FSAWWA Paper and Presentation Floridan Aquifer 12022015

FSAWWA 2015 Paper GJ Schers, et al. Date November 30, 2015 “Floridan Aquifer” Lessons Learned Page 1 of 32

Salinity Increases in the Upper Floridan Aquifer Wellfields in South Florida:

What have we learned and how do we plan new systems?

Authors: GJ Schers1 PMP, Ed Rectenwald1 PG, PMP, Jim Anderson2 PG, Andy Fenske3,

Amanda Barnes PE4, Howard Brogdon5, and Tom Uram6 PG

1 MWH Global Inc, 2 JLA Geosciences, Inc., 3 City of Cape Coral, 4 Town of Jupiter, 5 Collier County and

6 Palm Beach County

1. Introduction

The use of brackish ground water as a source for potable water supply has gained interest throughout the

country. Population growth in areas of fresh water scarcity, coupled with an affordable brackish water

reverse osmosis (BWRO) treatment technology, have led to implementation of many systems in Texas,

Florida and California. Production well and treatment technology have improved tremendously over the

past decades and long term operational experiences have provided the utilities with valuable information

on design and operational criteria and pitfalls. Three aspects need to be carefully considered during the

implementation of a BWRO system: (1) wellfield design and related source water productivity and quality,

(2) required pre‐treatment for RO and (3) disposal of the RO concentrate.

The wellfield design and related source water productivity and quality depend on aquifer conditions and

heterogeneity, which vary by region, but design criteria for production wells are commonly understood.

Criteria like well spacing, capacity, redundancy, depth and flow and withdrawal control of production

wells are nowadays carefully considered to provide a sustainable wellfield in terms of both production

capacity and water quality. Some wells have experienced water quality degradation over time and, if not

anticipated during design, treatment modifications can become expensive. Also treatment technologies

have improved over time and current low‐energy RO thin film composite membrane technology has

become the treatment of choice.

The type of pre‐treatment is dependent upon the local ground water chemistry where sand, iron,

hydrogen sulfide, organics, and silicates can be present. Each substance requires dedicated attention in

regards to pre‐treatment to avoid membrane fouling and/or scaling. In Florida the presence of sand in the

aquifer, without removal, can cause physical damage to the membranes and hydrogen sulfide and

dissolved iron, when oxidized, can cause membrane fouling. In parts of Texas, the major issue for BWRO

pre‐treatment includes radionuclides, arsenic, and heavy metals. In Southern California, elevated levels

of iron and manganese may require upstream media or greensand filtration.

Disposal of RO concentrate varies per region. Common methods of disposal, including surface water

discharge and deep well injection, are not readily available for landlocked areas. Areas with deep aquifers

or in close vicinity to the ocean, like coastal areas in Florida, can use more common methods of disposal.

Disposal methods in Texas are variations of surface disposal and while Southern California has access to

ocean disposal, there is a trend towards using regional concentrate transmission such to improve the

inland salt balance. Alternatives for concentrate treatment include evaporation ponds, membrane

distillation and thermal treatment, although these methods are typically only applied when zero‐liquid

discharge is required. While these technologies are effective, they are also expensive and the pros and

cons need to be evaluated prior to implementation.


This paper will focus on BWRO systems in Florida and source water quality. The Upper Floridan Aquifer

(UFA) is widely used here as a water source for potable and industrial water supply. The current (2014)

permitted capacity of BWRO systems for municipal use is 551 MGD, which represents close to 25% of the

total permitted capacity. In South Florida the aquifer contains brackish ground water and many systems

have used this source in the last 40 years. The Cities of Venice and Cape Coral along the west coast were

among the first to implement these systems successfully in the 1970’s and 1980’s. Many other water

utilities in that region followed their footsteps including Lee County, Collier County, Bonita Springs, and

the cities of Fort Myers and North Port. In 1990, the Town of Jupiter was the first east coast utility to

implement BWRO. Martin County followed shortly thereafter with the construction of its North plant in

Jensen Beach. In the 2000’s other utilities including the City of North Miami Beach, the Town of Davie,

and Palm Beach County implemented UFA wellfield and BWRO treatment systems.

Although there are many commonalities between east and west coast UFA systems, there are also distinct

differences. Aquifer salinity, expressed as total dissolved solids (TDS), typically ranges from 2,000‐5,000

mg/L on the west coast, and is often more saline (3,500‐8,000 mg/L TDS) on the east coast. The production

zone of the aquifer (or typical well depth), is deeper in east Florida because the aquifer dips steeply to the

Southeast. In general, the UFA is more productive on the Southeast coast. Typical well production rates

on the west coast are 350‐700 gpm as compared to 800‐2,000 gpm towards the Southeast. One

commonality for the UFA is that the wellfield salinity generally increases over time, requiring improved

wellfield management by including redundant production wells and treatment modifications, which have

increased the costs of producing potable water.

The salinity increases may impact the BWRO treatment system, which typically is limited by the raw water

salinity it can treat. The limits are caused by upper design criteria of individual components of the

treatment system, like the horsepower of RO feed pump motors or the pressure rating of membranes,

membrane vessels or pipework. In existing systems, the operation is restricted by the limits of the

materials and equipment and unless these are replaced, it may not be able to treat higher salinity raw

water. Fortunately, there have also been improvements in membrane technology which provide for better

salt rejections at lower feed water pressure and developments in scale inhibitor which allow for higher

system recoveries. In several cases these improvements have been implemented and have offset the

salinity increases allowing continued successful use of the BWRO systems. In terms of planning for a new

system, the designer needs to allow for some form of water degradation. This flexible design approach

has been documented by this author in several publications and focuses on selecting conservative design

criteria for RO feed pumps, chemical pre‐treatment feed systems, RO skid design, and RO bypass pipeline

and valving.

This paper will describe the general hydrogeology of the Floridan Aquifer in South Florida, will provide

details on wellfield and source water quality of certain BWRO systems and will present impacts of source

water salinity on the treatment.


2. Florida Aquifers

Three major aquifer systems underlie Florida: Surficial Aquifer System (SAS), Intermediate Aquifer System

(IAS), and Floridan Aquifer System (FAS). The IAS is only present in the Southwestern half of the state and

has variable permeability. The aquifer systems are composed of multiple, discrete aquifers separated by

low permeability “confining” units that occur throughout this Tertiary/Quaternary age sequence.

Brackish ground water in Florida is mainly found in the lower FAS, but is also present in the IAS and SAS

along coastal areas that have been impacted by lateral saline water intrusion. The brackish to highly saline

ground water found at depth in the lower FAS is connate water that was trapped in the marine limestones

as they were deposited. Very highly saline water with TDS concentrations exceeding 100,000 mg/L is

found below the Cretaceous‐aged anhydrite sequence underlying the FAS and is caused by the long term

dissolution of rock units.

2.1 Southwest Florida Aquifers

In Southwest Florida, freshwater resources occur within the SAS and IAS. The more abundant sources of

water occur within the FAS, with water quality that ranges from brackish to saline. The FAS is defined as

a vertically continuous sequence of permeable carbonate rocks of tertiary age that are hydraulically

connected in varying degrees, and whose permeability is generally several orders of magnitude greater

than that of the formations above and below (Miller, 1986). The system is subdivided into the Upper

Floridan aquifer (UFA), middle confining unit (MCU) and the Lower Floridan aquifer (LFA). The FAS in the

west coast study area is composed predominately of limestone with lower occurrences of dolomitic

limestone and dolomite (Miller, 1986).

The UFA is composed of a series of variable permeable carbonate formations including, in descending

depth, the Lower Hawthorn, Suwannee and Ocala. The MCU is composed of a series of low porosity

limestones and dolomites that consists of the Avon Park formation. The LAS consists of the highly

transmissive Boulder Zone found within the lower Avon Park and Oldsmar Formation. Potential sources

of drinking water are found in the UFA above the regulatory Underground Source of Drinking Water

(USDW), which is defined as having TDS levels of 10,000 mg/l or less and is expected to occur at

approximately 1,100 to 1,300 feet below land surface. Portions of the LFA are used for disposal of RO

concentrate and/or treated wastewater. The source water of BWRO systems is provided from the UFA

with water quality generally ranging between 1,500 and 4,000 mg/L TDS up to about 15,000 mg/L in

deeper and more coastal areas. Water quality in the LFA is likely saline, with a TDS concentration of

approximately 37,000 mg/L, based on sparse existing data. A summary of hydraulic, water quality and

potential use of these aquifers is shown in Figure 1.

Lateral saltwater intrusion occurs when seawater migrates inland from a natural reduction of freshwater

heads or pumping of wells. Increases in pumping lowers the hydraulic potential by stressing the aquifer

allowing the seawater to move inland at a faster rate. Fractures which are also evident in carbonate

aquifers of Florida may also increase movement of saltwater laterally into coastal wellfields. Vertical

saltwater intrusion occurs when saline water moves upward through fractures from underlying more

brackish aquifers. Production wells are occasionally drilled into fractures that are oriented vertically or at

high angles. These fractures may act as conduits for saline waters to move upward rapidly degrading the

water quality of some wells soon after they go into production (see Figure 2).


Figure 1: Southwest Florida Aquifer Hydrogeology

Figure 2: Sources and Migration Pathways of brackish ground water in Coastal Southwest Florida

[Source USGS Circular 1262, 2003]

Production Zone: 1500-3000 mg/L TDS


2.2 Southeast Florida Aquifers

In Southeast Florida, freshwater resources occur within the SAS and Biscayne Aquifer System (BAS). Water

resources that occur within the FAS, with water quality that ranges from brackish to saline. The FAS is

defined as a vertically continuous sequence of permeable carbonate rocks of tertiary age that are

hydraulically connected in varying degrees, and whose permeability is generally several orders of

magnitude greater than that of the formations above and below (Miller, 1986). The system is subdivided

into the Upper Floridan aquifer (UFA), middle confining unit (MCU) and the Lower Floridan aquifer (LFA).

The FAS in the east coast study area is composed predominately of limestone with lower occurrences of

dolomitic limestone and dolomite (Miller, 1986).

The UFA is composed of a series of carbonate formations with variable permeability including, in

descending depth, the Arcadia Formation of the Basal Hawthorn Unit, Suwannee, Ocala and Avon Park.

The MCU is composed of a series of low porosity limestones and dolomites that consists of the Avon Park

Formation. The LFA consists of the highly transmissive Boulder Zone found within the lower Avon Park

and Oldsmar Formation. Potential sources of drinking water are found in the UFA above the regulatory

Underground Source of Drinking Water (USDW), which is defined as having TDS levels of 10,000 mg/l or

less and is expected to occur at approximately 1,000 to 1,400 feet below land surface. Portions of the LFA

are used for disposal of RO concentrate and/or treated wastewater. The source water of BWRO systems

is provided from the UFA with TDS generally ranging between 3,000 mg/L to 6,000 mg/L up to about

15,000 mg/L in deeper and more coastal areas. The UFA within some parts of Palm Beach County and

northern Broward Counties exhibits a reversal in salinity, with higher TDS concentration in the upper

sections of the UFA and lower TDS concentrations in the lower sections (Reese and Memberg, 2000).

Water quality in the LFA is likely saline, with a TDS concentration of approximately 37,000 mg/L. A

summary of hydraulic, water quality and potential use of these aquifers is shown in Figure 3. The pathways

of lateral and vertical saltwater intrusion are similar than in Southwest Florida Aquifers.

Figure 3: East Florida Aquifer Hydrogeology


3. Wellfield and Water Quality

Water quality trends can occur in two fashions; slow trends over time within the wellfield, and fast

changes in individual wells. Certain mechanisms can be responsible for these trends or quick changes,

however, many times the exact answer is not known without intense and potentially expensive

investigations. There are however ways to minimize the effects of water quality changes.

Several wellfields tapping the UFA have shown some type of abrupt water quality declines, including the

North Collier Regional WTP UFA wellfield (CDM, 2005), North Lee County wellfield (SFWMD Permit

Information Files), the City of Cape Coral North RO wellfield (MWH, 2007,) and the City of Fort Myers

wellfield. Four utilities, the City of Cape Coral, Collier County, the Town of Jupiter, and Palm Beach County,

gave permission to use their water quality data in this report. Also current water quality data are provided

in this report from other utilities, including Bonita Springs Utility, City of Venice, City of North Miami

Beach, Town of Davie and Broward County, to complement the data from the four case studies and to

enable a presentation over a wide range of source water quality. However historical trends are not

provided. The next sections will present the data on the four featured case studies.

3.1 Southwest Florida Case Studies

City of Cape Coral

The City of Cape Coral is a pre‐platted community that relies on domestic self‐supply in areas not served

by the utility system. Limited fresh ground water sources from the IAS are available to supply domestic

users. The City is in the process of expanding the utility service again to meet the demands of a growing

population, after 6 years of decline between 2006 and 2012. The City operates two BWRO systems. The

Southwest BWRO system is the older system, which was originally put in operation in 1976 at a capacity

of 3 MGD. Expansions occurred in 1985 to 15 MGD and in 2008 to 18 MGD. In 2008, as part of plant

expansion, also the number of production wells was increased from 24 to 32, each with an approximate

depth of 700 ft. The North BWRO system is the newer system with a capacity of 12 MGD, put in operation

in 2010. This system includes 22 production wells to a similar depth than the Southwest wells.


A well location map and a photo of a North BWRO production well is included above. The City kept records

of well pumpage and water quality since inception. While pumpage of each well was recorded

continuously, water sampling for water quality was carried out monthly at each production well. Samples

were analyzed for hardness, alkalinity, chlorides, conductivity, TDS, pH, hydrogen sulfide, color, fluoride

and turbidity. Trends for each of these water quality parameters was developed for each well and for all

wells combined. Figure 4 shows the increasing raw water chlorides in all wells combined from each

wellfield. The Southwest wellfield combined chlorides increased from 600 mg/L in 1988 to 900 mg/L in

2014, while the North wellfield chlorides increased from 800 mg/L in 2010 to 1,100 mg/L in 2014. Similar

trends were established for sodium, TDS, hardness and conductivity. On the other hand, limited to no

variations were observed for other parameters, such as the source water alkalinity, pH, hydrogen sulfide,

radionuclides, fluoride and turbidity.

Water quality trends have occurred in two fashions; slow trends over time within the wellfield, and fast

changes observed in individual wells. The referenced figure includes two graphics depicting the chloride

increases in individual wells. In the Southwest wellfield, 21% of the wells have seen a chloride increase of

5% or higher during each 12 months of operation. The ‘bad’ wells are 105, 112, 114, 214 and 231 and are

spread randomly throughout the wellfield. In the North wellfield, 50% of the wells have seen a chloride

increase of 5% or higher during each 12 months of operation. The ‘bad’ wells are 301, 302, 303, 305, 306,

307, 318, 320, 322, 323 and 324 and are grouped in certain areas in North Cape Coral.

Collier County

The County has kept records of the North Collier County well pumpage and water quality since the

beginning. The North Collier brackish wellfield was originally constructed in 1998 with 10 production wells

in the lower Hawthorn aquifer and now currently has a total of 25 production wells. Six of the production

wells are constructed into the mid Hawthorn aquifer with casing and total depths approximately 400 feet

and 515 feet, respectively. With the other 19 production wells constructed in the lower Hawthorn aquifer

with casing and total depths of the wells approximately 750 and 950 feet below land surface (bls),

respectively.

Wells RO‐001 through RO‐004 at the western end of the wellfield which are producing from the lower

Hawthorn aquifer experienced rapid increases in salinity shortly after they were placed into operation.

The chloride concentration of water samples obtained from wells RO‐001 through RO‐004 during 2000

and 2001 (after they were initially constructed) ranged from approximately 2,000 to 3,000 mg/L. Chloride

concentrations in these wells increased to between 6,000 to 10,000 mg/L within two year of operation.

The membrane processes used at the NCRWTP are unable to adequately treat the higher salinity waters

so wells RO‐001 through RO‐004 have largely been unused since 2002. A total of 19 additional production

wells have been added to date to increase the raw water supply capacity to the RO WTP. Individual well

yields in the wellfield generally range from 500 to 700 gpm with chlorides concentrations ranging from

850 mg/L to 3,000 mg/L (see Figure 5) and have shown no signs of degradation. In the North wellfield,

21% of the wells have seen a chloride increase of 5% or higher during each 12 months of operation. The

‘bad’ wells are 001, 002, 003, 004 and 009.

The County also has kept records of the South Collier County well pumpage and water quality since the

beginning of operation. The South Collier brackish wellfield was originally constructed in 2001 to 2002

with 11 production wells in the mid Hawthorn aquifer and 4 production wells in the lower Hawthorn

aquifer for a total of 15 production wells. In 2006 through 2007, the County constructed 25 additional

production wells in the mid Hawthorn aquifer and 2 production wells in the lower Hawthorn aquifer for a


grand total of 42 production wells. All of the production wells constructed in the mid Hawthorn aquifer

have casing and total depths approximately 300 feet and 420 feet, respectively. And the production wells

constructed in the lower Hawthorn aquifer have casing and total depths of the wells approximately 630

and 1,000 feet below ground surface, respectively.

All production wells in the South County Wellfield show somewhat stable chlorides throughout the period

of record which typically average from approximately 2,000 to 3,000 mg/L. However, RO‐7, 10, 13, 14,

17, 39 and 41 all have average chloride concentrations between 3,000 to 4,600 mg/L within one year of

operation and have seen rapid inclines of chlorides over time. Individual well yields in the wellfield

generally range from 500 to 700 gpm with chlorides concentrations ranging from 2,000 mg/L to 3,000

mg/L and have shown no signs of degradation. In the South wellfield, 13% of the wells have seen a chloride

increase of 5% or higher during each 12 months of operation. The ‘bad’ wells are 007, 010, 013, 014 and

039.

3.2 East Florida Aquifers

Town of Jupiter

Information on the Town of Jupiter UFA wellfield was collected from historical pumping and water quality

data provided by the Town. The Towns UFA wellfield was originally constructed in 1995 with 4 production

wells in the UFA. The wellfield was periodically expanded westward as the Towns water demands

increased and as water quality changed in older production wells. The Towns UFA wellfield expansion

occurred in several phases with the most recent in 2003 with the newer wells being constructed away

from the Towns Water Treatment Plant along the C17 canal with well spacing of approximately 2,000 feet.

The average UFA well casing and borehole depths are approximately 1,215 and 1,560 feet bls,

respectively. Individual well yields range between 500 gpm and 1,600 gpm.

The Towns older UFA production wells RO‐2 through RO‐6 have experienced the most significant increases

in TDS with at the northern end of the wellfield experienced increases in salinity shortly after they were

placed into operation. The chloride concentration of water samples obtained from wells RO‐2 through

RO‐7 during 1995 and 1997 (after they were initially constructed) ranged from approximately 1,350 to

2,700 mg/L. Chloride concentrations have increased between 2,110 mg/L (RO‐2) and 556 mg/L (RO‐13)

with the largest increases (>1,000 mg/L) in wells RO‐2, RO‐3, RO‐5, RO‐6, RO‐7. Overall, chloride

concentrations have increased between 2% and 5% per year. This increase in salinity is directly related to

the well usage.

In 2004, JLA Geosciences suggested the need for uniform and equitable pumpage of the RO wells following

rapid increases in chloride and specific conductance in water produced from wells RO‐2 and RO‐3. At that

time as much as 46% of the water supplying the plant was being produced from RO‐2 and 58% from RO‐

3, with an average chloride of approximately 3,000 mg/L. With the installation of wells RO‐11, RO‐12 and

RO‐13 (2004), pumpage from RO‐2 and RO‐3 declined to level off the produced water quality. The leveling

off of chloride on the RO‐2 is likely the result of reduced stress on the well from the even distribution of

well pumpage and the increased number of wells available for use with the installation of RO‐11, RO‐12

and RO‐13.


Palm Beach County

Information on the Palm Beach County Water Utilities Department (PBCWUD) Water Treatment Plan No.

11 (WTP‐11) UFA wellfield was collected from historical pumping and water quality data provided by

PBCWUD. The WTP‐11 was originally placed into service in 2008 with 7 UFA production wells producing

approximately 10 MGD raw water. The wellfield extends northward with well spacing of approximately

800 feet. Average well casing and borehole depths are approximately 1,150 feet below land surface (bls)

and 1,450 feet bls, respectively. Due to declines in well performance and rapid water quality degradation

the WTP‐11 UFA wellfield was expanded in 2012 (Well PW‐8) and in 2014 (Wells PW‐9 and PW‐10). The

newly constructed wells were completed with boreholes at shallower depths of approximately 1,350 feet

bls, to target sections of the UFA with better water quality. Additionally, individual well withdraw rates

were reduced from an average of 2 MGD per well to 1MGD per well to reduce the rate of water quality

degradation.

The existing Upper Floridan Aquifer wells in the Lake Region WTP11 wellfield have lost capacity since they were constructed in 2005 (TP‐1 and TP‐2), 2007 (PW‐3 – PW‐7) and 2012 (PW‐8) due to a combination of over pumping and water quality degradation. Wells in the northern portion of the wellfield, TP‐1, TP‐2, PW‐4 through PW‐7, have experienced upconing of higher saline water with a current range of 5,000 to 10,000 mg/L TDS in the source water.


Figure 4: Summary of City of Cape Coral Floridan Aquifer production wells

Summary of Production Wells

Southwest Wells North Wells

Start of Operation 1976 (3 MGD) 2010 (12 MGD)

Capacity 15 MGD (exp. in ’85)18 MGD (exp. in ‘08)

12 MGD (original)

Number Average Capacity

34 500 gpm

24500 gpm

Specific capacity 10‐100 gpm/ft 10‐100 gpm/ft

Depth Diameter

700 ft 12 inch FRP

700 ft12 inch FRP

Original TDS Current TDS

1,400 mg/L (1988)2,200 mg/L (2014) 2% increase/year

2,000 mg/L (2010)2,500 mg/L (2014) 5% increase/year

Other source water quality parameters

Chloride 900 mg/L, Hardness 575 mg/L as CaCO3, H2S 3 mg/L

Chloride 1100 mg/L, Hardness 625 mg/L as CaCO3, H2S 3 mg/L

0%

5%

10%

15%

20%

25%

30%

35%

40%

101

103

105

107

109

111

211

213

215

217

219

221

223

225

227

229

231

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

Chlorides increase (%/Year of Operation)

Southwest Wellfield ‐Well ID

Chlorides Concentration (mg/L)

Chlorides (avg first 6 m)

Chlorides Increase During Ops

Chlorides Increase (%/Yr Ops)

0%

5%

10%

15%

20%

25%

30%

35%

40%

301

303

305

307

309

311

313

317

319

321

323

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000


North Welllfield ‐Well ID





SW Wells: 21% bad (>5% incr/yr)

N Wells: 50% bad (>5% incr/yr)

0

4

8

12

16

20

24

28

0

500

1,000

1,500

Total Pumpage (mgd)

Concentration Chloride (mg/L)

Chlorides SouthChlorides NorthTotal Pumpage North + South


Figure 5: Summary of Collier County Floridan Aquifer production wells

Summary of Production Wells

South Wells North Wells


Capacity 12 MGD (exp. in ’07) 8 MGD (exp. In ‘03)

Number Average Capacity

42 300 gpm

25300 gpm


Depth Diameter

300‐400 ft (mid Hawtorn)600‐1000 ft (low Hawtorn)

12 inch FRP

400‐500 ft (mid Hawtorn)700‐900 ft (low Hawtorn)

12 inch FRP

Original TDS Current TDS

4,500 mg/L (2002)5,500 mg/L (2013) 2.5% increase/year

5,500 mg/L (2000)4,780 mg/L (2013)

No increase

Other source water quality parameters

Chloride 2,750 mg/L, Hardness 1,500 mg/L as

CaCO3, H2S 3 mg/L

Chloride 2,000 mg/L, Hardness 1,100 mg/L as

CaCO3, H2S 3 mg/L

0

2,000

4,000

6,000

8,000

Concentration Chloride (mg/L)

Chlorides South

Chlorides North

12 per. Mov. Avg. (Chlorides South)

12 per. Mov. Avg. (Chlorides North)

0%

5%

10%

15%

20%

25%

30%

35%

40%

001

003

005

007

009

011

013

015

017

019

021

023

025

027

029

031

033

035

037

039

041

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000


South Wellfield ‐Well ID





0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

001

003

005

007

009

011

013

015

017

019

101

114

116

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000


North Welllfield ‐Well ID





S Wells: 13% bad (>5% incr/yr)

N Wells: 21% bad (>5% incr/yr)


Figure 6: Summary of Jupiter Floridan Aquifer production wells


Figure 7: Summary of Palm Beach County Floridan Aquifer production wells


3.3 Solutions to Minimize Saline Water Migration

As presented in the previous sections, water degradation due to migration of poor water quality is quite

common, in particular in the larger wellfields at both the east and west coast. Upward migration of poor

quality water can have detrimental effects not only for the individual well, but for the entire wellfield. As

presented by the authors in other publications, there are some solutions that can be imposed on brackish

water wells that are experiencing impacts from poor quality water. At times, multiple solutions need to

be considered simultaneously to achieve the desired effect. Some of these solutions were discussed in

the sections on the four case studies.

Back Plugging with Cement

It is possible that wells may be usable in the future if they are back plugged. Since wells can have multiple

distinct flow zones in the borehole, they can be back plugged to minimize the connection to poorer quality

water. Grout can be forced into small fractures, thus plugging off even small conduits outside of the

borehole wall. Back plugging reduces the borehole stresses from the previously exposed vertical conduits,

decreasing the possibility of upward migration. During construction of the southwest wells, the City of

Cape Coral used this method effectively to ‘save’ one of wells without affecting the wellfield.

Hydraulic Control and Water Quality Blending

Since a well may be connected or in close proximity to a vertical conduit, it is quite possible that back

plugging may not be completely successful and the well will continue to show signs of degradation since

any vertical conduit will always be within the capture zone (drawdown cone) of this well. Further

operational analysis may show that the well can be used at lower flow rates thus reducing drawdown and

stresses on the aquifer and the water produced can be diluted into the raw water stream to the res

treatment system. The wellfields exhibited in the case studies use this method, although in several cases

wells were retrofitted with variable frequency drives and flow meter control since their construction. A

recent example is the Palm Beach County Region 11 wells retrofit.

Well Abandonment

A facility could also plug and abandon a well and look for new location where production could be as good

or better without the degrading water quality. If the poor water quality is originating from the upper half

of the production interval just below the casing then this may be the only option available. This solution

is also not a guarantee and could be more costly if the new location identified the same water quality

issues as the well that was plugged and abandoned. Well abandonment due to severe water quality

impacts within an active wellfield will have a much lower chance of success. If the conduit connection to

the poor quality water is not severed and abandonment activities are not successful, then the poor quality

water can be pulled or captured by the next closest operating well if the poor quality water is allowed to

enter the raw water aquifer. The option of well abandonment should be considered as the last option

available for many reasons. Besides the capital investment in the construction of the well and pipeline,

abandonment removes all potential for hydraulic control of the migration of poor water quality, and may

artificially migrate through the wellfield by pumping stresses alone. The exhibited case studies have wells

which are very infrequently used or have even by abandoned because of severe water quality.


Redundant Production Wells

Some facilities are moving forward to better manage changing water quality occurrences by adding

additional wells to their system for redundancy. The poor water quality may be occurring due to stressing

a particular area for too long and not allowing the aquifer to recharge naturally. If property is available

and location is favorable within the proximity of the main raw water lines, a facility may want to include

redundant production wells, with sufficient spacing to existing wells. This will allow the facility to cycle

the use of the wells with better management to keep from stressing the aquifer in one specific area for a

long period of time. The facility could take wells offline in areas where the aquifer is being stressed more

than usual and operate the redundant production wells. In particular, the exhibited wellfield in Southwest

Florida have used this method extensively to control water degradation. Both the City of Cape Coral and

Collier County have a number of standby wells in each wellfield.

Pro‐active Wellfield Management System

On‐line monitoring of certain well parameters to allow the implementation of a pro‐active wellfield

management system is another method to create a sustainable wellfield operation in terms of production

capacity and water quality. Several utilities have implemented this method successfully in the last 10

years. Multiple parameters are collected on‐line, in real time, to monitor the operations of a well,

including the well drawdown, production flow and water quality. This information can be used in a pro‐

active program to control the wellfield. All exhibited case studies use on‐line monitoring extensively and

have some form of pro‐active wellfield management system. For instance the City of Cape Coral keeps

careful track of drawdown, flow, total flow and conductivity and use that for development of a series of

trends for each production well.

4. Reverse Osmosis Treatment

4.1 General Description of RO Treatment

One of the most critical factor for a successful operation of a BWRO system is the quality of the RO feed

water, which is determined by the source water quality and the pre‐treatment effectiveness. For the

treatment system design, a good understanding of source water quality and chemistry is needed including

future water quality trends, variability between wells and wellfields, and seasonal variations.

The typical process used in Florida to treat brackish ground water uses chemical pretreatment, cartridge

filtration, reverse osmosis (RO), degasification, disinfection, and corrosion control (see Figure 8). Sand

separation, upstream of the chemical pre‐treatment, is optional and depends on the expected sand

production from wells. Pretreatment chemicals are used to manage scaling in the RO membranes and

reduce the pH to optimize hydrogen sulfide stripping in the degasifiers. Post treatment chemicals are used

to perform primary disinfection and to adjust the pH, hardness and alkalinity for corrosion control.

Pre‐treatment

Sand separators are used to separate sand and other solid matter larger than a certain diameter (generally

≥0.1 mm) from the source water that could either plug the downstream cartridge filters, or damage or

foul the downstream RO membrane systems. Sand separators are typically used as a pretreatment step,


treating the full ground water wellfield flow, in addition to the pretreatment steps of chemical treatment.

There are different types of sand separators and its selection depends on the sand/silt characterization

and plant operation. Typically, RO manufacturers require the feed water turbidity and silt density index

(SDI) to be below specific maximums as a requirement for the RO membrane warranty; in this regard, the

sand separators may help the pretreatment system comply with these warranty requirements.

The primary function of cartridge filters is to remove particulate and colloidal matter larger than a certain

diameter from the source water that could either damage or foul downstream RO membranes. Cartridge

filters serve two important roles: 1) to remove relatively large particulates in order to protect the RO/NF

membrane integrity, and 2) as an inexpensive backup in the event of a failure in one of the other upstream

pre‐treatment systems.

Figure 8: Typical Schematic of BWRO Treatment Process

Iron is naturally present in shallow limestone aquifers in South Florida and, if exposed to air and allowed

to oxidize, the ferrous bicarbonate forms an insoluble ferric hydroxide that is difficult to remove from the

membrane surface. The approach here is to keep the iron in the reduced form (Fe2+) by minimizing air

intrusion and applying an acid to the feed water. Hydrogen sulfide, at low concentrations though, is

present in the shallow aquifers in South Florida. The deeper, brackish aquifers contain elevated levels of

hydrogen sulfide and, if exposed to air and allowed to oxidize, elemental sulfur (S0) will be formed, which

will block the cartridge filters and RO membranes and is difficult to remove. In this case also, minimizing

air intrusion is very important.

Chemical pre‐treatment with acid and scale inhibitor is practiced to minimize membrane scaling, with a

general trend in the industry to limit or even eliminate acid and to rely mostly on a scale inhibitor.

Acidification is often achieved with sulfuric acid (and less commonly hydrochloric acid) to prevent


precipitation of carbonate scale on the membranes. An additional impact of the sulfuric acid addition is

the conversion of carbonate ion (alkalinity) to carbonic acid in the source water (see Figure 9), which

passes as carbon dioxide gas through the membranes. Sulfuric acid is preferred over hydrochloric acid due

to cost and safety reasons, but the addition of sulfuric acid may increase the scaling potential for sulfate

salts and lower the recovery. All polyamide membranes are intolerant to chlorine and may result in

breakdown of the membrane material with prolonged use.

Figure 9: (L) Carbon Dioxide and (R) Hydrogen Sulfide Dissociation in Water as Function of pH

As the RO permeate passes through the membranes, the remaining concentrate becomes increasingly

concentrated with dissolved solids. At a certain point, the solubility of various salts can be exceeded,

causing precipitation onto the membranes, called “scaling.” Scaling can reduce the flow or flux of

permeate and can also damage the membrane itself. As mentioned, scale inhibitor is commonly used in

conjunction with acidification to inhibit the formation of phosphate and sulfate scaling on membranes,

and reduce the acid dosage required to inhibit formation of carbonate scales. Threshold scale inhibitors

suppress precipitation by interrupting the kinetics of normal crystallization, thus delaying precipitation

beyond the residence time in the membrane system. Scale inhibitors allow the membrane system to

increase recovery beyond limiting salt saturation limits. The sparingly soluble salts of concern in most

waters include calcium carbonate (or LSI), calcium sulfate, strontium sulfate, barium sulfate and silica

dioxide. It is important to select the appropriate scale inhibitor for the design application – some scale

inhibitors may act as coagulants, facilitating the accumulation of organic carbon on the membrane

surfaces and increasing fouling potential. There are well‐documented industry guidelines on the limits of

the saturation indexes of these sparingly soluble salts with or without the use of scale inhibitors.

0.00

0.20

0.40

0.60

0.80

1.00

4 5 6 7 8 9 10 11 12 13 14

Molar Fraction

pH

Carbonic Acid (H2CO3)

Bicarbonate Ion (HCO3‐)

Carbonate Ion (CO32‐)

0.00

0.20

0.40

0.60

0.80

1.00

4 5 6 7 8 9 10 11 12 13 14

Molar Fraction

pH

Hydrogen Sulfide (H2S)

Bisulfide Ion (HS‐)

Sulfide Ion (S2‐)


Main treatment

High pressure pumps are used to increase the

feed water pressure to overcome osmotic

pressure and hydraulic losses in the connecting

pipework and RO trains. Two methods exists for

the high pressure pump arrangement: (1)

decentralized system with a dedicated feed

pump per RO train or (2) centralized system with

one common pump station and a common

header and flow control valves to feed each of

the RO trains. Both methods have their specific

advantages and disadvantages with a decision

mainly driven by the owner.

RO trains in South Florida typically consist of two single‐pass stages with the 1st stage concentrate used as

the feed for the 2nd stage. Permeate flows of both stages join for the total permeate while the 2nd stage

concentrate flow is the total train concentrate. Each train consist of multiple, parallel pressure vessels

which can either hold six or seven standard 40‐inch long and 8‐inch diameter RO elements. The train array,

including the number of pressure vessels in each stage and related flow and flux distribution, is dependent

upon the feed water quality and the overall train design concept.

The RO process produces a high‐pressure concentrated waste stream. When this concentrate stream is

depressurized, the energy that is lost can be recovered using energy recovery devices (ERDs), which can

reduce overall energy requirements by 10 to 50 percent. Energy recovery is not economically justified for

all RO membrane systems; those with tight membranes, low recovery and high concentrate flow and

pressure (e.g. high TDS brackish ground water) are more likely to find energy recovery beneficial with low

pay‐back periods and improved flow balancing between the membrane train stages. Those systems with

lose membranes, high recovery and low concentrate flow and pressure (e.g. low TDS brackish ground

water) will find 1st stage permeate throttling more cost‐beneficial for flow balancing between the

membrane train stages. ERDs will minimize the feed pressure required, which in turn minimizes the

operating power costs.

There are different devices available with direct‐

transfer pressure exchangers for high efficiency (90‐

95%) –see picture to the right–, turbo‐chargers

which combine recovery device and pump in one for

medium efficiency (50‐70%) and more‐traditional

turbines with medium‐high efficiency (80‐85%). The

devices are typically installed on a membrane train

to transfer pressure/energy in the concentrate to

the 2nd stage feed water, reducing the size of the RO

feed and/or inter‐stage pumps.


Bypassing a portion of the feed water around the membrane process, treating it with cartridge filters only,

and blending it with the membrane permeate may allow the system to meet treated water quality goals

while treating less of the incoming flow stream. Blending of the RO bypass water with permeate may be

beneficial when the permeate water quality requires re‐mineralization. The bypass will reduce the amount

of post‐treatment chemicals and the amount of feed water that needs to be treated, both reducing the

capital and O&M costs for the plant.

Post treatment

Degasifiers are typically used as post‐treatment to remove hydrogen sulfide (H2S) and carbon dioxide

(CO2) from the RO permeate by counter current mass transfer of air and water. The removal of H2S and

CO2 is optimized by a reduction of the pH to levels around 5.8‐6.0 (see Figure 11). The addition of chlorine

to the degasified water will oxidize any remaining dissolved sulfide in the water into colloidal sulfur,

increasing the finished water turbidity. Elemental sulfur is a sticky substance that is difficult and

cumbersome to remove from the downstream clearwell and can create variations in finished water

turbidity by sloughing off of sulfur sediments. Degasifiers serve several important roles: 1) reduction of

effluent turbidity by removing sulfide from the water stream before chlorination; 2) odor control for the

treated effluent; and 3) corrosion control through

the removal of excess CO2.

Odors from degasifiers can result in complaints

from neighbors in the vicinity of the installation

which may present community relations

challenges for the owner. Odor control through

dilution and dispersion, or odor removal systems

(like wet chemical scrubbers or biological filters)

may need to be added to degasifier systems to

meet local air quality and odor limit requirements

at the site of the installation.

Depending on the downstream requirements chemical addition to the treated water may be required for

primary disinfection, pH adjustment and corrosion control. The need for these chemicals depends on

specific raw water quality and treated water quality goals, and should be evaluated on a case by case

basis. The concept of removing excess carbon dioxide in the degasifier and then having to dose it again

for corrosion control in the clearwell may be considered inefficient. Some utilities have been studying

alternative technologies for hydrogen sulfide removal, such as oxidation.

4.2 Specific Considerations to address Salinity Increases

As presented before, some degradation of raw water quality can be expected in Floridan Aquifer water

supply and treatment systems. A conservative design of a RO treatment system will accommodate some

degradation while maintaining the required capacity and finished water quality. Also the design will be

specific to the raw water quality anticipated for that system and in South Florida the salinity can vary by

region, with TDS levels of around 2,000 mg/L in certain parts going up to levels around 8,000 mg/L in other

parts. As can be expected, this variance has a significant influence on the system design and equipment

specifications. As part of this work, raw water quality data was obtained from different systems, as

summarized in Table 1 to create several case‐studies, with the following observations:


In general, TDS are higher along the east coast with some exceptions

Some parameters relevant to chemistry equilibrium of sparingly soluble salts, like barium, silica

and strontium appear relatively higher on the west coast

Hardness is higher along the east coast, with a higher relative contribution from magnesium

whereas alkalinity is relatively constant throughout the region despite varying TDS levels

Sulfate levels, important for saturation indexes of sparingly soluble salts, in general follow the TDS

trend

Hardness and sulfate levels are extremely high in the Sarasota County wellfields which may impact

the RO recovery rate or led to alternative technologies, such as electro dialysis reversal (EDR)

Potassium appears to be higher on the east coast

Though not included in the table, the Floridan Aquifer water also contains hydrogen sulfide, with

concentrations varying from 2 to 5 mg/L, and radionuclides with gross alpha concentrations varying

between 5 and 40 pCi/L.

The raw water quality data sets, presented in the table, were used to model the RO treatment process for

each particular case study. Proprietary membrane software was used to develop the system design and

predict the treatment performance. Subsequently, also proprietary scale inhibitor software was used to

determine the required chemical pre‐treatment (type and dose) to control membrane scaling. The

following design assumptions and goals were used as guidelines for the design efforts:

Pre‐Treatment

Sand separators and cartridge filters to bring the Silt Density Index below 3

Sulfuric acid dosing to suppress the feed water pH to 6.0. A few scenarios were run with reduced

acid dose in the RO feed water and relying on a scale inhibitor, while maintaining a Langelier

Saturation Index (LSI) of 1.8 or less

RO Train Design

Dedicated feed pump per train

2‐stage array, single pass

Dependent upon design need, with or without ERDs. In our particular case studies, a direct

transfer pressure exchanger was selected

Average, conservative system flux is 12.5 gfd

2‐1 vessel array, with 7 elements per vessel

Capacity of RO train and associated bypass is 3 MGD

TFC HR RO membranes, 40‐inch long and 8‐inch diameter with 5 year element age and 25% fouling

allowance

1st and 2nd stage back pressure 20 psi, interstage pressure loss 5 psi

Use of software design warnings to guide the design effort

Post Treatment

Blend permeate with RO bypass

Sulfuric acid dosing to suppress the feed water pH to 5.8‐6.0.

Degasifier to remove carbon dioxide and hydrogen sulfide


Liquid lime (Ca(OH)2) and carbon dioxide (or gas CO2) dosing in clearwell to meet the finished

water quality goals, including a pH of 8.0, hardness of 80 mg/L as CaCO3 and total alkalinity of 60

mg/L as CaCO3. This is somewhat arbitrary and has to be evaluated on a case by case basis and

depends largely on the corrosion control strategy.

A summary of the results of the membrane modeling efforts is included in Table 2, with increasing TDS

levels in the wellfield presented from left to right. The table does not include the modeling results of the

City of Venice due to different water chemistry composition here and its negative impact on possible RO

recovery rates, which would skew the trends presented further in this report.

General observations from the modelled case studies with TDS levels varying from 1500 mg/L to 9500

mg/L as summarized in the table are:

RO recovery rate reduces from 85% to 75%

RO bypass reduces from 20% to 2%, with the permeate flow making up the difference

Train array increases from 48‐24 vessels to 60‐30 vessels to produce the additional required

permeate flow at higher raw water TDS

Feed pressure increases from 188 to 392 psi, without ERD, and from 188‐282 with ERD

Post treatment chemicals to adjust alkalinity and calcium are necessary across the full range of

raw water quality however for instance CO2 dosages increase from 14 to 44 mg/L

Electricity and chemical costs for RO treatment only increase from $0.32 to $0.72 per 1000 gallons

treated water

The table can facilitate in quick assessments in the following conditions: (1) by a utility planning a new

BWRO system to verify typical design criteria and operational parameters based on initial well water

quality and (2) by a utility operating an existing BWRO to estimate impacts of degrading raw water quality

on the design configuration and operations.

In the sections below, specific aspects of the BWRO treatment system will be described in regards to

different TDS levels in the wellfield.


Table 1: Water Quality Data Sets from Case Studies and Others

Parameter

(current ‐ 2015, or

otherwise noted)

UnitsCape Coral,

South RO 2000

Cape Coral,

North RO 2010

Cape Coral,

North ROVenice RO

North Miami

Beach RO

Town of Davie

RO

Collier County

SouthJupiter RO

Broward

County

System 1

Palm Beach

County

System 11

Total Dissolved Solids mg/L 1,516 2,000 2,623 3,000 3,200 4,950 5,350 7,880 7,470 6,100

Barium mg/L 0.03 0.04 0.04 0.03 0.01 0.01 0.03 0.02 0.01 0.04

Fluoride mg/L 1.4 1.4 1.4 1.7 0.9 1.1 1.4 1.8 0.8

Nickel mg/L U U U U U U U 0.01U 0.01U 0.01U

Nitrate mg/L as N 0.01U 0.01U 0.01U U U U 0.50 0.02U 0.01U 0.50U

Sodium mg/L 343 420 600 265 780 1,450 1,370 2,310 2,100 1,590

Chloride mg/L 589 890 1,210 528 1,400 2,500 2,530 4,110 3,850 2,660

Iron mg/L 0.01U 0.01U 0.01U U U 0.01U 0.02U 0.02U 0.05 0.02U

Manganese mg/L 0.01U 0.01U 0.01U U U U 0.01U 0.01U 0.01U 0.012U

Sulfate mg/L 269 280 330 1,315 460 510 720 595 935 530

pH mg/L 7.7 7.7 7.7 7.6 7.8 7.7 7.5 7.6 7.8 7

Ammonium‐N mg/L 0.32 0.35 0.39 0.49 0.35 0.50 n/a 0.74

mg/L as CaCO3 141 142 145 105 115 132 152 120

mg/L 172 173 177 128 140 0 161 n/a 146

Calcium mg/L 90 108 125 415 105 150 270 197 210

Hydrogen Sulfide mg/L 3.0 3.0 3.0 3.0 3.5 3.2 4.0

Magnesium mg/L 86 92 105 178 110 205 280 290

Potassium mg/L 19 22 25 9 35 55 68 131

Silica mg/L 15 16 18 25 15 19 10

Strontium mg/L 17 19 21 14 3 15 15 12

Temperature °C 25 25 25 25 25 25 25 23 25 22

Bicarbonate Alkalinity


Table 2: Membrane Modeling Results

Modeling Scenarios

Cape Coral,

Southwest RO

(1990)

Cape Coral,

North RO

(2010)

Cape Coral,

North RO

North Miami

Beach

Cape Coral,

North RO

(projected)

Town of DavieCollier County

South RO

Palm Beach

County System

11

Jupiter RO

Broward

County WTP1

(projected)

Raw Water TDS (mg/L) 1516 2000 2623 3250 3850 4690 5350 6060 7600 9356

Permeate TDS 43 51 76 90 112 169 153 197 244 282

Finished Water TDS Contrib. Bypass (mg/L) 405 400 375 360 337 281 296 251 202 163

Bypass Limit as % of Finished Water 20% 20% 14% 11% 9% 6% 6% 4% 3% 2%

Bypass Flow (gpm) 438 402 313 242 191 131 120 90 58 38

Permeate Flow (gpm) 1,753 1,754 1,878 1,945 1,988 2,054 2,056 2,084 2,115 2,135

Finished Water Flow (gpm) 2,191 2,156 2,191 2,187 2,179 2,185 2,176 2,175 2,173 2,172

Array Recovery 85% 80% 80% 80% 80% 80% 80% 75% 75% 70%

Stage 1 Pressure Vessels 48 50 52 54 56 58 58 58 58 60

Stage 2 Pressure Vessels 24 25 26 27 28 29 29 29 29 30

Average Flux (gfd) 12.5 12.0 12.4 12.3 12.2 12.1 12.2 12.3 12.5 12.2

Feed Water pH 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0 6.0

Feed Water Pressure w/ Fouling Allowance 188 195 215 210 220 234 243 251 270 282

Interbank ERD Pressure Gain (psi) 0 0 0 43.7 58.9 82.8 99.1 102 137 153

Design Warnings None None None None None None None None None None

Saturation Indices (If Scaling)

Langlier Index 0.57 0.29 0.36 0.41 0.43 ‐ 0.65 ‐ ‐ ‐

CaSO4 ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐

CaF2 2.5 1.4 1.3 1.4 2.4 ‐ 1.2 ‐ ‐ ‐

BaSO4 14.3 11.9 11.35 10.82 10.42 2.5 9.41 1.8 2.36 1.6

SrSO4 6.1 4.2 4.2 4.0 3.9 1.8 2.8 1.9 1.6 1.1

SiO2 ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐ ‐

Degasified Water Quality (range)

Ca. Hardness (mg/L as CaCO3) 59.3 52.8 44.7 36.8 32.2 22.4 35.3 15.8 14.4 10.1

Mg. Hardness (mg/L as CaCO3) 92.8 74.0 61.8 53.1 49.3 44.5 44.1 43.4 32.6 22.9

Total Hardness (mg/L as CaCO3) 152 127 106 90 81 67 79 59 47 33

TDS (mg/L) 434 421 428 414 430 434 415 443 435 433

pH 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0

Alkalinity (mg/L as CaCO3) ‐ Treated Water 44 35 28 24 21 15 15 13 11 10


Table 2: Membrane Modeling Results (Continued)

Modeling Scenarios (Table Continued)

Cape Coral,

Southwest RO

(1990)

Cape Coral,

North RO

(2010)

Cape Coral,

North RO

North Miami

Beach

Cape Coral,

North RO

(projected)

Town of DavieCollier County

South RO

Palm Beach

County System

11

Jupiter RO

Broward

County WTP1

(projected)

Finished Water Quality Goals

Total Hardness (mg/L as CaCO3) ‐ Goal 80 80 80 80 80 80 80 80 80 80

Alkalinity (mg/L as CaCO3) ‐ Goal 60 60 60 60 60 60 60 60 60 60

Post‐Treatment Chemicals

Additional Hardness Dosed (mg/L as CaCO3) 0 0 0 0 0 13 1 21 33 47

Calflo (Ca(OH)2) (mg/L) 0 0 0 0 0 10 0 15 24 35

Carbon Dioxide (mg/L) 0 0 0 0 0 12 1 18 29 41

Additional Alkalinity Dosed (mg/L as CaCO3) 16 25 32 36 39 45 45 47 49 50

Additional Bicarbonate Dosed (mg/L) 19 31 39 44 47 55 54 58 60 61

Calflo (Ca(OH)2) (mg/L) 12 19 23 26 29 33 33 35 36 37

Carbon Dioxide (mg/L) 14 22 28 31 34 40 39 42 43 44

Electricity Use 0 0 0 0 0 0 0 0 0 0

Feed Flow Per Train (gpm) 2062 2192 2347 2431 2485 2568 2569 2779 2821 3049

Feed Pressure (psi) 188.2 195.3 215 209.6 220.2 234 243.4 251 270 282

Feed Pump Suction Pressure (psi) 30 30 30 30 30 30 30 30 30 30

Feed Pump Efficiency 78% 78% 78% 78% 78% 78% 78% 78% 78% 78%

Interbank Flow (gpm) 675 806 811 778 741 703 683 900 872 1,094

Interbank ERD Pressure Gain (psi) 0 0 0 44 59 83 99 102 137 153

Energy Cost ($/kWh) $0.12 $0.12 $0.12 $0.12 $0.12 $0.12 $0.12 $0.12 $0.12 $0.12

Annual Energy Cost ($/year) $191,461 $213,992 $256,477 $257,906 $279,132 $309,360 $323,841 $362,753 $399,806 $453,862

Energy Cost ($/1000 gallons) $0.17 $0.19 $0.22 $0.23 $0.25 $0.27 $0.29 $0.33 $0.36 $0.41

Chemical Use $0 $0 $0 $0 $0 $0 $0 $0 $0 $0

Sulfuric Acid Dose (mg/L) 100 101 104 108 107 92 93 93 97 102

Sulfuric Acid Unit Cost ($/gallon) $2.19 $2.19 $2.19 $2.19 $2.19 $2.19 $2.19 $2.19 $2.19 $2.19

Sulfuric Acid Cost ($/1000 gallons) $0.11 $0.12 $0.13 $0.15 $0.15 $0.13 $0.14 $0.15 $0.16 $0.18

Anti‐scalant Dose (mg/L) 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

Anti‐scalant Unit Cost ($/lb) $1.25 $1.25 $1.25 $1.25 $1.25 $1.25 $1.25 $1.25 $1.25 $1.25

Anti‐scalant Cost ($/year) $5,653 $6,008 $6,434 $6,665 $6,811 $7,038 $7,043 $7,618 $7,732 $8,359

Calflo Dose (mg/L) 11.7 18.8 23.5 26.5 28.5 33.5 33.0 35.1 36.2 37.0

Calflo Unit Cost ($/lb) $0.24 $0.24 $0.24 $0.24 $0.24 $0.24 $0.24 $0.24 $0.24 $0.24

Calflo Cost ($/year) $26,665 $42,863 $53,579 $59,267 $62,116 $74,984 $71,914 $76,457 $78,887 $80,458

Carbon Dioxde Dose (mg/L) 17.4 27.9 34.9 39.3 42.4 49.8 49.1 52.2 53.9 54.9

Carbon Dioxide Unit Cost ($/lb) $0.11 $0.11 $0.11 $0.11 $0.11 $0.11 $0.11 $0.11 $0.11 $0.11

Carbon Dioxide Cost ($/year) $18,775 $30,181 $37,726 $41,732 $43,738 $52,798 $50,637 $53,835 $55,546 $56,652

Chemical Cost ($/1000 gallons) $0.16 $0.19 $0.22 $0.24 $0.26 $0.25 $0.26 $0.27 $0.29 $0.31

Total Energy + Chemical Cost ($/1000 gallons) $0.32 $0.38 $0.44 $0.47 $0.51 $0.53 $0.55 $0.60 $0.65 $0.72

The chemical costs covers only the chemicals explicitly mentioned above. The electricity costs only covers RO feed pump, and does not cover the well pumps,

transfer pumps and high service pumps; neither does is include other electrical consumers, such as the degasifier blowers.


Pre‐Treatment – Acid

In the last ten to fifteen years significant improvements have been made in terms of scale inhibitors.

Different types of scale inhibitors have been developed with each a specific objective in mind, like

inhibition of calcium carbonate scaling, inhibition of certain sparingly soluble salts scaling and/or

reduction of ferrous fouling. Currently, many utilities operating BWRO treatment systems have reduced

or even eliminated the use of sulfuric acid in the RO feed stream for cost and/or safety reasons. As the

optimal pH of the degasifier influent is around 5.8 to 6.0, utilities still need to dose sulfuric acid to the

degasifier influent or have converted to carbonic acid (or carbon dioxide gas), dosed to the blend stream

of RO permeate and RO bypass, to reduce the pH. In considering this the utility needs to consider the total

system chemistry, including the use of post treatment chemicals as liquid lime and carbon dioxide to meet

the finished water hardness and alkalinity goals. This has been done for three case‐studies: (1) Cape

Coral’s North RO, (2) Town of Davie and (3) Town of Jupiter RO. The results are presented in Figure 10.

Based on modeling, sulfuric acid is still required in the RO feed to maintain a LSI of 1.8 or less and acid is

also needed in the degasifier inlet to maintain optimal pH conditions of 5.8‐6.0, although the overall acid

consumption reduces. The figure shows that acid reduction is partly offset with an increase in the scale

inhibitor, liquid lime and carbon dioxide dose, although overall chemical costs are lower for all case studies

with limited sulfuric acid addition.

Figure 10: Chemical Costs, with Acid and Limited Acid, for some Case Studies

$0.13

$0.05

$0.13

$0.05

$0.16

$0.05

$0.01

$0.02

$0.01

$0.02

$0.01

$0.03

$0.03

$0.05

$0.05

$0.05

$0.05

$0.05

$0.05

$0.06

$0.07

$0.07

$0.07

$0.08

$0.00

$0.05

$0.10

$0.15

$0.20

$0.25

$0.30

$0.35

2623 2623(limited acid)



Chem

ical Costs ($ per 1000 gallons treated)

Raw Water Total Dissolved Solids (mg/L)

Sulfuric Acid Cost ($/1000 gallons) Anti‐scalant Cost ($/1000 gallons)

Carbon Dioxide Cost ($/1000 gallons) Calflo Cost ($/1000 gallons)


RO Bypass

As mentioned before, bypassing a portion of the raw water around the membrane process and blending

it with the RO permeate helps meeting the finished water quality goals through re‐mineralization, while

treating less of the incoming flow stream. The ability to bypass and the amount of the bypass flow is

primarily dependent on the raw water TDS and finished water quality goals, but can also be influenced by

the membrane salt rejection performance. As is expected, the bypass flow will need to be reduced when

the raw water TDS increases or is higher. This concept is presented in Figure 11 illustrating the reduction

in the bypass flow from just below 20% to around 6% of the raw water flow when the TDS in the raw water

increases from 2,000 to 4,700 mg/L. Consequently, and to maintain the treatment plant capacity (at

100%), the permeate flow will need to be increased from 80 to 94%. This requires modifications to the RO

trains to accommodate more pressure vessels and RO elements for higher production while maintaining

the same flux rate. The RO Bypass flow at raw water TDS levels of above 5,000 mg/L is low and practicing

RO bypass becomes less attractive.

Figure 11: Bypass Water and Permeate Flows as Function of Raw Water TDS

RO Train

Using the design assumptions and goals presented before, a RO train design was developed for each case

study as summarized in Table 2. The concept of increasing the number of pressure vessels and RO

elements to maintain the same finished water flow is evident. For a 3 MGD system, the RO array needs to

be increased from 48‐24 to 58‐29 pressure vessels in the first and second stage respectively, when the

TDS in the raw water increases from 1,500 to 7,600 mg/L. At the same time the RO recovery rate decreases

from 85% to 75% and the feed pressure increases from 188 to 392 psi without ERD (and 188 to 282 psi

0%

20%

40%

60%

80%

100%

120%

1,500 2,500 3,500 4,500 5,500 6,500 7,500 8,500 9,500

Flow (% of Design Flow)


Bypass Flow WTP (%) Permeate Water Flow WTP (%)

Rated Finished Water Capacity WTP (%)


with ERD). The RO feed pumps are the single largest electrical consumers at any BWRO treatment system

and ERDs to obtain the residual pressure from the second stage concentrate, which would be lost when

concentrate is discharged, and to convert that into feed pressure energy to the second stage feed has

become very popular. For each case study, the payback period was calculated and the results are provided

in Figure 12. As illustrated the payback period of ERDs is less than 10 years and will become interesting

when the raw water TDS exceeds 3,000 mg/L. Below 2,500 mg/L TDS, the payback period of an ERDs is

above 15 years and is therefore less interesting. The combination of number of pressure vessels and the

inclusion of an ERD in the BWRO treatment system are provided in Figure 13.

Figure 12: Payback Period of an ERD as Function of Raw Water TDS

0

5

10

15

20

25

30

35

40

45

1,500 2,500 3,500 4,500 5,500 6,500 7,500 8,500 9,500

Interstage ERD Payback Period (Years)



Figure 13: RO Array Design as Function of Raw Water TDS

Post Treatment

The RO permeate is combined with the RO bypass and treated in the degasifier vessels to remove the

hydrogen sulfide and excess carbon dioxide. The chemistry of the blend stream will change as a function

of the membrane performance but also as a result of the RO bypass flow percentage. Therefore, also the

chemistry of the degasified water will change as a function of the raw water TDS content. This concept is

illustrated in Figure 14. The alkalinity of the degasified water will decrease from just above 40 to around

15 mg/L CaCO3 when the raw water TDS increases from 1500 to 5000 mg/L, while total hardness drops

from 120 to 70 mg/L CaCO3. (The little blip on the hardness curve is due to the relatively higher hardness

in the Collier County raw water at 5350 mg/L TDS.) The alkalinity is a measure of the buffering ability of

the water and a certain minimum threshold or goal is recommended for corrosion control and stable

chlorine residual. The exact alkalinity goal will need to be defined by each owner and is dependent upon

many factors like distribution system, finished water quality and corrosion control strategy. In this

particular report, alkalinity and hardness goal of 60 and 80 mg/L CaCO3 respectively are assumed

requiring the addition of post–treatment chemicals to add alkalinity and hardness to the treated water.

Different chemicals can be used for this purpose but over the last couple of years the combination of

carbon dioxide and liquid lime has become popular. As can be found in Table 2, for a raw water TDS of

5,000 mg/L about 30 mg/L liquid lime and 40 mg/L carbon dioxide is needed to recondition the degasified

water to the set points for corrosion control.

48 50 52 54 56 58 58 58 60

2425

2627

2829 29 29

30

0

20

40

60

80

100

120

140

160

180

200

0

10

20

30

40

50

60

70

80

90

100

1,516 2,000 2,623 3,250 3,850 4,690 6,060 7,600 9,356

Interstage ERD Pressure Gain (psi)

RO Train Array


Stage 1 Pressure Vessels Stage 2 Pressure Vessels Interbank Booster Pump (psi) ‐ Max


Figure 14: Finished Water Quality, prior to Post‐Treatment Chemicals as Function of Raw Water TDS

Electricity and Chemical Costs

For each case study, electricity and chemical costs were calculated as shown in Figure 15. The costs

increase from $0.33 to $0.72 per 1000 gallons of water treated over the range of raw water TDS levels

from our case studies. The increase in electricity is more significant than the increase in chemical costs,

despite the use of ERDs for TDS levels of 3000 mg/L and above.

Four different chemicals make up the BWRO system chemical costs as shown in this figure, e.g. sulfuric

acid, scale inhibitor, liquid lime and carbon dioxide. Further details are depicted in Figure 16, which shows

that the costs of the scale inhibitor is relative constant across the range of TDS levels while the cost of

sulfuric acid increases slightly. However the largest relative increase to the chemical costs is due to the

increased dosages of liquid lime and carbon dioxide to maintain stable finished water.

0

20

40

60

80

100

120

140

1,500 2,500 3,500 4,500 5,500 6,500 7,500 8,500 9,500

Total H

ardness and Alkalinity in RO Permeate (mg/L as

CaC

O3)


Total Hardness (mg/L as CaCO3) ‐ Treated Water

Alkalinity (mg/L as CaCO3) ‐ Treated Water

Total Hardness (mg/L as CaCO3) ‐ Goal

Alkalinity (mg/L as CaCO3) ‐ Goal

Increase Alkalinity

Increase HardnessNeeded


Figure 15: Chemical and Electricity Costs as Function of Raw Water TDS

Figure 16: Chemical Costs as Function of Raw Water TDS

$0.17 $0.19 $0.22 $0.23 $0.25 $0.27 $0.29 $0.33 $0.36$0.41

$0.16$0.19

$0.22 $0.24$0.26 $0.25

$0.26$0.27

$0.29

$0.31

$0.00

$0.10

$0.20

$0.30

$0.40

$0.50

$0.60

$0.70

$0.80

1,516 2,000 2,623 3,250 3,850 4,690 5,350 6,060 7,600 9,356

Operating Cost ($ per 1000 gallons treated)

Total Dissolved Solids (mg/L)

Energy Cost ($/1000 gallons) Chemical Cost ($/1000 gallons)

$0.11 $0.12 $0.13 $0.15 $0.15 $0.13 $0.14 $0.15 $0.16

$0.18

$0.01 $0.01 $0.01

$0.01 $0.01 $0.01 $0.01

$0.01 $0.01 $0.02

$0.04 $0.04 $0.05

$0.05 $0.06

$0.07 $0.07 $0.07

$0.07

$0.07

$0.03 $0.03

$0.03 $0.04

$0.04 $0.05 $0.05 $0.05

$0.05

$0.05

$0.00

$0.05

$0.10

$0.15

$0.20

$0.25

$0.30

$0.35

1,516 2,000 2,623 3,250 3,850 4,690 5,350 6,060 7,600 9,356

Chem

ical Costs ($ per 1000 gallons treated)


Sulfuric Acid Cost ($/1000 gallons) Anti‐scalant Cost ($/1000 gallons)

Calflo Cost ($/1000 gallons) Carbon Dioxide Cost ($/1000 gallons)


5. Conclusions

Important lessons have been learned over 40 years of operation of this brackish source. The UFA is

complex and heterogeneous aquifer where well specifics may differ moving from the west to east coast.

However there are many commonalties in the wellfield design and operation as well. Many wellfields have

experienced degrading water quality due to migration of pockets and areas of poor water quality due to

human‐induced changes in the aquifer. This paper has presented several case studies of BWRO systems

with degrading raw water quality impacting wellfield and treatment operations. The paper can provide

guidance to utilities who are either operating or planning a BWRO system in terms of best practices of

wellfield and treatment plant design.


Literature References

GJ Schers, and Andy Fenske, Reducing Adverse Impacts of Declining Water Quality on RO WTP by

Implementing Operational Changes to the Wellfield; paper presented at FWRC 2007

Stefan Schuster, GJ Schers, and Michael Weatherby, Brackish Ground Water Supply in the US: 40 Years of

Experience with RO Design and Operation; paper presented at Texas Water 2014

Ron Cass, GJ Schers et al (2006); Optimizing existing facilities: finding another three MGD at the existing

Cape Coral RO WTP; paper presented at FWRC 2006

FDEP Division of Water Resource Management, Desalination in Florida: Technology, Implementation, and

Environmental Issues 2010

Karla Kinser, GJ Schers and Andy Fenske (2007 and 2008); Chemical optimization for a new brackish

ground water RO WTP, paper presented at the FSAWWA, FWRC and AMTA conferences in 2007 and 2008,

and article was published in the FWRJ 2008

MWH (2011), Best Practice Design Guides for Sand Separators, RO and Degasifiers

Ed Rectenwald, Mike Weatherby, Significant water quality trends observed in the lower Hawthorn Aquifer

of Southwest Florida, occurrences and solutions, paper presented at FSAWWA 2007

GJ Schers et al; Designing a RO plant for changing raw water quality, paper presented at the FWRC 2007

GJ Schers et al; Reducing adverse impacts of declining water quality on RO WTP by implementing

operational changes to the wellfield; paper presented at the FSAWWA and FWRC conferences 2007

USGS, National Brackish Groundwater Assessment 2013

U.S. Department of the Interior Bureau of Reclamation, Desalination and Water Purification Research and

Development Program Report No. 155 Treatment of Concentrate 2009

U.S. Geological Survey. 2003. Desalination of Ground Water: Earth Science Perspectives. Fact Sheet 075‐

03, October 2003.

To Presented at:

FSAWWA 2015

Wed 3A: Dec. 2, 2015

Presented by: GJ Schers PMP

2

• Introduction in Florida Aquifer

• Featured Wellfields and Salinity Trends

• Floridan Aquifer Water Quality

• Treatment Evaluations

• Conclusions*Cape Coral

Collier County *

*Jupiter*

Palm Beach County*

Venice

*Bonita Springs*

Broward (TW)DavieNorth Miami Beach

**

Floridan Aquifer Wellfield and Treatment

3

4

• Relatively deep; brackish source

• Used since 1970’s– Cape Coral, Venice

• Represents 25% of permitted capacity

• Benefits:– Is considered an Alternative Water Supply (AWS)

– Requires localized/regionalized only permitting

– (can be) Small scale, easy expandable

– Therefore, Floridan Aquifer is good supplemental source

5

• Important Aspects for Implementation of BWRO systems:1. Wellfield productivity and water quality2. Pre-Reverse Osmosis (RO) treatment3. RO4. Post-RO treatment5. Concentrate disposal Red: addressed in paper

• Objective paper: develop best practices for BWRO based on existing operational systems

6

• Collect operational data on performance of 4 case studies:• West coast: Cape Coral, Collier County (Venice, Bonita)

• East coast: Jupiter and Palm Beach County (NMB, Davie, Broward)

• Analyze and compare wellfield data

• Develop ‘wide’ set of source water quality

• Evaluate impacts on water treatment

• Develop best practices on salinity impacts


7

8

Floridan Well

9

Item West Coast East Coast

In operation since 1970’s 1990’s

Productivity expressed in typical well capacity (gpm)

350-700 800-2,000

Depth wells (ft fls) 500-1,000 1,000-1,600

Aquifers Upper FloridanMid, Lower Hawthorn

Upper Floridan

Water Quality in TDS (mg/L) 1,500-4,000 3,000-7,500Salinity reversal in area

Concentrate Disposal (ft bls) Boulder Zone(>2,000)

Boulder Zone(>2,000)

Several publications addressed experiences with water quality trends• Slow trend over time in wellfield• Fast change in individual wells

10

Summary of Production

Wells

Southwest Wells North Wells


Capacity 15 MGD (exp. in ’85)

18 MGD (exp. in ‘08)

12 MGD (original)

Number

Average Capacity

34

600 gpm

24

600 gpm


Depth

Diameter

700 ft

12 inch FRP

700 ft

12 inch FRP

Original TDS

Current TDS

1,400 mg/L (1988)

2,200 mg/L (2014)

2% increase/year

2,000 mg/L (2010)

2,500 mg/L (2014)

5% increase/year

Other source water quality

parameters

Chloride 900 mg/L

Hardness 575 mg/L as CaCO3

H2S 3 mg/L

Chloride 1100 mg/L,

Hardness 625 mg/L as CaCO3

H2S 3 mg/L

11

0

4

8

12

16

20

24

28

0

200

400

600

800

1,000

1,200

1,400

Tota

l Pu

mp

age

(mg

d)

Co

nce

ntr

atio

n C

hlo

rid

e (m

g/L

)

Chlorides SouthChlorides NorthTotal Pumpage North + South

12

-5%

0%

5%

10%

15%

20%

25%

30%

35%

101

103

105

107

109

111

211

213

215

217

219

221

223

225

227

229

231

-1,000

0

1,000

2,000

3,000

4,000

5,000

6,000

TD

S i

ncr

ease

(%

/100

MG

)

Well ID

TD

S C

on

cen

trat

ion

(m

g/L

)

TDS increaseAvg first 6 mIncrease (%/100 MG)

13

Summary of Production

WellsTown of Jupiter RO Production wells

Start of Operation 1995 (6 MGD)

Capacity8 MGD (exp. in ‘03)

10 MGD (exp. in ‘15)

Number

Average Capacity

12

1,400 gpm

Specific capacity 50 gpm/ft

Depth

Diameter

1,400 ft

17.4 inch PVC/FRP

1,600 ft

16, 12 inch PVC/FRP

Original TDS

Current TDS

3,000 mg/L (1995)

Avg. 6,500 mg/L (max 9,700 mg/L)

3%‐12% increase/year

Other source water

quality parameters

Chlorides 4,000 mg/L

Hardness 1,650 mg/L as CaCO3

H2S 3 mg/L

14

0

2

4

6

8

10

12

0

2,000

4,000

6,000

8,000

10,000

12,000

Total P

um

pag

e (mg

d)

Ave

rag

e C

on

cen

trat

ion

TD

S (

mg

/L)

TDS AVERAGE

Total Pumpage

15

0%

5%

10%

15%

20%

25%2 3 5 6 7 8 9 10 11 12 13

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

10,000

TD

S In

crease (%/100M

G)

Well ID

TD

S C

on

cen

trat

ion

(m

g/L

)

TDS avg (first 6m) TDS increase TDS Increase (%/100 MG)

• Similar patterns were developed for other case studies

16

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

Co

nce

ntr

atio

n C

hlo

rid

e (m

g/L

)

Chlorides South

Chlorides North

12 per. Mov. Avg. (Chlorides South)

12 per. Mov. Avg. (Chlorides North)

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

9.00

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

9,000

10,000

To

tal Pu

mp

age (m

gd

)

Ave

rag

e C

on

cen

trat

ion

TD

S (

mg

/L)

TDS Average

PumpingAverage

• Back plugging with cement

• Hydraulic control and water quality blending

• Redundant production wells

• Pro-active wellfield management system

• Well abandonment

17


18

19

Parameter(current - 2015, or otherwise noted)

UnitsCape Coral,

South RO

Cape Coral,

North RO

North Miami

Beach RO

Town of Davie RO

Collier County South

Jupiter RO

TDS mg/L 1,516 2,623 3,200 4,950 5,350 7,880

Barium mg/L 0.03 0.04 0.01 0.01 0.03 0.02

Fluoride mg/L 1.4 1.4 0.9 1.1 1.4

Sodium mg/L 343 600 780 1,450 1,370 2,310

Chloride mg/L 589 1,210 1,400 2,500 2,530 4,110

Sulfate mg/L 269 330 460 510 720 595

Bicarbonate Alkalinity mg/L 172 177 140 0 161 182

Calcium mg/L 90 125 105 150 270 197

Hydrogen Sulfide mg/L 3.0 3.0 3.5 3.2

Magnesium mg/L 86 105 110 205 280

Potassium mg/L 19 25 35 55 68

Strontium mg/L 17 21 3 15 15


20

21

22

0.000.100.200.300.400.500.600.700.800.901.00

4 5 6 7 8 9 10 11 12 13 14

Mol

ar F

ract

ion

pH

Carbonic Acid (H2CO3)

Bicarbonate Ion (HCO3-)

Carbonate Ion (CO32-)

0.000.100.200.300.400.500.600.700.800.901.00

4 5 6 7 8 9 10 11 12 13 14

Mol

ar F

ract

ion

pH

Hydrogen Sulfide (H2S)

Bisulfide Ion (HS-)

Sulfide Ion (S2-)

• Use range of source water quality

• Apply generic design assumptions BWRO

• Utilize membrane, scale inhibitor and chemistry software models to develop conceptual designs BWRO

• Table the results

23

Snapshot of Output Table

• LSI in RO feed < 1.8 (scale inhibitor)

• pH degasifier inlet < 6.0– Acid in RO feed or

– Limited acid in RO feed and additional acid in permeate

• Treated water Hardness > 80 mg/L, Alkalinity > 60 mg/L CaCO3

• Meet the treated water goals (for corrosion control) by:– RO Bypass and addition of carbon dioxide and liquid lime

24

25

$0.13

$0.05

$0.13

$0.05

$0.16

$0.05

$0.01

$0.02

$0.01

$0.02

$0.01

$0.03

$0.03

$0.05

$0.05

$0.05

$0.05

$0.05

$0.05

$0.06

$0.07

$0.07

$0.07

$0.08

$0.00

$0.05

$0.10

$0.15

$0.20

$0.25

$0.30

$0.35




Che

mic

al C

osts

($

per

1000

gal

lons

trea

ted)


Sulfuric Acid Cost ($/1000 gallons) Anti-scalant Cost ($/1000 gallons)

Carbon Dioxide Cost ($/1000 gallons) Calflo Cost ($/1000 gallons)

• Bypass limited by treated water TDS < 440 mg/L

• Upper limit 20%

• Total capacity system remains at 100%

• Reduction in RO Bypass compensated by increase permeate

26

27

0%

20%

40%

60%

80%

100%

120%

1,500 3,500 5,500 7,500 9,500

Flo

w (

% o

f D

esig

n F

low

)


Bypass Flow WTP (%)

Permeate Water Flow WTP (%)

Rated Finished Water Capacity WTP (%)

48 50 52 54 56 58 58 58 60

24 25 26 27 28 29 29 29 30

0

20

40

60

80

100

120

140

160

180

200

0

10

20

30

40

50

60

70

80

90

100

1,516 2,000 2,623 3,250 3,850 4,690 6,060 7,600 9,356

Inte

rsta

ge E

RD

Pre

ssur

e G

ain

(psi

)

RO

Tra

in A

rray


Stage 1 Pressure Vessels Stage 2 Pressure Vessels

Interbank Booster Pump (psi) - Max

• Use of direct pressure exchanger from concentrate to second stage feed

• Electricity $0.12/kWh

28

0

5

10

15

20

25

30

35

40

45

1,500 3,500 5,500 7,500 9,500

Inte

rsta

ge E

RD

Pay

back

Per

iod

(Yea

rs)


29

• Hardness and Alkalinity goal of 80 and 60 mg/L CaCO3

• Bypass as before

0

20

40

60

80

100

120

140

1,500 2,500 3,500 4,500 5,500 6,500 7,500 8,500 9,500

Tota

l Har

dnes

s an

d A

lkal

inity

in R

O P

erm

eate

(m

g/L

as C

aCO

3)


Total Hardness (mg/L as CaCO3) - Treated Water

Alkalinity (mg/L as CaCO3) - Treated Water

Total Hardness (mg/L as CaCO3) - Goal

Alkalinity (mg/L as CaCO3) - Goal

Increase Alkalinity Needed

Increase HardnessNeeded

30

$0.17 $0.19 $0.22 $0.23 $0.25 $0.27 $0.29 $0.33 $0.36$0.41

$0.16$0.19

$0.22 $0.24$0.26 $0.25

$0.26$0.27

$0.29

$0.31

$0.00

$0.10

$0.20

$0.30

$0.40

$0.50

$0.60

$0.70

$0.80

1,516 2,000 2,623 3,250 3,850 4,690 5,350 6,060 7,600 9,356

Ope

ratin

g C

ost (

$ pe

r 10

00 g

allo

ns t

reat

ed)


Energy Cost ($/1000 gallons) Chemical Cost ($/1000 gallons)


31

• Wellfield:– Aquifer is complex and heterogeneous– Many wellfields have experienced water quality degradation

• In wellfield• Individual wells

– Best practice of wellfield design and operation available

• Treatment:– Membrane and scale inhibitor technology improved– Design should accommodate some form of degradation– Impacts/trends on treatment assessed

32


33

FSAWWA Paper and Presentation Floridan Aquifer 12022015

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