populated region of Charlotte County in proximity to
CCDOH. Finally, the third criterion was the willingness of the selected residents to
participate in this study. Below is a detailed description of how participants were
randomly selected and recruited to participate in this study.
37
Subtask 2A: Participant Selection
Location of systems in Charlotte County
The Charlotte County GIS (CCGIS) Department located homes with OSTDSs from
sewer line data and property parcel information obtained from the Charlotte County
Property Appraisers. The property parcel information and the sewer line data were then
transferred to a map of the region to identify which homes have sewer services or
OSTDSs. Properties that were in contact or adjacent to the sewer lines were considered
sewer-using properties. The remaining properties were then identified as OSTDS-using
properties.
Identification of OSTDS density by soil type
A map showing which residences possessed OSTDSs was subsequently overlayed
with soil types in Charlotte County using ARC GIS software (ESRI, Redlands, CA,
USA). The resulting map, Figure 3-1, allowed counting of the number of OSTDSs in
each soil type and, ultimately, the determination of which soil types contained the most
systems.
Random selection of potential participants
In response to a request by CCDOH, one specific area of Charlotte County was
chosen for the study to ease accessibility for pump-outs and solid measurements. This
region, Mid-County 2 (MC2), was selected for its proximity to CCDOH and high density
of OSTDSs (refer to Figure 3-1). Forty-five percent of the county’s systems (and nearly
50% (20,822) of the systems when considering “most populated” soils only) are in MC2.
The CCGIS Department randomly extracted 2,300 systems from their database using
ARC GIS utilities. The property identification number was cross-referenced with the
property appraiser’s database to extract the owner’s name and mailing and physical
38
addresses, and this information was transferred to a master Access (Microsoft, Mountain
View, CA, USA) database.
Subtask 2B: Recruiting of Participants
Recruitment efforts began in October 2001 and ended in June 2002 using a
combination of methods as described below.
Newspaper article
In March of 1999, the Charlotte Sun Herald published an article announcing the
pilot program that will involve 400 OSTDS users. The article provided contact
information for interested homeowners. Also, residents were informed of a free septic
tank pump out for their participation.
Mailing campaign
The mailing campaign was focused on residents with OSTDSs selected by the
CCGIS Department. The volunteers who responded to the newspaper article were also
included in the mailing campaign. The mailings consisted of a postcard followed by a
delivered package one week later. The postcard informed the residents of the study and
that they would soon be receiving a package within the following week. The packages
consisted of the following items:
1. a flier describing what was requested of the participants and how the gathered information would be used (Appendix A),
2. two consent forms: one for the resident’s record and one to sign and mail back (Appendix A),
3. the initial survey (to be discussed below), 4. a self-addressed and stamped return envelope, and 5. a pamphlet on system care.
39
Figure 3-1 Selected Systems in Mid-County 2 Basin per Soil Type
40
A followup postcard was mailed a month after the survey to remind those
participants not responding to the mailings to do so as soon as possible. Five mailings
totaling 1,512 packages were mailed from October 2001 to May 2002 in order to recruit
participants.
County fair
A booth at the Charlotte County Fair was used for recruiting from February 15-23,
2002. CCDOH and UF staff manned the fair booth and provided educational information
on system operation and maintenance and was used to invite residents to participate in the
study. Residents who volunteered to participate completed the consent form and initial
survey at the booth and received a pamphlet, flier, and a copy of the consent form.
Although the mass mailings and newspapers were distributed to many, the results
were relatively poor for participant recruiting. The fair booth was the best method for
recruiting participants, most likely because most people feel more comfortable receiving
information in a personal conversation where they can have their questions answered
with an immediate response rather than from passive sources, such as the fliers mailed
and articles printed in the newspaper that placed the burden on them to call for additional
information.
Subtask 2C: Division of Recruits into Phase 1 and Phase 2 of the Study
The list of participants in the study was divided into two phases, and, in doing so,
care was taken to make certain that Phase 1 and Phase 2 contain an even distribution of
the studied performance (independent) variables. The groupings of the participants were
randomly prepared taking into account only information received in the initial survey of
household characteristics. To ensure even distribution of characteristics, both qualitative
data denoted by “yes” or “no” (e.g., washing machine draining to septic tank) and
41
quantitative data (e.g., number of people per home) were divided. The quantitative data
were first converted to two levels, high (H) and low (L), depending where they fell in
relation to a median value. The resulting percent distribution of high and low levels of
each was maintained for both Phases 1 and 2. These results will be discussed in Chapter
4.
The pump out date was also used to separate the phases so that participants whose
systems were the first to be pumped would be in Phase 1, while those whose systems
pumped last would be in Phase 2. The pump out date ensured that Phase 1 final
measurements would be completed in a timely manner for subsequent analysis. There
were 179 Phase 1 participants and 42 Phase 2 participants.
Subtask 2D: Data Collection and Databasing
As previously described, the independent variables for the constructed model are
the performance variables that have potential to significantly impact system performance,
represented in this study as accumulation rates. The performance variables were
collected by means of surveys, permits, and inspections. Survey information was mostly
completed by mail, but could have been completed in person or by telephone.
Initial survey
With consultation with Dr. Chris McCarty of the UF Survey Research Center in the
Bureau of Economic and Business Research and reference to previous CCDOH surveys,
special consideration was taken in the design of the initial survey in order to enable each
OSTDS owner to accurately provide as many of the potential performance variables for
each system as possible. As previously described, the initial surveys were sent to
prospective participants in Charlotte County from December 2001 to June 2002 after
receiving approval from the UF Institutional Review Board (IRB). The initial survey,
42
shown in Appendix B, allowed collection of information on residency, home activity,
home description, and surrounding environment.
Follow-up survey
A followup survey, also approved by UF’s IRB, was sent to the participants to
clarify answers to questions that were asked in the initial survey, to ask more detailed
questions, and to note any changes in habit. Most participants mailed their surveys back,
while others were called to complete the survey by phone. The followup survey was sent
to participants within 6 months to one year after they had received the initial survey, and
a copy of the followup survey is provided in Appendix B.
Permit information
Permits of most of the participants’ OSTDSs were obtained from CCDOH, and
information concerning system description and characteristics of the surrounding
environment was gleaned from these documents and transferred to the Access database.
It is important to note that, of the 221 participants, 25 permits could not be located. As a
result, the system and environmental data were either left blank or estimated.
Inspection information
Initial inspections of each participant’s OSTDS were performed by CCDOH as a
means of judging eligibility for participation in the project. Their criteria for eligibility
was that the system could not be experiencing a failure, must operate properly, and have
an intact outlet-tee. While at each home, CCDOH was also able to collect valuable
information for the modeling phase, including the corrosion condition and water
consumption. Also during the initial inspection, the septic tank was pumped out, the tank
condition evaluated, and water use determined.
43
Height measurements
After the initial inspection, two measurement inspections were performed at 6 and
12 months to obtain the change in height of scum, sludge, and liquid in the tank. The
CCDOH personnel used a raven meter to measure sludge height and a measuring tape to
measure the height of scum. A raven meter is a device that uses light to detect solids.
When light is blocked by solid particles in the tank, it is assumed that the sludge blanket
begins at that elevation. The scum is not uniformly thick in the tank, and the average
thickness must be estimated. Measurements were taken at the exit end of the tank, unless
the septic tank was a multiple chamber tank where measurements were taken in the first
chamber.
Databasing of all variables collected
An Access database was created to contain the data collected for each participant.
The database was first created to include the 2,300 OSTDS homes randomly selected by
the CCGIS Department. Mailing labels were created from the database for recruitment
from those 2,300 OSTDS homes. Separate forms in Access were created for entering
data from the initial and the followup surveys for ease and efficiency.
Task 3: Calculation of Solids Accumulation
A primary focus of this project was assessment of the relative significance of the
collected performance variables (independent variables) on the measured solids
accumulation (dependent variables). Before regression analysis could be performed, the
data needed to be converted to the proper format. The description below provides details
of how the solid height data were used to calculate 6- and 12-month accumulation rates.
In order to begin creating a model, it was necessary to translate a change in sludge
and scum heights to a volume change. With these volume changes in hand, an
44
accumulation rate of sludge and scum was determined. A discussion of how these
volume rates were calculated follows.
Subtask 3A: Determination of Tank Dimensions
The tank volume and manufacturer for each participant was collected from permit
information. Tank schematics were collected from the CCDOH and Mr. Paul Booher of
the Florida Department of Health located in Gainesville, Florida. A total of 10 different
manufacturers’ tanks were present in the study; however, only 8 of the manufacturers’
tank schematics, giving dimensions, were located. Twenty-seven participants either had
no tank information or had unavailable schematics, making it necessary to estimate tank
dimensions. Two methods were used to estimate dimensions depending on the tank
volume. Figures 3-2 and 3-3 illustrate the dimensions of V-bottom and taper-side tanks
(respectively) with the appropriate nomenclature. The taper-side tank maintains a
constant sidewall slope from the inlet to the bottom of the tank, unlike the V-bottom tank
that has 2 different sidewall slopes with the slope being greater at h3 than at h2.
Those tanks with no available dimensions and less than 600 total gallons volume
(representing 2.3% of the 221 OSTDSs in this study) were assumed to have a 2:1 length-
to-width ratio, a 2-inch taper to the bottom, and a 15% airspace volume. The tank’s
average surface area was calculated from the tank’s volume divided by the liquid height.
The method used to calculate the tank dimensions shown in Figure 3-2 and 3-3 is
provided in Appendix D.
Those tanks with no available dimensions (22 systems total) and volumes of 750,
900, and 1050 gallons were assumed to have the same average dimension ratios as the
same size tanks of known dimensions. The height from the entrance baffle to the lid and
the outer wall width were estimated and assumed constant for each corresponding tank
45
volume. The estimated dimensions were calculated using lid width and length, liquid
height, along with the dimension relationships for each tank volume. The following
dimension relationships noted in Figures 3-2 and 3-3 were calculated:
w1
w2
w3
l1
l2
l3
h2
h3
h1
HL
a. b.
Figure 3-2 V-bottom Septic Tank Schematic with Dimensions (a) width profile (b) length
profile (w1 is the width at the top of the tank; w2 is the width at the top of the sharp bottom taper; w3 is the width at the bottom of the tank; l1 is the length at the top of the tank; l2 is the length at the top of the sharp bottom taper; l3 is the length at the bottom of the tank; h1 is the height from the top of the tank to the liquid level, h2 is the height from the liquid level to the top of the sharp bottom taper; h3 is the height from the top of the sharp bottom taper to the bottom of the tank)
h2 + h3
w1
w3
l1
l3
h1
HL
a. b.
Figure 3-3 Taper-Side Septic Tank Schematic with Dimensions (a) width profile (b)
length profile (w1 is the width at the top of the tank; w3 is the width at the bottom of the tank; l1 is the length at the top of the tank; l3 is the length at the bottom of the tank; h1 is the height from the top of the tank to the liquid level, h2+h3 is the height from liquid level to the bottom of the tank)
46
1. R1 = w2/w3 2. R2 = h2/h3 3. R3 = (w2-w1)/(h1+h2) 4. R4 = (l3-l1)/(h1+h2+h3) 5. h1 6. t (thickness of wall)
The dimension parameters, w1, w2, and w3, represent the inner width dimensions,
w1 at the top of the tank, w2 in the middle, and w3 at the bottom of the tank. As noted in
Figure 3-4, the w2 inner width dimension was chosen at the change in slope for V-bottom
profiles, and at a set distance from the bottom in taper profiles. This established distance
used in taper profile tanks was 10 inches. The inner depth was also presented at these
distances, defined as l1, l2, and l3, and l2 being the length in the tank at the break in slope
like w2. The height dimension parameter is based on the inlet baffle. The distance from
the top of the tank to the center of the inlet baffle is termed h1. The distance from the
center of the inlet baffle to the break where l2 and w2 is measured is termed h2. The
distance from the bottom of the tank to the break where h2 ends is termed h3.
Subtask 3B: Calculation of Solids Volumes in Each Tank
Solids accumulation was calculated using height measurements obtained during
measurement inspections. Scum and sludge heights in each participant’s tank were
measured by CCDOH in inches and converted by UF to a volume in gallons based on
tank dimensions.
47
w1
w2
w3
l1
l2
l3
hsl
h2
h3
h1 hsc
HL
Figure 3-4 Height Dimensions for Liquid Height (HL), Scum Height (hsc), and Sludge
Height (hsl)
The analytical solutions to calculate solids volumes in a V-bottom septic tank and a
taper-side septic tank are given in Equations 3-1 and 3-2. The dimensions used in the
analytical solutions are illustrated in Figure 3-4, which also demonstrates liquid height
(HL), sludge height (hsl), and scum thickness (hsc). All dimensions and solid and liquid
heights arepresented in units of inches, and the resulting volume is in units of gallons.
(EQN 3-1) if hsl < h3 use
( ) ( )231/*2*2
4 3
323
3
323
−+
−+= ss
sSL h
hll
lhh
www
hV
and if hsl > h3
( ) ( )
( ) ( )( ) ( ) ( )
( ) ( )231/
2*2*4
4
312
2123
21
212
3
2323
3
−
+−
+
−
+−
+−
+
++
=hh
hhll
lhhhhww
whh
llww
h
V
sss
SL
(EQN 3-2) ( ) ( )
231/*321
311
321
311
++−
−
++−
−=hhhwwH
whhh
llHlhV LL
scsc
48
The sludge calculation in the tapered-side tanks was based on the trapezoidal width
profile. Sludge heights less than h3 were calculated with the top of the sludge blanket as
the top of the trapezoid and the bottom of the tank, and the trapezoid area is multiplied by
the average length of the tank between the top and bottom of the trapezoid. For sludge
depths greater than the h3, the volume is calculated by adding the volume of the lower
trapezoid (top and bottom of h3), and the volume of the trapezoid between the top of h3 to
the top of the sludge blanket. The scum volume was calculated by the scum thickness
multiplied by the width and length of the tank at the liquid height (HL).
Subtask 3C: Calculation of Accumulation Rates
Two different methods were used to determine solids accumulation rates. An
accumulation rate was calculated for the first six months and second six months (using
the change in volume over time) while another accumulation rate was determined using
all three data points (0-, 6-, and 12-month solids volumes) using the least squares method.
Accumulation rates were calculated on a per system basis and a per person basis.
Subtask 3D: Assessment of Trends in Accumulation
The accumulation of solids during the first year of service was evaluated for trends,
by plotting the volume of solids from the time the septic tank was pumped. The
accumulation trends were assessed for a decrease, no change, or increased change in
volume from 6 months to 12 months and grouped accordingly. The accumulations that
increased were grouped by comparing if the change in volume during the first 6 months
was equal to, greater than, or less than the change in volume during the second six
months. The first-year accumulation rate was evaluated for the number of people in the
home, the septic tank size, and the capacity of the septic tank.
49
Task 4: Interpretation of Phase I and Phase II Model Results
The accumulation rates and performance variables were subsequently subjected to
correlation analysis in order to quantify the influence of each performance variable on
OSTDS performance. The correlation analysis and model development were performed
by Dr. Gary Stevens of the Biostatistics Consulting Laboratory (BCL) in the Department
of Statistics at the University of Florida. The specific methods used are described below.
Subtask 4A: Assess Sample Size
The mean and standard deviation values of the linear accumulation rates were
evaluated to determine if the sample size used was statistically significant. There are two
considerations in determining the appropriate sample size: the tolerable error that
establishes the width of the interval and the level of confidence. The tolerable error was
based on the smallest increment of measure.
In most cases, a confidence interval of the mean was too wide. The solid
accumulation is based on a vertical measurement and tank dimensions. However, the
measurement is only accurate to ¼-inch. A ¼-inch error can cause error in the solid
volume accumulation rate that can range from 0.002 to 0.009 gallons per day, depending
on the tank dimensions. An average of possible accumulation rate error for each tank
was used for the margin of error when calculating the significant sample size needed for a
95% confidence of the population mean. The sample size is calculated from Equation 3-
3 (Ott and Longnecker, 2001). The sample size equation is based on an estimated
standard deviation (σ), confidence interval (α), the tolerable error (W), and the Z statistic
(Zα/2).
50
(EQN 3-3) ( )
( )2
222/
2WZn σα=
Subtask 4B: Assessment of Correlations
Correlations determine if a relationship exists between two variables. A
correlation was done for the potential performance variables to predict accumulation and
trends. A correlation measures the strength of the linear relationship between the two
properties and the stronger a correlation is; the better the variable is in predicting the
accumulation. The relationship is determined by the coefficient of determination, r-
square. A value of zero indicates that no predictive value in using the variable and a
value of 1 indicates perfect predictability (Ott and Longnecker, 2001).
Correlation analysis to determine the relationships between the accumulation rates
and performance variables was conducted using accumulation rates of all the systems
together and using rates divided into two specific groups. Dr. Stevens used the Statistical
Analysis Software (SAS) to perform linear correlations of the performance variables to
the accumulation rate.
The two groups rates, termed as “excessive” and “not excessive,” were chosen
based on the differences in accumulation observed. Distribution plots provided a visual
means of dividing the rates into these two categories.
Subtask 4C: Assessment of Model Results
The accumulation model was calculated by multiple linear regressions that relate
the accumulation rate to a set of performance variables. The multiple regression model
that best predicts the accumulation rate is indicated by the coefficient of determination.
A coefficient of determination, r-square value, of 0.400 was the assumed cutoff value that
51
shows a relation when correlating variables, but a 0.700 was the cutoff value for a
significant model. The r-square value used as the cutoff limit is based on the 0.6 being
the minimum value that a scatter plot shows a relationship between the variables and
accumulation rates. Using a 0.4 value for correlations ensures that all correlating
variables will be included. A 0.700 value used as an r-square cutoff value makes the
model have a mandatory relationship without being too stringent
52
CHAPTER 4 RESULTS AND DISCUSSION
Task 1: Identification of Relevant Variables
As discussed in Chapter 3, the purposes of the literature review stage of this project
were to not only to assess current knowledge of OSTDSs and their best management but
also to identify any potential variables that may impact their performance. A large group
of these “performance” variables were assembled and grouped into three basic categories,
wastewater quality/hydraulics, systems design and condition, or the surrounding
environment, as previously described. Figure 4-1 provides a breakdown of these
variables into their respective groupings. For example, occupancy characteristics, the
source and amount of water used, the type of water-using appliance, and the type of
disposal practices may impact wastewater quality and hydraulics. It should be noted that,
of all of products disposed of (solids and liquids) listed under Disposal Practices, none
were considered in the modeling portion of this project; however, information on these
specific disposal activities (e.g., disposing of medications in the toilet) was collected
from each participant via the surveys. These activities provided a general overview of
the awareness that the participants had on the impact of their activities on OSTDS
performance.
53
Number of residentsOccupancy per yearLength of time in homeAge of residents
Occupancy
Water sourceWater quantity used
Water Use
capacitydetergentloads
Dishwasher
capacitywater-conservingdetergentfabric softenerbleachloads
Washing Machine
Garbage Disposal
Water-Using Appliances
ShowerToilet
Plumbing Fixtures
feminine productsdisposable diapersplastic productsnylonmedicationrubber productscoffee grindscigarette filtersfood particlespaper towels
Solids
paint thinnerpesticidescooking oilsour milkdisinfectantsbleachsour milk
Liquids
Disposal Practices
Wastewater Quality and Hydraulics
System Age
VolumeChambersNumber of tanksAverage lengthAverage width
Septic Tank
AreaConfigurationElevation
Drainfield
Repairs
Knows system's locationDrives or parks on systemPlants on systemSystem additive
Care and Maintenance
System Design and Condition
Site Elevation
Normal water tableSeasonal High water table
Water Table
FloodingNear surface water
Surrounding Environment
Potential Performance Variables
Figure 4-1 Divisions of Performance Variables into the Three Characteristic Groups
These variables were broken down further into those that were quantitative in
nature (requiring an actual numerical value) or qualitative in nature (e.g., yes/no, on/off),
as shown in Tables 4-1 to 4-3 for all three larger categories of variables. Those variables
that were qualitative in nature were designated by a “0” to represent “no,” or “off,” or a
“1” to represent “yes,” or “on.” For example, if the participants did not drive on their
54
drainage location, this would be signified by a “0,” and, if their systems experienced
flooding over the OSTDS after rainfall events, this variable was denoted by a “1,”
meaning “yes.” In some instances, additional numbers were used for qualitative
variables. For example, dishwasher capacity is a qualitative variable used, where 0
signified that the homeowner does not have a dishwasher, 1 signified a compact capacity,
and 2 signified a standard capacity.
Table 4-1 Potential Performance Factors Describing Wastewater Quality and Hydraulics
Description* Data Type Quantitative Units Residents (R1) Quantitative Capita Residents under 11 years (R2) Quantitative Percent Residents 11 to 19 years (R3) Quantitative Percent Residents 20 to 69 years (R4) Quantitative Percent Residents over 70 years (R5) Quantitative Percent Occupancy (HM1) Quantitative Months/year Length in home (HM2) Quantitative Years Water Source (HD1) Qualitative 1/2/3 Water Consumption (HM21) Quantitative Gallons per day Dishwasher-Capacity (HD11) Qualitative 0/1/2 Dishwasher-Detergent (HM8) Qualitative 0/1/2/3/4 Dishwasher-Loads (HM7) Quantitative Loads/week Washing Machine-Capacity (HD9) Qualitative 0/1/2/3/4 Washing-Machine-Detergent (HM5)
Qualitative 0/1/2/3
Washing Machine-Water Conserving (HD10)
Qualitative 0/1
Washing Machine-Fabric Softener (HM6)
Qualitative 0/1
Washing Machine-Bleach (HM4) Quantitative Cups/week Washing Machine-Loads (HM3) Quantitative Loads/week Garbage Disposal Use (HM9) Qualitative 0/1/2/3 Plumbing Fixtures-Toilet (HD5) Qualitative 0/1 Plumbing Fixtures-Showers (HD6) Qualitative 0/1 Disposal Practice-Solids (HM12) Qualitative 0/1 Disposal Practice-Liquid (HM13) Qualitative 0/1
* Items in parenthesis are abbreviations used to denote these factors
55
Table 4-2 Potential Performance Factors Describing System Design and Condition Description* Data Type Quantitative Units System Age (SD1) Quantitative Years Septic Tank –Volume (SD2) Quantitative Gallons Septic Tank – Chambers (SD3) Quantitative Chambers/tank Septic Tank –Quantity (SD4) Quantitative Tanks/home Septic Tank-Average Length (SD9) Quantitative Inches Septic Tank-Average Width (SD8) Quantitative Inches Drainfield-Surface Area (SD5) Quantitative Square Feet Drainfield-Configuration (SD6) Qualitative ½ Drainfield-Elevation (SD7) Quantitative Feet Known Repairs to System (HD7) Qualitative 0/1 Care-System Location (HM10) Qualitative 0/1 Care-Drives or Parks on System (HM11)
Qualitative 0/1
Care-Plants on System (HD8) Qualitative 0/1 Care- System Maintenance (HM20) Qualitative 0/1
*Items in parenthesis are abbreviations used to denote these factors. Table 4-3 Potential Performance Factors Describing Surrounding Environment
Description Data Type Qualitative Units Site Elevation Quantitative Feet Water table-Normal Quantitative Feet Water table-Seasonal Quantitative Feet Flooding of OSTDS After Rain Event
Qualitative 0/1
Within 75 ft to surface water Qualitative 0/1
Task 2: Selection and Recruitment of Participants from a Select Population of Charlotte County OSTDS Owners
Subtask 2A: Selection of Potential Participants
As previously described, the CCGIS Department used ARC GIS methods to locate
OSTDSs throughout the county. Figure 4-2 shows a map of Charlotte County’s Mid-
County Basin 2 (MC-2) of the selected homes having OSTDSs. A total of 43,311 homes
using OSTDSs in 2001 were located, and this database was layered with the county soil
information to determine the amount of OSTDSs for each soil type present in Charlotte
56
County. Soil types with 2% or more of the County’s OSTDS population were considered
to be the “most populated” soil types (defined as those types with the greatest number of
OSTDSs), and Table 4-4 shows those soil types with the greatest number of OSTDSs.
For a single soil type, the greatest number of systems (18.7%) was located in Oldsmar
Sand with 8094 systems. In alignment with the criterion that the project focus on systems
in these “most populated” soils, potential participants were subsequently randomly
selected from these areas in MC2.
Table 4-4 “Most Populated” Soils (with Largest Number of OSTDSs) in Charlotte
County Soil ID
Soil Name County Population
Percent of Population
36 Immokalee-Urban Land Complex 1866 4.2 % 43 Smyrna Fine Sand 4009 9.3 % 11 Mayakka Fine Sand 2451 5.7 % 28 Immokalee Fine Sand 3887 9.0 % 69 Matlacha gravelly fine Sand 2531 5.8 % 26 Pineda Fine Sand 2747 6.3 % 7 Matlacha-Urban Land Complex 6005 13.9 % 33 Oldsmar Sand 8094 18.7 % 12 Fedla Fine Sand 1114 2.6 % 13 Boca Fine Sand 4425 10.2 % 34 Malabar Fine Sand 1341 3.1 % 42 Wabasso Sand, limestone substratum 2931 6.8 %
Subtask 2B: Recruit Participants
Because many of the addresses of the randomly selected homes obtained from the
ARC GIS database were not updated or did not contain OSTDSs or even homes, it was
necessary to repeatedly select new participants and mail initial information concerning
this program (see Appendix A). Another method of approaching potential participants
was by recruiting methods, as previously discussed. Recruitment ended in June of 2002
with a total of 292 willing residents. Table 4-5 illustrates the number of participants from
57
Figure 4-2 Selected Homes having OSTDSs within Mid-County-2
58
each recruitment event. All volunteers who approached CCDOH after hearing or reading
advertisements or who were recruited during initial inspections of neighboring homes
(except the ones recruited from the county fair) were combined with the randomly
selected group for the mailing campaign. At one participant’s home during the
inspection, five interested neighbors joined the study and had their systems inspected and
pumped the same day. The best recruiting method was by face-to-face contact (at the fair
and during inspections), and the fair booth also enabled contact with residents that
received information from the mailing campaign.
Table 4-5 Recruitment Results Recruitment Event No. Recruited Participants Mailing Set #1 32 Mailing Set #2 15 Mailing Set #3 61 County Fair 98 Mailing Set #4 51 Mailing Set #5 35 Total 292
At the end of the first inspection, 238 participants remained in the study. The 54
participants that were no longer in the study either withdrew or were not eligible because
of the condition of their system or the conversion to sewer in the next year. At the end of
the 12 months, 221 participants remained with 179 in Phase 1 and 42 in Phase 2. The 29
participants dropped from the program between June 2002 and June 2003 because tank
information could not be obtained (preventing volumes to be calculated), system failures,
or the property owner had their system pumped during the study.
During the initial inspections by the CCDOH, 25 percent of the systems needed
repairs. Ten percent of the systems had broken off or missing outlet tees. Seventeen
59
percent of the systems had bad seals between either the manhole and the lid or the lid and
the top of the tank. Ten percent of the systems had holes in the tank, a result of the
sulfate content of the wastewater entering into the septic tank, where anaerobic
conditions, high-strength wastewater, low-flow velocity, and long retention time
promotes the production of H2S, thus leading to the corrosion of tank walls (Fan et al.,
2001).
Subtask 2C: Separation of Recruits into Phase 1 and 2
As mentioned previously, 221 residents were recruited or volunteered to participate
in this study, as a result of the vigorous recruiting and advertising campaigns. Of this
total count, 42 were selected to participate in Phase 2, designed specifically to validate
the results of Phase 1. As a result, their systems were initially inspected and pumped out
subsequent to the Phase 1 systems.
The distribution of the collected quantitative and qualitative data obtained for all
238 initial recruits and their systems is presented in Tables 4-6 and 4-7. As previously
described, care was taken in separating Phase 1 and 2 groups to maintain these
percentage breakdowns of the number of systems in the low and high ranges of each
factor. For example, as shown in Table 4-6, the percentage of all homes with 1-2
occupants was 63% ((149/238)*100), and this was the target percentage used in both
Phases 1 and 2, as shown in Table 4-8 with 125 out of 198 for Phase 1 and 24 out of 40
for Phase 2.
The median number of occupants per house was 2, and each home prepared a
median number of washing machine and dishwasher loads of 4 and 2, respectively (Table
4-6). Of interesting note in Table 4-7 is the large number of systems with plants on or
near the drainfield, flooding, drainage from the washing machine, and the large numbers
60
of occupants who dispose of liquids and solids down their sink drains. On the other hand,
46% of the participants had water-conserving toilets and showerheads, and 95% reported
to have knowledge of the location of their OSTDSs. Refer to Appendix C for further
detail on the separation of the recruits into the 2 phases.
Table 4-6 Distribution for Quantitative of Household Characteristic for All Recruits Factor ID
Factor Count Median Low Value Range
Low Count
High Value Range
High Count
R1 People per home 238 2 1-2 149 3-12 89 HM1 Time of Occupancy
(months) 238 12 0-11 19 12 219
HM3 Washing machine loads 238 4 0-4 121 5-28 117 HM7 Dishwasher loads 238 2 0-2 123 3-10 114
Table 4-7 Distribution for Qualitative of Household Characteristics for All Recruits Factor ID
Factor Count
Qualitative Factors City Well BothHD1 Water Source 238 195 35 8 Qualitative – Yes/No No Yes HD2 Washing machine drains to septic tank 238 33 205 HD5 Water-conserving toilets 235 126 109 HD6 Water-conserving showers 235 126 109 HD7 System repair or replacement 234 97 137 HM10 Location knowledge 237 10 226 HM11 Parking or driving on system 237 235 2 HD8 Plants on drainfield 235 153 81 E5 Systems within 75’ of surface water 236 219 17 HM12 Disposal of liquids 238 121 117 HM13 Disposal of solids 238 63 174 E4 Flooding 238 22 216
Subtask 2D: Data Collection and Databasing
As previously described, the independent variables to be used in the modeling
phase were obtained from an initial and follow-up survey, permits, and inspections. The
initial survey was successfully completed and returned from the participants, and the
61
Table 4-8 Frequencies for Each level of Household Characteristics for Each Phase
follow-up survey was completed and returned by all but 27 participants. Many of the
participants had to be contacted in person or by telephone to have the follow-up survey
completed. Permits were not available for 6 participants; therefore, their systems had
missing factor data for the system design and surrounding environment factors. The
height measurements used for the dependent variable in the subsequent statistical analysis
were gathered during inspections approximately 6 and 12 months after pump out, as
previously described. One home’s OSTDS was not measured at 6 months because the
water consumption was less than 200 gallons. All data, both the solids management and
performance factors, were transferred and stored in an Access database.
Factor ID Phase 1 Phase 2 Low High Low High R1 125 73 24 16 HM1 17 181 2 38 E4 179 19 37 3 HM3 103 95 18 22 HM7 100 98 23 17 City Well Both City Well Both HD1 164 29 7 31 8 1 No Yes No Yes HD2 30 168 3 37 HD5 105 90 21 19 HD6 83 111 14 26 HD7 181 10 38 2 HM10 7 190 3 37 HM11 196 1 39 1 HD8 127 68 26 14 E5 184 12 35 5 HM12 51 147 12 28 HM13 100 98 21 19
62
Task 3: Calculation of Solids Accumulation
Subtask 3A: Calculation of Tank Dimensions
Of the total of 221 tanks studied, 27 did not have readily available dimension
information, thus necessitating calculation of these parameters. As previously described,
two methods were used to calculate dimensions. Method 1 simply required the tank
volume and liquid height and was used for calculating dimensions of 5 tanks with
volumes less than 600 gallons. Method 2 was more complex (and, perhaps more
accurate) using dimension ratios and was used on the remaining 22 tanks with volumes
greater than 750 gallons. The manufacturer’s tank dimensions and the estimated
dimensions using the two methods described above are presented in Appendix D of this
document.
Subtask 3B: Calculation of Solid Volumes Septic Tank
The tank dimensions and measured heights of liquid, scum, and sludge taken
during inspections were subsequently used to calculate solids volumes after 6 and 12
months of accumulation time. Tables 4-9, 4-10, and 4-11 provide the scum, sludge, and
total solid volumes accumulated along with their accompanying standard deviations and
coefficients of variation.
The mean scum accumulation during the first 6-month period was 6.1 gallons,
ranging from 0 gallons (in 58 tanks) to 90.7 gallons (in 1 tank). Considering the 12-
month scum data, the mean accumulation increased to 9.4 gallons, showing that the rate
of accumulation during the latter 6 months was somewhat lower than the first 6 months;
however, care should be taken in making conclusions given the magnitude of the standard
deviations. A similar trend was observed for the sludge. The more critical time period in
terms of accumulation appears to be during the months immediately following pump out.
63
Why faster rates in accumulation occur in the earlier stages is not known; however, it can
be surmised that the biological community is not well established and less capable of
solid degradation soon after pump out as reported in the Bounds (1996) study. The mean
total solids volume increased after 12 months to 49.4 gallons, or 9.8% of the smallest
tank volume and 3.3% of the largest tank volume.
Table 4-9 Scum Volume 6 and 12 Months After Pump out Accumulation (Gallons)
Mean Standard Deviation
Coefficient of Variation
6 months 6.1 13.2 2.15 12 months 9.4 17.6 1.88
Table 4-10 Sludge Volume 6 and 12 Months After Pump out Accumulation (Gallons)
Mean Standard Deviation
Coefficient of Variation
6 months 28.5 24.1 0.845 12 months 41.1 29.8 0.726
Table 4-11 Total Solids Volume 6 and 12 Months After Pump out Accumulation (Gallons)
Mean Standard Deviation
Coefficient of Variation
6 months 34.3 26.7 0.779 12 months 49.4 33.2 0.673
Subtask 3C: Calculation of Accumulation Rates
The accumulation rate for scum, sludge, and total solids was calculated. The
accumulation rate for the solids was calculated for 0-6 months, 6-12 months, and 0-12
months by dividing the volume data by the appropriate time period. The accumulation
rate was also calculated on a per person basis. A linear accumulation rate per system and
a linear accumulation rate per person were determined for the first year of service, which
used the 6- and 12-month measurements. The number of people in a home, the tank
64
volume, and the tank capacity were evaluated to determine if these variables would affect
the accumulation rate.
Accumulation rate per system
The accumulation rate for scum, sludge, and total solids during the first six months,
second six months, and one year are presented in Tables 4-12, 4-13, and 4-14. The
accumulation rate during the second six months of service is less than the accumulation
rate during the first six months. Scum accumulation in the second six months is about
two-thirds as the first six months, while sludge accumulation in the second six months is
less than half than the first six months.
Table 4-12 Scum Accumulation Rates Accumulation (Gallons/Day)
Mean Standard Deviation
Coefficient of Variation
0-6 months 0.031 0.066 2.170 6-12 months 0.019 0.071 3.810 0-12 months 0.025 0.048 1.849
Table 4-13 Sludge Accumulation Rates Accumulation (Gallons/day)
Mean Standard Deviation
Coefficient of Variation
0-6 months 0.141 0.121 0.86 6-12 months 0.072 0.120 1.82 0-12 months 0.109 0.078 0.70
Table 4-14 Total Solids Accumulation Rates Accumulation (Gallons/day)
Mean Standard Deviation
Coefficient of Variation
0-6 months 0.171 0.134 0.791 6-12 months 0.091 0.137 1.621 0-12 months 0.134 0.086 0.670
The accumulation rates of the measured solids during the first year determined by
linear regression are presented in Table 4-15. Compared to the “rise over run” method
65
used to calculate the data in Tables 4-11, 4-12, and 4-13, these values are quite similar
with relatively large R2 values (a measure of goodness of fit).
Table 4-15 Scum and Sludge Accumulation Rate Data Determined Using Linear Fits Accumulation (Gallons/day) Slope
Standard Deviation
Coefficient of Variation
R2 Mean
Scum 0.011 0.020 1.818 0.564 Sludge 0.116 0.082 0.707 0.825 Total Solids 0.142 0.089 0.627 0.853
Accumulation rate per person
The accumulation rates for the solids were also normalized on a per person basis
for each time interval, and these values are presented in Tables 4-16, 4-17, and 4-18.
Table 4-19 presents the accumulation rates determined using linear fits and normalized
on a per person basis.
Table 4-16 Scum per Person Accumulation Rate Accumulation (Gallons/day/capita)
Mean Standard Deviation
Coefficient of Variation
0-6 months 0.010 0.020 1.978 6-12 months 0.007 0.026 3.735 0-12 months 0.009 0.016 1.909
Table 4-17 Sludge per Person Accumulation Rate Accumulation (Gallons/day/capita)
Mean Standard Deviation
Coefficient of Variation
0-6 months 0.062 0.055 0.883 6-12 months 0.032 0.058 1.967 0-12 months 0.048 0.039 0.835
Table 4-18 Total Solids per Person Accumulation Rate Accumulation (Gallons/day/capita)
Mean Standard Deviation
Coefficient of Variation
0-6 months 0.072 0.056 0.778 6-12 months 0.039 0.062 1.706 12 months 0.057 0.040 0.723
66
Table 4-19 Accumulation Rates of Solids per Person Determined Using Linear Fits Accumulation
(Gallons/day/capita) Slope Standard Deviation
Coefficient of Variation
R2 Mean
Scum 0.004 0.007 1.817 0.562 Sludge 0.050 0.040 0.791 0.825 Total Solids 0.060 0.040 1.024 0.853
Subtask 3D: Trends in Accumulation
Accumulation trends during the first year
The accumulation trends were evaluated by comparing the change in solid
volumes during the first and last 6-month periods. Table 4-20 provides the number of
systems that showed an increase, decrease, or no change in scum and sludge volumes
from the first to the last 6-month period, and Table 4-21 shows the number of systems in
which the accumulation rate changed from the first and last 6 month periods. Volumes
that remained the same or decreased from 6 months to 12 months were considered to
have a decreasing accumulation rate, as shown in Table 4-21. Of the 140 systems that
had decreased scum accumulation rate, only 17 systems increased in scum volume after 6
months. While 145 systems experienced a decrease in sludge accumulation rate, only 71
systems experienced an increased volume. A majority of those systems with decreases in
accumulation rate did not experience an increase in the accumulation volume. The result
of this lack of accumulation after 6 months shows that the systems’ accumulation rates
begin to decrease after 6 months for just over half of the participants.
Table 4-20 Solids Volume Change from 6 months to 12 months No. of Phase 1 Systems No. of Phase 2 Systems Scum Sludge Scum Sludge
Decrease 44 40 8 9 No Change 60 22 11 3 Increase 75 117 23 30
67
Table 4-21 Solids Accumulation Rate Change from 6 months to 12 months No. of Phase 1 Systems No. of Phase 2 Systems Scum Sludge Scum Sludge
Rate Increase 62 60 19 16
Rate Constant or Decrease 117 119 23 26
Phase 1 had 18 participants and Phase 2 had 6 participants with tanks where scum
and sludge accumulation rates increased during the second 6 months compared to the first
6 months. A total of 51 participants’ tanks showed a decrease in scum volume, and 44
participants’ tanks had a decrease in sludge volume during the second six months. The
decreasing accumulation of solids in the first year is not likely to be caused by microbial
degradation, as it has been reported that microbial activity impacts the accumulation rates
only after two years (Bounds, 1996). Sludge settling resulting in an increase in density
may cause the decrease in sludge volume.
Accumulation trends in relation to people and septic tank volume
The linear accumulation rate at one year was analyzed based on number of people
in the home, volume of the septic tank, and septic tank capacity. The septic tank capacity
was calculated by dividing the total tank volume by the number of people in the house.
The mean and standard deviation were determined for each set of accumulation rates
within each level of number of people per home (Table 4-22), septic tank volumes (Table
4-23), and septic tank capacity (Table 4-24). The accumulation rates of scum and sludge
were plotted against the number of people, the septic tank volume, and the septic tank
capacity (Figure 4-3 to Figure 4-5, respectively).
The accumulation rates based on the number of people in the home
graphically show that the accumulation rate increases as the number of people increase
68
Table 4-22 Accumulation Rates Based on the Number of People in the Home
Number of People Solids Count Mean
Standard Deviation
Coefficient of Variation
1 Scum 28 0.006 0.009 1.482 Sludge 28 0.073 0.054 0.7412 Scum 107 0.016 0.036 2.345 Sludge 107 0.116 0.077 0.6653 Scum 33 0.027 0.030 1.115 Sludge 33 0.119 0.064 0.5364 Scum 33 0.056 0.080 1.429 Sludge 33 0.121 0.098 0.8085 Scum 13 0.054 0.055 1.010 Sludge 13 0.103 0.060 0.5776-10 Scum 7 0.058 0.072 1.242 Sludge 7 0.181 0.174 0.962
for both scum and sludge. The accumulation rates for both solids were statistically tested
using the t-test to determine if the accumulation rate means increased with occupants.
For the sludge accumulation rate, the accumulation rate beyond 2 or more people was
statistically the same. However, an increase in the sludge accumulation rate was
statistically significant once there was more than 1 occupant in the home. The scum
accumulation rate statistically increased for each occupant from 1 to 4 occupants in the
home. For 4 or more occupants in the home the scum accumulation rate was statistically
the same. The increase in accumulation based on number of people in the home may be a
result of a higher volume of solids entering into the septic tank.
The accumulation rate based on septic tank size graphically showed that solids
accumulation increased as the septic tank volume increased. The accumulation rates
were statistically tested using the t-test to determine if the accumulation rate means
increased with the larger volume septic tanks. The sludge accumulation rate increased
for septic tank volumes larger than 750 gallons when compared to smaller tanks. The
scum accumulation rate showed to not be statistically different for any septic tank
69
y = 0.0112x - 0.0042
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0 2 4 6 8 10People per Home
Scum
Acc
umul
atio
n R
ate
(Gal
lons
/day
)Scum MeanScum ValuesLinear (Scum Values)
A
y = 0.0137x + 0.0764
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 2 4 6 8 10People per Home
Slu
dge
Acc
umul
atio
n R
ate
(Gal
lons
/day
)
Sludge MeanSludge ValuesLinear (Sludge Values)
B
Figure 4-3 Accumulation Rate Trends Based on number of People in the Home A) Scum
Accumulation B) Sludge Accumulation
Table 4-23 Accumulation Rate Based on Septic Tank Volume Tank
Volume (gallons) Solids Count Mean
Standard Deviation
Coefficient of Variation
<750 Scum 6 0.012 0.020 1.654 Sludge 6 0.060 0.066 1.107750 Scum 54 0.020 0.049 2.460 Sludge 54 0.085 0.062 0.723900 Scum 136 0.024 0.044 1.811 Sludge 136 0.123 0.080 0.651>900 Scum 25 0.048 0.065 1.358 Sludge 25 0.128 0.107 0.835
70
y = 6E-05x - 0.0274
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
400 600 800 1000 1200 1400Septic Tank Volume (Gallons)
Scum
Acc
umul
atio
n R
ate
(Gal
lons
/day
)
Scum MeanScum ValueLinear (Scum Value)
A
y = 0.0002x - 0.0757
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
400 600 800 1000 1200 1400Septic Tank Volume (Gallons)
Slu
dge
Acc
umul
atio
n R
ate
(Gal
lons
/day
)
Sludge MeanSludge ValueLinear (Sludge Value)
B
Figure 4-4 Accumulation Rate Trends Based on Septic Tank Volume A) Scum
Accumulation B) Sludge Accumulation
volume. The increase of solids accumulation with respect to septic tank size is likely
retention of more solids by larger tanks than compared to smaller tanks.
The septic tank capacity was plotted to further evaluate the previous two results.
The scum accumulation trend graphically shows that, as a system increased in capacity, a
larger tank with fewer residents, the accumulation of scum decreases and sludge
accumulation has an overall decreasing pattern. The t-test was used to statistically test if
the accumulation rates decreased as septic tank capacity increased. The sludge
71
Table 4-24 Accumulation Rate based on Septic Tank Capacity Tank
Capacity (gallons/person) Solids Count Mean
Standard Deviation
Coefficient of Variation
<200 Scum 21 0.048 0.052 1.092 Sludge 21 0.129 0.054 0.741201-250 Scum 27 0.058 0.084 1.447 Sludge 27 0.123 0.107 0.871251-300 Scum 13 0.045 0.060 1.131 Sludge 13 0.102 0.059 0.584301-350 Scum 26 0.026 0.030 1.131 Sludge 26 0.127 0.063 0.493351-400 Scum 32 0.020 0.059 2.903 Sludge 32 0.091 0.064 0.699451-500 Scum 70 0.014 0.021 1.524 Sludge 70 0.125 0.079 0.630501-550 Scum 4 0.607 0.009 1.358 Sludge 4 0.137 0.097 0.702701-750 Scum 14 0.005 0.008 1.810 Sludge 14 0.061 0.044 0.719851-900 Scum 13 0.008 0.010 1.217 Sludge 13 0.091 0.060 0.660
accumulation rate was inconclusive. The scum accumulation rate was shown to increase
with decreasing septic tank capacities. The preliminary trends in accumulation show that
accumulation decreases when larger septic tanks and fewer people are taken into
consideration.
Task 4: Interpretation of Phase 1 and Phase 2 Model Results
Subtask 4A: Assess Sample Size
As previously described, the number of participants finally chosen for this study was
restricted to a total number that would accommodate a severely limited number of
CCDOH staff assigned to pump out and solids measurement duties. The final number of
72
y = -7E-05x + 0.05540
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
400 600 800 1000Septic Tank Capacity (Gallons/person)
Scu
m A
ccum
ulat
ion
Rat
e (G
allo
ns/d
ay)
Scum MeanScum ValueLinear (Scum Value)
A
y = -7E-05x + 0.1394
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
400 600 800 1000Septic Tank Capacity (Gallons/person)
Slu
dge
Acc
umul
atio
n R
ate
(Gal
lons
/day
)
Sludge MeanSludge ValueLinear (Sludge Value)
B
Figure 4-5 Accumulation Rate Trends Based on Septic Tank Capacity A) Scum Accumulation B) Sludge Accumulation
systems studied was 221. In order to assess whether this number was significantly high
enough to represent all of the Charlotte County systems (and, therefore, produce robust,
statistically significant results), the smallest significant sample size was calculated using
the sludge and scum accumulation rates determined by linear fits. Table 4-25 presents
values of significant sample sizes for both accumulation rates, along with parameters
used to estimate these values. The tolerable error was a conservative value that accounts
73
for the average error of each measurement by using the smallest increment of measure on
each side of the mean as the width. This average “smallest increment of measure” was
determined by taking the average gallons of solids at a quarter of an inch and dividing by
365 days calculated the average smallest increment of measure. The Zα/2 term, taken
from the tabulated values (Ott and Longnecker, 2001), is the area under a normal curve
for a 95% confidence interval.
Table 4-25 Significant Sample Size Values
Solids
Significant sample
Size Mean Standard deviation
Average Smallest
increment Tolerable
error Zα/2 Scum 150 0.026 0.048 0.008 0.015 1.96 Sludge 526 0.114 0.08 0.007 0.014 1.96
The sample size used in this study of 221 systems is sufficient to provide robust
analysis of scum accumulation but falls short for sludge accumulation. Therefore, care
should be taken in interpreting this latter data. It is important to note here that there was
no freedom to add additional participants in this study. The primary reason for this lack
of freedom was the previously mentioned inability of CCDOH to perform individual
pump outs and measurements beyond the final number 221. Also, because this project
was constrained by fixed deadlines set by FDEP and CCDOH in their original agreement,
new participants could not be added once Phase 1 of the project had begun. Despite not
having a sample size significantly sufficient to warrant robust correlation of sludge
accumulation with collected performance variables, meaningful trends between the two
variable types were still discerned.
74
Subtask 4B: Development of Correlations
Correlations of the potential performance variables to the solids accumulation
rates were constructed to determine their influence on system performance. As
mentioned previously, the Statistical Analysis Software (SAS) was used to construct
multiple linear and linear correlations. The robustness of the correlations was initially
judged based on R2 values, where 0.7 was arbitrarily set as a cutoff value.
If a correlation between the solids accumulation rates and performance variable(s)
was not shown to have a high enough R2 value (and corresponding low p value), evidence
of influencing trends between the rates and performance variable was assessed. A
performance variable was determined to have significant influence on an accumulation
rate if the probability value (or p value) was less than 0.05. The p value is the portion of
the population that will not be included in the statistical prediction.
The MLR analysis using the scum, sludge, and total solids accumulation over 6 and
12 months did not result in correlations of statistical significance (with r-square values
greater than 0.7). However, consistent trends between the independent performance
variables and the dependent accumulation rates were observed by the correlation’s p
values less than 0.05. While a p value of less than 0.1 was deemed as significant but not
statistically.
Sludge accumulation rate correlations
Excessive sludge accumulation, defined as an accumulation rate greater than 2.41 x
10-4 gallons per person per day, was impacted by the washing machine activity
(specifically machine capacity, use of a water-conserving model, detergent type, and
drainage) during the first six months. The severe sludge accumulation appeared when
using liquid detergent, a large or extra large capacity, water-conserving (front-loading)
75
washing machine that drains to the septic tank. The washing machine detergent, washing
machine drainage, type of water-conserving washing machine model, and the washing
machine drainage were the only factors to noticeably correlate with sludge accumulation
during the first six months. The scatter plots in Figure 4-6 show the trends for washing
machine capacity, water conserving models, washing machine detergent, and washing
machine drainage against the sludge accumulation rate at 6 months. After one year,
excessive sludge accumulation did not significantly correlate to any of the variables. The
variables, use of low-flow showerheads, and the presence of plants on the drainfield
showed trends influencing sludge accumulation, but the resulting correlations were not
statistically significant.
Scum accumulation rate correlations
Unlike what was observed with the excessive sludge accumulation, excessive scum
accumulation, defined as an accumulation greater than 4.11 x 10-5 gallons per person per
day, correlated well with some performance variables using the 12-month data but not the
6-month data. Using accumulation rates at 6 months, only the occurrence of flooding
correlated with the scum accumulation rate since the probability was less than 0.10, but
was not statistically significant since it did not have a probability of less than 0.05.
However, only 14 participants reported flooded conditions in their OSTDS area, leaving
this result in question.
Correlations between the 12-month data and performance variables showed that
garbage disposal use, washing food down the sink drain, dishwasher detergent and
washing machine capacity significantly influenced the rate of scum accumulation. The
most significant correlation was obtained with the capacity (large-to-extra-large) of
washing machine (p=0.0017). As a matter of fact, 62% of the participants with
76
“excessive” scum used large-to-extra-large capacity washing machines. The dishwasher
detergent type showed an influencing trend with a probability of 0.0437. Sixty-one
percent of the participants with excessive scum accumulation used liquid detergent,
compared to participants that did not have excessive scum accumulation (37% of whom
used liquid detergent). Furthermore, 43% of participants that did not have excessive
scum accumulation used powder detergent, and 28% for excessive scum accumulation.
Likewise, garbage disposal use yielded a significant correlation with the 12-month scum
accumulation rates (p=0.0245) with 74% of the participants that experienced excessive
scum accumulation regularly used their garbage disposal. Whether a participant disposed
of food scraps on their dishes before washing influenced scum accumulation (p=0.0168),
and it was found that 46.4% of those participants that experienced excessive scum
accumulation did not scrape their dishes before rinsing. When pooling Phase 2 data with
Phase 1 data, the use of low-flow showerheads was shown to decrease scum
accumulation with 68.5% of the participants that did not have excessive scum
accumulation used these showerheads. The scatter plots further illustrate the trend in
scum accumulation for these performance variables (Figure 4-7).
Solids disposal and flooding trends influenced excessive scum accumulation at 12
months, but the resulting correlations were not statistically significant. The practice of
improper solids disposal showed an influencing trend for scum accumulation (p=0.0561)
with 63% of participants that had excessive scum accumulation improperly disposed of
solids while 44.8% of participants that did not have excessive accumulation also
improperly disposed of solids. The occurrence of flooding, again, correlated with
excessive scum accumulation with a p value of 0.0711. Although, only 15.2% of
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participants that had excessive scum accumulation experiencing flooding and 5.6% of
who do not have excessive scum accumulation experience flooding. The low percent
value does not seem evident in supporting the significant of flooding at 12 months.
Subtask 4C: Development of Model Results
A model was attempted for both solids at six months and at one year. At six
months, the largest R2 achievable was 0.405 using all the potential performance variables,
and the largest R2 at one year was 0.56 using all the potential performance variables. The
inability to produce a model from the data is a result of both too few data points and the
large amount of variability present in the solids measurements. The short time in which
these measurements were collected most likely is not long enough to adequately measure
system performance.
78
0
0.15
0.3
0.45
0 1 2B
0
0.15
0.3
0.45
0 1 2 3 4A
Slud
ge A
ccum
ulat
ion
Rat
e at
6 m
onth
s(g
allo
ns/c
apita
/day
)
0
0.15
0.3
0.45
0 1 2 3
C
Slud
ge A
ccum
ulat
ion
Rat
e at
6 m
onth
s(g
allo
ns/c
apita
/day
)
0
0.15
0.3
0.45
0 1
D
Figure 4-6 Scatter Plots for Sludge Accumulation Rates at 6 Months with the lines
representing the trend A) Washing Machine Capacity (0 = No Washing Machine; 1 = Compact; 2= Medium; 3 = Large, 4 = Extra Large) B) Water Conserving Model (0 = No Washing Machine; 1 = Top-loading; 2 = Front-loading) C) Washing Machine Detergent (0 = No Washing Machine; 1 = Liquid; 2 = Powder; 3 = Both) D) Washing Machine Drainage (0 = Does Not Drain to OSTDS; 1 = Drains to OSTDS)
79
0
0.1
0.2
0.3
0.4
0 1 2 3A
Scum
Acc
umul
atio
n R
ate
at 1
2m
onth
s (g
allo
ns/c
apita
/day
)
0
0.1
0.2
0.3
0.4
1 2 3 4B
0
0.1
0.2
0.3
0.4
0 1 2 3C
Scum
Acc
umul
atio
n R
ate
at 1
2m
onth
s (g
allo
ns/c
apita
/day
)
0
0.1
0.2
0.3
0.4
0 1 2 3 4D
Figure 4-7 Scatter Plots for Scum Accumulation Rates at 12 Months with the lines
representing the trend A) Garbage Disposal Use (0 = Not Applicable or Never; 1 = Occasionally; 2 = After Dinner; 3 = Always) B) Food Down Sink Drain (1 = Food is Scraped from Plates and the Sink is Strained; 2 = Food is Scraped from Plates and the Sink is Not Strained; 3 = Food is Not Scraped from Plates and the Sink is Strained; 4 = Food is Not Scraped from Plates and the Sink is Not Strained) C) Dishwasher Detergent (0 = Dishwasher is Not Used; 1 = Liquid; 2 = Powder ; 3 = Both) D) Washing Machine Capacity (0 = No Washing Machine; 1 = Compact; 2 = Medium; 3 = Large; 4 = Extra Large)
80
CHAPTER 5 SUMMARY AND CONCLUSIONS
The primary goals of this project were to determine what characteristics of
household activities, system design, and environmental conditions impact the
performance of OSTDSs to a significant extent and to provide CCDOH with a list of
recommended changes to current practices in OSTDS management by Charlotte County
residents. A secondary goal was to develop a model to allow prediction of solids
accumulation over time. In order to achieve these goals, data on current OSTDS
practices, systems, and environmental conditions were obtained by means of surveys,
existing state/county documentation, and inspections, and then evaluated against the
accumulation rate determined from measurements taken at 6 and 12 months.
The budget and time restrictions limited the sample size to be 300 or less
participants. Although, extreme efforts were taken to recruit OSTDS using homeowners,
only 221 residents were willing and qualified to participate. All data collected through
out the study were databased to maintain the data, while being able to be easily converted
into a useable format for statistical analysis.
The solids measurements at 6 and 12 months from pump out were converted into
sludge and scum volumes using the specific septic tank dimensions. The accumulation
rates were calculated using the number of days between measurements, and the
accumulation rates were determined with linear fits to take into account for the change in
the accumulation rate through out the year.
81
In order to best determine the that characteristics of household activity, system
design, and environmental conditions impact on the accumulation rates, the performance
variables were linearly correlated against the accumulation rates. The linear correlation
between the accumulation rate and a specific performance variable assesses the
relationship between the two. The performance variables that correlate to the
accumulation rates were used to develop a model for predicting the accumulation rates.
Before a model is created, the performance variables were related to the accumulation
rate using multiple linear correlations. An R2 value is determined for that multiple linear
correlation, when the R2 value is less than 0.7 a significant model cannot be predicted
using only these performance variables.
The resulting conclusions at one year are that scum and sludge accumulation was
too random to be used in creating an accumulation model for predicting pump out
frequencies. Trends in sludge accumulation proved to be ineffectively significant with
the inadequate sample size. The number of participants for scum accumulation trends
was significant enough.
Scum accumulation rates showed to be impacted by wastewater quality and
hydraulic characteristics than system design and environmental conditions. During
correlations with the performance variables, the scum accumulation rate at 12 months
significantly related to the frequent use of garbage disposals, allowing food particles to be
disposed of down the sink drain, the washing machine capacity, and the dishwasher
detergent.. Sludge accumulation did not have an adequate sample size, resulting in an
inconclusive statistical analysis.
82
While some potential variables were significant in the study, other performance
variables were not, possibly because of the short measurement period of one year may
not provided enough time to completely characterize system performance. Previous
studies used a time period of up to eight years. The increase of solids accumulation
during the last six months showed that for these systems the maximum accumulation rate
in these systems did not occur within the first year of service. While not completely
understood, the cause of the drastic decrease in solids volumes from six to 12 months of
service was possibly caused by system failure, unexpected early microbial activity, or
settling of stored solids, resulting in a denser sludge blanket.
The variables that significantly impacted accumulation rates of scum and sludge are
the following:
• Washing machine capacity; • Garbage disposal use; • Low-flow showerheads; • Direct disposal of food particles; • Washing machine detergent; • Water-conserving washing machine model; • Flooding. Given the trends observed with the above-mentioned household activities, the following
practices are recommended to ensure optimal performance of OSTDSs in Charlotte
County:
• Promote use of water-conserving fixtures and appliances; • Discourage the use of unnecessary over-sized capacity washing machines; • Limit garbage disposal use to extend the pump-out interval; • Encourage occupants to limit the amount of solids and foods particles that exit
through the drain and enter the septic tank; • Promote unsaturated soil conditions to prevent high ground water levels from
infiltrating into the septic tank via the drainfield or leaky tank.
83
The results of this study have potential to significantly impact the public health of
Charlotte County residents by ensuring prevention of pollution from OSTDSs into its
numerous water bodies. Providing information concerning the effects of certain
household activities on system performance to OSTDS owners would result in a
heightened awareness of the impact of their activities not only on their system behavior
(and frequency of pump-outs) but also on the health of the surrounding environment.
Given that the literature has reported that the majority of system owners are unaware of
these connections and that many of the participants polled in this study were also
unaware of appropriate maintenance procedures, transfer of this information to the public
is imperative.
84
CHAPTER 6 RECOMMENDATIONS FOR FUTURE WORK
The work performed in this study is vital for the improvement of management and
maintenance for OSTDSs. Further analysis of accumulation rates will help lead the
progress for OSTDS management and ultimately, public and environment health. The
purpose of this chapter is to provide recommendations for future studies that may attempt
to extend this project or repeat the methods reported herein. The results and conclusions
brought forth from this study show that data collection is among the most important
aspect for the analysis of solids accumulation. A majority of the recommendations to
follow will discuss the importance of various aspects of sample size, monitoring of
accumulation, and collection of performance variables collected.
Identification of Relevant Variables
In this study, three major groupings were used to identify potential performance
variables. In identifying variables for this study, anything that could possibly impact a
accumulation was gathered, thus resulting in a large set of independent variables with
unknown links to wastewater generation. Additional research of each variable and its
possible relationship to wastewater generation may have provided more informaed
insight in development of survey questions. An example of this in this study is garbage
disposal use. Given that a closer study of the variability that exist in use of garbage
disposals, the different levels of use (never, occasionally, only after meals, and always)
could have been more accurately defined. In doing so, the survey questions could have
85
been better posed to elicit more accurate responses and, ultimately, more robust
correlations.
Selection, Recruitment, and Monitoring of Participants
Other factors to consider in studies such as these include the ability to monitor
solids accumulation over the entire pump out interval and to obtain a significant sample
size. Monitoring over the entire pump out interval will create a model that describes the
accumulation that determines the need to perform maintenance pumping. An adequate
sample size will ensure that the statistical analysis performed will bring forth more
meaningful results. The sample size needed for this study over one year was 150
participants for scum accumulation and 526 participants for sludge accumulation.
However, it is important to remember that the sample size required could change for
different monitoring periods.
Participant Selection
The participants used in an accumulation rate study should be selected on the basis
of soil type, tank size, and number of occupants. Selecting participants based on the
designated soil type is viable if it is possible to recruit strictly from the desired soil types.
Participant selection should use a nested design based on number of people in the home
and the septic tank size. (Further information on nested design can be found in statistical
experimental design textbooks.) Participants should have available septic tank
manufacture and dimensions with no current failures.
Recruiting Participants
The most successful recruitment technique was the booth at the county fair. The
booth allows initial questions to be answered that would otherwise inhibit residents from
participating. If mailing is desired, using a two-part postcard that can be mailed and a
86
portion of it returned that is sent to every potential participant in the desired area is
recommended. A two-part postcard would allow the study to be introduced to the
prospective participants, and the returned portion can provide information on the number
of occupants, contact information, and other information needed before collecting
performance variable data. The two-part postcard would save both time and money
(especially since survey packets would only be sent to interested recruits).
Division of Recruits for Two Groups
Maintaining two groups, one for creating a predictive model and the second for
validating the model, is recommended. Using the nested design discussed above would
prevent the need to separate the recruits, since the recruits would be placed into the
appropriate model, and the tank volume and number of occupants would be used in
selection and separating the recruits.
Data Collection
The potential performance variables were collected by means of surveys, permits,
and inspections. The use of surveys was essential for collecting the household activities
variables, which were mailed to the participant. Most of the participants mailed the
surveys back promptly but many of the participants did not, leading to phone calls or
having the participants complete the survey during an inspection.
Monitoring solids accumulation was performed for one year in 6-month intervals. It
is recommended to monitor systems for the entire pump-out interval after the septic tank
is pumped. Having a longer interval between monitoring would allow enough time for
more participants to be included. The changes in accumulation observed in this study
lead to the recommendation that monitoring intervals do not exceed 1 year.
87
Calculation of Solids Accumulation and Model Development
The calculation of solids accumulation is performed in several steps, tank
dimension collection, volume calculation, and accumulation rate calculation. Selecting
participants based on having known septic tank dimensions, if possible, would prevent
the introduction of error incurred from calculating tank dimensions. It is also
recommended that the investigators have or receive sufficient training in statistics prior to
the modeling phase to ensure that all considerations have been made prior to and during
analysis.
88
APPENDIX A CONTENTS OF PACKAGE INITIALLY MAILED TO PARTICIPANTS
The mailing campaign consisted of a flier, 2 consent forms, an initial survey, and a
pamphlet. The 2 consent forms allowed the homeowner to keep one for their records and
return the other with the survey. The initial survey is presented in Appendix B. The flier,
consent form, and pamphlet are presented in this Appendix.
89
U n ive r s it y o f F lo r id a (U F ) a nd
C h a r lo t t e C ou n ty H e a lt h D e p a r tm e n t (C CH D )
O ns ite S ew ag e T rea tm ent a nd D isp o sa l S y s tem
(O S TD S ) S tud y B ro ch u re
P re p a re d B y T h e U n ive rs it y o f F lo r id a
E n v iro nm e n ta l E n g in e e r in g S c ie n c e s
H e re is w h a t w e a re re q u e s t in g th a t y o u d o : S T E P 1 : • R e a d a n d s ig n th e c o n s e n t fo rm : R e a d th e fo rm c lo s e ly s o y o u u n d e rs ta n d h o w th is s tu d y c a n b e n e f it y o u a n d C h a rlo tte C o u n ty . S ig n th e fo rm in th e s ig n a tu re b o x .
S T E P 2 : • C o m p le te th e S u rv e y : M a k e s u re to f ill it o u t a s a c c u ra te ly a s y o u c a n ! M a il th e s u rv e y a n d c o n s e n t fo rm u s in g th e s ta m p e d , s e lf -a d d re s s e d e n v e lo p e p ro v id e d . S T E P 3 : • A w a it c o n ta c t fro m C C H D : If y o u c h o o s e to p a rt ic ip a te in th is im p o rta n t s tu d y , p e rs o n n e l f ro m th e C C H D a n d C h a rlo tte C o u n ty U t ilit ie s (C C U ) w ill b e s to p p in g b y y o u r h o u s e . O n c e y o u r s y s te m h a s b e e n d e te rm in e d re a d y fo r s tu d y , C C H D a n d C C U w ill in s ta ll m o n ito r in g p o rts a n d r is e rs in y o u r ta n k a n d d ra in f ie ld . T h e y w ill th e n m o n ito r y o u r O S T D S o r s y s te m le v e ls tw ic e a y e a r.
W h a t w il l w e b e d o in g w it h a l l o f t h is in fo r m a t io n ?
∗ M e a s u r e s o l id s o v e r th e c o u r s e o f th e y e a r : W e w i l l ta k e th e d a ta th a t C C H D g a th e rs w h e n i t m o n i to rs y o u r ta n k tw ic e d u r in g th e y e a r .
∗ C re a t e a m o d e l : U s in g
th e d a ta c o l le c te d d u r in g m o n i to r in g a n d f r o m y o u r s u rv e y , w e w i l l c r e a te a m o d e l th a t w i l l a l lo w th e p re d ic t io n o f p e r fo rm a n c e a n d re q u ir e d p u m p in g o f a n y O S T D S .
∗ U s e t h e m o d e l : F ro m
th is m o d e l , w e w i l l b e b e t te r a b le to g u id e O S T D S o w n e rs l ik e y o u in m o re e f f ic ie n t p ra c t ic e s th a t w i l l r e s u l t in lo w e r re p a ir c o s ts fo r y o u a n d m in im iz e th e O S T D S p u b l ic h e a l th r is k s a n d e n v i ro n m e n ta l im p a c t o n lo c a l w a te r re s o u rc e s .
W e th a n k y o u fo r c o n s id e r in g p a r t ic ip a t in g in th is s tu d y . If y o u
h a v e a n y q u e s t io n s , p le a s e d o n o t h e s i t a te to c o n ta c t M r . B o b V in c e n t (C C H D )
a t ( 9 4 1 )7 4 3 -1 2 6 6 o r
D r . A n g e la L in d n e r (U F ) a t (3 5 2 )8 4 6 -3 0 3 3 o r a t a l in d @ e n g .u f l .e d u .
Figure A-1 Flier (a) front (b) back
90
INFORMED CONSENT FORM - Official Copy to be Returned with Survey
Dear Septic Tank Owner: We are writing you to ask for your help in a study of septic tank systems. The study is a multi-year project to explore how maintenance and daily household activities can affect septic system performance. The ultimate goal of this project is to better able to assist you in properly maintaining your septic system. You are one of two hundred septic tank owners in Charlotte County who have been asked to participate in our study. This is a two-tiered study. First, we ask that you take approximately twenty minutes to complete the enclosed survey, and mail it back to us using the stamped, self-addressed envelope also enclosed in this package. The survey asks you questions about daily household activities and your household occupancy. Protecting the confidentiality of your answers is very important to us. We assure you that your survey responses will be kept strictly confidential to the extent provided by law, and the results will only be publicly reported in aggregate statistical format. There is an identification number on the survey for tracking purposes, so we know not to send you another survey. Upon completion of the survey, the second portion of the study involves monitoring your septic system for approximately one year. First, Charlotte County Health Department (CCHD) personnel will inspect your system. The CCHD personnel will identify any necessary repairs, which you will then need to have made at your expense. Next, CCHD personnel will install a drainfield monitoring port, and Charlotte County Utilities personnel will install a riser in your septic tank. Twice a year, CCHD personnel will monitor your septic tank levels. You must be at least 18 years of age to participate, and your participation is voluntary. We anticipate no risks to you. If you agree to participate in both portions of this project, you will receive a septic tank pumpout for a nominal fee of $15-20. We are especially grateful for your help because it is only by asking septic tank users like you to share information about their septic system that we can understand how septic tank maintenance and household practices can affect septic system performance. If you have any questions or concerns, please feel free to contact me. In addition, please feel free to contact the University of Florida’s Institutional Review Board at (352) 392-0433 if you Figure A-2 Consent Form Page 1
91
have any questions about your rights as a study participant. If you wish to participate in this program, please indicate by signing your name and placing today’s date in the space provided below. Please return this form, along with your completed survey, in the enclosed self-addressed envelope. Thank you in advance for your help with this important study. Sincerely yours, Angela S. Lindner, Ph.D. Environmental Engineering Sciences, University of Florida
Figure A-3 Consent Form Page 2
______________________________________ _____________________Your Signature Today’s Date
APPENDIX B INTIAL AND FOLLOWUP SURVEYS
93
On-Site Sewage Treatment Disposal Survey Charlotte County Health Department* and
The University of Florida Department of Environmental Engineering Sciences#
*Port Charlotte, Florida 33948 #Gainesville, Florida 32611
Your Name: _________________________________________________________ Phone number: HOME: (______)_____________________________ WORK: (______)_____________________________ Email address: _____________________________________ Mailing address: ______________________________________________________________________ ______________________________________________________________________ Physical address: (If different from above.) ______________________________________________________________________ ______________________________________________________________________ QUESTIONS CONCERNING OCCUPANCY IN YOUR HOME: 1. Number of years at current address: ______________________ 2. Number of people living in your home: _______________________ 3. Number of bedrooms in your home: _______________________ 4. Do you live here year round? (Circle One.) YES NO
If no, number of months of the year you live here: _______________ QUESTIONS CONCERNING YOUR DAILY ACTIVITIES IN YOUR HOMETHAT MAY AFFECT YOUR SEPTIC SYSTEM’S PERFORMANCE: 5. Has your yard ever flooded in the vicinity of the septic tank after rains? (Circle One.) YES NO 6. Do you have well water or city water? (Circle One.) Figure B-1 Initial Survey Page 1
94
If you answered city water, what is your normal monthly usage?____________ If you answered city water, do you irrigate the yard with this water? (Circle One.) YES NO If you answered well water, do you use water softener for your drinking water? (Circle One.) YES NO 7. Do you use a clothes washing machine in your house? (Circle One.) YES NO
If yes, does the washing machine drain into the septic tank?________
8. How many loads are washed in a week?_____ most loads in a day? _____ 9. Do you have and use an automatic dishwasher? (Circle One.) YES NO
How many loads are washed in a week?______ In a day?______ 10. Do you have a garbage disposal? (Circle One.) YES NO
If yes, how often do you use the disposal? (Circle One.) Don’t use Occasionally use Use after meals Always use 11. Do you have a (Circle one):
Low-Flow Toilet YES NO Low-Flow Showerhead YES NO QUESTIONS CONCERNING YOUR SEPTIC SYSTEM: 12. What year was your home built? ______Has the septic system been replaced? ____ If it has been replaced, in what year was it replaced?______ 13. Do you know where your septic tank and drain field are located? (Circle One) YES NO 14. Does anyone park or drive on your drain field? (Circle One.) YES NO 15. Are there any plants planted on or near the drain field? (Circle One) YES NO 16. Have you ever had problems with your septic system? (Circle One.) YES NO
If so, explain the problems and how it was corrected in the space provided below: ________________________________________________________________________ ________________________________________________________________________ Figure B-2 Initial Survey Page 2
95
_________________________________________________________________________ 17. When was the last time your septic tank was pumped out? ________________________________________________________________________ 18. Is the septic tank or drainfield within 75 feet of surface water (lake, pond orother)? (Circle One.) YES NO If you responded YES, how far (in feet) is the water from the septic tank or drainfield:_________
19. Check the following that you dispose of in your drains or toilets: paint thinner plastic items disinfectants bleach pesticides nylon products coffee grinds food grease feminine products medications cigarette filters paper towels disposable diapers rubber products any type of food item salad oil sour milk Comments: ______________________________________________________________________ _________________________________________________________________________ _________________________________________________________________________ _________________________________________________________________________
(You may continue on the reverse side of this page if necessary.) 20. Further input by you will help researchers guide septic tank users like you in more efficient practices, resulting in lower repair costs and minimal environmental impact on local water bodies. Would you be willing to participate further in our study? (Circle One.) YES NO If so, what is the best means of contacting you? (Circle One.)
Mail Email Phone If by phone, when is the best time of day to contact you? _______ Figure B-3 Initial Survey Page 3
96
On-Site Sewage Treatment Disposal Follow Up Survey Charlotte County Health Department* and
The University of Florida Department of Environmental Engineering Sciences# *Port Charlotte, Florida 33948
#Gainesville, Florida 32611
Name __________________________________ Participant ID #_______ Primary Phone Number: ____________________ home/work/mobile Secondary Phone Number: __________________ home/work/mobile
Occupancy 1. Please indicate the number of people living in your home that fit in the following age ranges:
___ younger than 11 ___ 11- 19 ___ 20-60 ___older than 60 House 2. Number of bedrooms: ______ 3. Number of bathrooms: ______ 3a. How many of your toilets are:
______ Water saving toilets ______ Regular flow toilets
3b. How many of your showers have a:
______ Water saving showerhead ______ Regular showerhead
Appliances 4. Please check the box that best describes your washing machine.
No, I do not have a washing machine in my home. Go to question 5.
4a. What is your washing machine capacity? Compact (1.7 – 2.3 cubic feet) M edium (2.31 – 2.5 cubic feet) Large (2.51 – 3 cubic feet) Extra Large (>3.1 cubic feet)
4b. What is the loading location?
Front Top
4c. Which detergent do you primarily use? Liquid Powder Both
4d. Do you use bleach?
No Yes If yes, Number of loads per week _____ Amount of bleach _________
4e. Do you use liquid fabric softener? No Yes
4f. How many loads of laundry are typically done each week?______
5. Please check the box that best describes your dishwasher. No, I do not have a washing machine in my home. Go to question 6.
5a. What is the size of your dishwasher?
Standard Size (24 inches wide) Compact Size (18 inches wide)
5b. W hich detergent do you primarily use? Liquid Powder Both
5c. How many dishwasher loads are typically done each week?___
Figure B-4 Follow-up Survey Page 1
97
H o u s e h o ld H a b i t s 6 . H o w m a n y o f t h e fo llo w in g m e a ls a r e t y p ic a lly p r e p a r e d in y o u r h o m e d u r in g t h e w e e k : _ _ _ _ B r e a k fa s t _ _ _ _ L u n c h _ _ _ _ D in n e r 6 a . D o y o u s c r a p e y o u r p la t e s o ff in t o t h e g a r b a g e c a n a ft e r m e a ls b e fo r e r in s in g t h e m ?
N o Y e s
6 b . W h e n r in s in g d is h e s in t h e s in k , w h a t d o y o u d o w it h t h e r e m a in in g fo o d p a r t ic le s in s t r a in e r :
le t t h e m g o d o w n t h e s in k d r a in p u t t h e m in t h e g a r b a g e c a n o r o t h e r 7 . H o w o ft e n d o y o u fr y fo o d s d u r in g a m o n t h : _ _ _ _ _ _ _ 7 a . D o y o u p o u r u s e d g r e a s e d o w n t h e s in k d r a in ? N o Y e s 8 . D o y o u u s e s e p t ic s y s t e m a d d it iv e s fo r y o u r s y s t e m ? N o Y e s
I f y e s , 8 a . P r o d u c t N a m e :_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
8 b . H o w o ft e n d o y o u u s e it : O n c e e v e r y _ _ _ _ _ _ _ M o n t h ( s ) 9 . H a v e y o u r h a b it s c h a n g e d a s a r e s u lt o f t h e e d u c a t io n a l m a t e r ia l p r o v id e d t o y o u b y t h e D e p a r t m e n t o f H e a lt h ? N o Y e s I f y e s , p le a s e in d ic a t e w h ic h h a b it s _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Figure B-5 Follow-up Survey Page 2
98
APPENDIX C DESCRIPTION OF METHOD USED TO DIVIDE PARTICIPANTS INTO PHASE 1
AND 2 GROUPING
Variable information collected in the initial survey was used to separate recruits
into two similar samples and presented in this appendix. Frequency charts and statistical
descriptions are presented for quantitative variables. The predicted and actual
distributions of the variables for each phase are also presented.
Variable Descriptions
The descriptions of variables used to divide the recruits are presented for all of the
recruits. Figure C-1, C-2, C-3, and C-4 are frequency charts of the quantitative variables.
The range, mean, and median are also presented for quantitative variables, Table C-1.
0
20
40
60
80
100
120
140
1 2 3 4 5 6 7 8 9 10 11 12People/ home
freq
uenc
y
Figure C-1 Number of Occupants per Home Frequency Chart
99
0
50
100
150
200
250
1 2 3 4 5 6 7 8 9 10 11 12months per year
Freq
uenc
y
Figure C-2 Occupancy in Home during the Year Frequency Chart
0
5
10
15
20
25
30
35
40
45
0 1 2 3 4 5 6 7 8 10 11 12 14 15 16 17 20 28loads per week
Freq
uenc
y
Figure C-3 Washing Machine Loads per Week Frequency Chart
100
0
10
20
30
40
50
60
0 1 2 3 4 5 6 7 9 10loads per wk
Freq
uenc
y
Figure C-4 Dishwasher loads per Week Frequency Chart
Table C-1 Quantitative Data Description Variable Count Low
rangeHigh range
Mean Median Low High Not available
Occupants (capita) 238 1 12 2.6 2 149 89 0 Occupancy (mo./yr.) 238 3 12 11.6 12 19 219 0 Washing machine loads
238 0 28 5.2 4 121 117 0
Dishwasher loads 238 0 10 2.62 2 123 114 0
Phase Distribution
The recruits were separated into two phases using the distribution of the high and
low levels. The desired sample size of the two samples was Phase 1 with 198 and Phase
2 with 40. The quantity of low and high levels for each phase was according to the
distribution and sample size of each phase. Table C-2 shows the predicted quantity of
low and high levels and the actual quantity of both levels after the recruits were
101
separated. Qualitative variables used the high level to signify yes and the low level to
signify no.
Table C-2 Variable Phase Distribution Phase 1 Phase 2 Predicted Actual Predicted Actual Variables Low High Low High Low High Low HighCapita 124 74 125 73 25 15 24 16 Occupancy per year 16 182 17 181 3 37 2 38 Washing machine loads 101 97 103 95 20 20 18 20 Dishwasher loads 102 95 100 98 21 19 23 17 Flooding 180 18 179 19 36 4 37 3 Washing machine drains to septic
27 171 30 168 6 34 3 37
Water conserving toilets 105 91 105 90 21 18 21 19 Water conserving showers 105 91 83 111 21 18 14 26 System repair or replacement 81 114 181 10 16 23 38 2 System location Knowledge 8 188 7 190 2 38 3 37 Parking or driving on system 196 2 196 1 39 0 39 1 Plants on Drainfield 127 68 127 68 26 14 26 14 Systems within 75’ of surface water
182 14 184 12 37 3 35 5
Water Source- Well 169 29 171 27 34 6 32 8 Water Source-City 36 162 34 164 7 33 9 31 Water Source- Both 191 7 191 7 39 1 39 1
102
APPENDIX D TANK DIMENSIONS
This appendix presents the tank dimensions for the known manufacturers, tanks
dimensions calculated with Method 1 and Method 2, and the relationship values used in
Method 2.
Manufactured Tank Dimensions
Table D-1Manufactured Tank Dimensions Manufacturer Volume
(gal) N w1
(in) w2 (in)
w3 (in)
l1 (in)
l2 (in)
l3 (in)
h1 (in)
h2 (in)
h3 (in)
T (in)
V-bottom Taylor 750 1 45 43 23 93 92 91 6.5 35 13 3 Stans 750 33 44.5 39 21 92 88 87 8 35 16 3 Nokomis 750 2 51 46 29 98 95 94 9 32 19 3 Southern 750 8 45 42 22 93 93 93 8 34 13 3 Stan’s 900 60 45 40 22 111 107 106 8 35 16 Nokomis 900 6 55 47 28 110 107 106 9 33 18 3 Southern 900 26 43 40 22 110 110 110 8 34 15 3.5 Sebring 900 36 55.6 50 33 98 97 94 10 28 19 2 Taylor 950 4 42.5 40 22 110 108 108 7.3 34 16 2.5 Stan’s 1050 2 45 40 22 115 111 110 8 40 16 3 Southern 1050 4 48 40 22 121 121 121 8 36 15 3.5 Southern 1200 1 58 49 36 116 116 116 8 30 22 4 Sebring 1500 1 57 53 43 120 120 120 7 40 21 4 Taper Side Crews 900 1 53 47 113 106 9.5 40 10 2.5
Method 1 Tank Dimension Calculation
Method 1 tank dimension calculations were used to determine the tank dimensions
for septic tanks with less than a 600-gallon volume. The dimensions calculated are
presented in Table D-2 for each system. The calculations used in this method are listed
below.
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1. Surface Area (in2) = (Tank Volume (gallons) /Liquid Height (in))*231 2. Average Width (in) = square root (Surface Area (in2)/3) 3. Average Length (in) = 2*Average Width (in) 4. h1 = (0.15*Tank Volume (gallons) *231)/ Surface Area (in2) 5. Total Tank Height (in) = h1 (in) + HL (in) 6. w1 = Tank Lid Width (in) – 6 in 7. l1= Tank Lid Length (in) – 6 in 8. w3 = w1 – 4 9. l3 = l1
Table D-2 Method 1 Dimension calculations ID Volume Surface
Area HL Avg.
Width Avg. Length
w1 w3 l1 l3 h1
2283 500 2333 49.5 34 68 41 37 81 81 7.4 2688 500 2264 51 33.6 67 32 28 72 72 7.65 2007 560 3058 42.3 40 80 35 31 73 73 6.3 2812 600 2665 52 36.5 73 38.5 34.5 73 73 7.8 2813 600 2917 47.5 38 76 40 36 76 76 7.1
Method 2 Tank Dimension Calculations
Method 2 tank dimension calculations were used for septic tanks with a volume of 750
and 900 gallons. The calculation was based on four relationships. The relationships were
calculated for each tank volume (Table D-3). All dimensions that were calculated for
each system are presented in Table D-4. The following calculations were used to
determine the dimensions:
1. w1 =W-2t 2. l1 = L – 2t 3. h2 = R2 * HL 4. h3 = (1-R2) * HL 5. w2 = w1 – R3 * (h1+h2) 6. w3 = w2/R1 7. l2 = L – R4* (h1+h2) 8. l3 = L – R4* (h1+h2+ h3)
Table D-3 Method 2 Ratios and Dimensions Tank Volume R1 R2 R3 R4 h1 t 750 1.8 2.3 0.12 0.057 8.25 3 900 1.9 2.3 0.13 0.07 8.57 3
104
Table D-4 Method 2 Calculated Dimensions ID Tank
Volume (gal)
Lid Length (in)
Lid Width (in)
HL (in)
w1 (in)
w2 (in)
w3 (in)
l1 (in)
l2 (in)
l3 (in)
h1 (in)
h2 (in)
h3 (in)
1059 750 97 51 51 45 44 24 91 90 88 8.2 36 15 10 750 98 51 51 45 40 22 92 89 89 8.2 36 15 596 750 98 48 51 42 41 23 92 89 89 8.2 36 15 949 750 102 45 50 39 34 19 96 93 93 8.2 35 15 1924 750 99 50 51 44 39 21 92 89 89 8.2 35 16 2287 750 98 47 50 41 36 20 92 89 89 8.2 35 15 2779 750 97 49 50 43 38 21 91 88 88 8.2 35 15 2782 750 99 64 51 58 53 30 93 90 90 8.2 36 15 2795 750 99 48 49 42 37 21 93 91 89 8.2 34 15 2740 750 95 48 50 42 38 21 89 86 86 8.2 35 15 2870 750 101 50 48 44 39 22 95 93 92 8.2 34 15 2916 750 99 50 53 44 39 21 93 90 89 8.2 37 16 2822 750 90 48 52 42 36 21 90 87 87 8.2 36 16 2879 750 90 48 51 42 37 21 84 82 81 8.2 36 15 229 900 102 53 47 48 42 25 104 101 100 8.6 36 16 1905 900 99 53 49 47 41 24 93 90 89 8.6 34 15 2658 900 98 49 52 43 37 22 92 89 88 8.6 36 16 2679 900 99 53 46 47 42 25 93 90 89 8.6 32 14 2712 900 98 46 52 40 34 21 92 89 88 8.6 36 16 2736 900 99 53 51 47 41 24 93 90 89 8.6 35 15 2874 900 110 54 52 48 42 25 104 101 100 8.6 36 16 298 950 102 54 48 48 42 41 96 93 92 8.6 34 14
105
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BIOGRAPHICAL SKETCH
I was born and raised in Charlotte County, Florida, in 1978 to Susan Peters and
Daniel Lurtz. After leaving Charlotte High School in 1996, I continued my education at
Edison Community College in Punta Gorda, where I earned my Associate of Science in
1998. I transferred to the University of Florida and earned my Bachelor of Science from
the College of Engineering in the Environmental Engineering Sciences Department in
2001. I stayed in the Environmental Engineering Science Department for my Master of
Engineering degree to work on a project that was based out of my hometown. During
my work on my master’s degree, I married the love of my life, Mr. Erik Lee Howard.
