Clarification (Settling): Theory and Applications Plant Operations Seminar Thursday, July 22, 2010 DeKalb, Illinois Krishna Pagilla, Ph.D., P.E. Illinois Institute of Technology Chicago, Illinois [email protected]
Oct 24, 2015
Clarification (Settling): Theory and Applications
Plant Operations SeminarThursday, July 22, 2010
DeKalb, Illinois
Krishna Pagilla, Ph.D., P.E.Illinois Institute of Technology
Chicago, [email protected]
Principles of Settling
Secondary Clarifiers
Settling in Grit Tanks
Primary Clarifiers
Principles of Settling
Principles of Settling
Forces acting on particles during settling
Particle Settling Velocity
Types of Settling
Surface Overflow Rate, Surface Area and
Depth of Tank
Shape of the Clarifier or Settling Tank
Flow Field in the Tank
Particles must settle by gravity to the bottom of the tank and stay there
Forces acting on the particle include:
• Gravity Fgravity Fdrag +• Overflow up velocity Fbuoyancy +• Drag force by fluid Fflow
•Buoyancy – Pushing by the water being displaced
Settling velocity of the particle, Vs, is very important and is dependent on the size, shape, and density of the particle
Forces acting on the particle
Vs = g(ρs-ρ)dp2
18µ
Particle Settling VelocityEffect of Diameter on Particle Settling Velocity
0
50
100
150
200
250
300
350
0 2 4 6 8 10 12 14 16 18
Normalized Diameter
Nor
mal
ized
Set
tling
Vel
ocity
Settling Classes or Types
IDISCRETE
I“Non-Sticky” Particles, e.g., Grit
FLOCCULENTII
Sticky Particles, e.g., Flocs, Primary Solids
HINDEREDIII
Concentrated Flocs, e.g., Mixed Liquor Sludge
COMPRESSIVEIV
Sludge, e.g., Return Activated Sludge
Surface Overflow Rate
SOR = Qo
A
Clarifier Overflow
Clarifier Surface Area
636 gpd/ft2 = 1.08 m/hr
Typical Operating Values:
Primary clarifiers: 1,000 gpd/ft2 averageSecondary clarifiers: 600 gpd/ft2 average
This is a VELOCITY!
Settling in a TankInlet OutletZone Zone
Sludge Zone
Water SurfaceD
, Dep
th
L, Length
Particles must settle depth D in θ, Hydraulic Retention Time = V/Q
Vs > D/θ = DQ/V == DQ/DWL == Q/WL = Q/A
Independent of depth; Surface Loading Rate = Q/A = vo
Removal of smaller particles, if vs < vo
Surface Area and Depth in Settling Non-sticky particles (e.g., grit) – Depth is not as
important, Surface Area is important
Sticky particles (e.g., primary solids, secondary
clarifier top layer, coagulated/flocculated solids) – Both
Depth and Surface Area are important
Activated sludge blanket – Surface Area is important
and Depth may be important
Overflow Rate
No Overflow Low Overflow High Overflow
Rapidly SettlingParticle
Slowly SettlingParticle
• Process Concerns– Particle Settling Velocity Distribution (PSVD) and flow field in
the tank determine primary clarifier performance.– Develop ‘reasonable’ flow field and maximize performance.
• Mechanical Concerns– Longitudinal Symmetry vs. Radial Symmetry– Rectangular requires bicycle chain, pulley system (or traveling
bridge).– Circular permits central drive and radial mechanism – more
mechanical development• Site Footprint Concerns
– Rectangular permits common-wall construction.– Circular requires less concrete and steel with reduced wall
thickness (hoop stress).
Rectangular vs. Circular ClarifiersTank shape does not matter to particles, only flow field matters
Flow Field in the Tank
Flow Field in the Tank is established by the hydraulics
of the tank
Inlet-Outlet Structures, Mechanical Equipment within
the Tank, and Size/Shape of the Tank
The operator has very little control over flow field once
the tank is in-place, except to reduce or increase flow
Determine flow field by drogue tests, doppler
measurement, computational fluid dynamics
Perhaps a sludge judge will provide same info
Summary of Theory-Principles
The larger the particle the better for settling
“Sticky” versus “Non-sticky particles”
Four types of settling behavior
Overflow rate and surface area and/or depth
are important
Tank hydraulics and flow field play a major role
Settling in Grit Tanks
Basics of Grit Settling Sandy material, mostly inorganic, is grit
Mostly between 0.05 and 1 mm in diameter
Considered to be “non-sticky” particles, but are mostly
“sticky” particles. Coated with fats, oils and grease in the
sewers, increasing buoyancy (upward force)
Need to know true grit size to determine the settling
velocity – wet sieving versus dry sieving
If not removed in grit tanks, ends up in digesters
Grit characteristics change with flow rates to the plant
Wet Sieving versus Dry Sieving
Eutek Systems, 2006
Cut-Point Particle (CPP) Tank Design
Eutek Systems, 2006
Text book design – 200 – 300 micron Sand Equivalent Size
Recommended – 60 micron SES
4.5 gpm/ft2 = 6500 gpd/ft2
Aerated Grit Tanks - Operations Usually see very low <10% grit removal in some cases
Best method is see the Fixed Solids (FS) mass
balance around the tank – In minus Out = Grit Removed
Check FS content in the influent at different flows
throughout the year
First flush is high in grit content
Check grit accumulation in the digester
Where does the grit settle along the length of the
tank?
Settling in Primary Clarifiers
Primary Clarifiers
Purpose and Capacity
Performance Comparison
Performance Improvement
Influent Characteristics Impact
Enhanced and High Rate Primary Clarification
Summary
Primary Clarifiers
Remove organic particles by gravity instead of costly
biological treatment
Recovery organic solids for biogas production instead
of “burning” them in the aeration tanks
Capacity available in the primary clarifiers is a function
of the secondary treatment available and settleability of
the influent solids
Performance varies from plant to plant and from day to
day
Comparing Primary Clarifier PerformanceFacility/Data Set TSS Removal, % BOD Removal, %
Metro North Complex 59% 28%Metro South Complex 53% 30%Metro Total Primary Process 57% 28%Littleton Englewood WERF PCPP 53% 27%Edmonds, WA Square PCPP 57% 34%Edmonds, WA Squircle PCPP 59% 32%LOTT WW Characterization 54% 34%Lincoln, NE 1 Historical 63%Lincoln, NE 2 Historical 64%Lincoln, NE 3 Historical 66% 35%Lincoln, NE 4 Historical 67% 38%Lincoln, NE 5 Historical 67% 34%Lincoln, NE 6 Historical 66% 36%
Archie Elledge: 2001 – 2006 69% 28%Archie Elledge: 2006 69% 30%
Dr. David Kinnear, HDR
Comparing PerformanceFacility/Data Set
TSS Removal, % BOD Removal, %
Bowery Bay Battery 6 46% 44%
Bowery Bay Battery 9 44% 30%
Coney Island 53% 44%
Jamaica 41% 28%
North River North Battery 62% 29%
North River South Battery 58% 29%
Oakwood Beach 59% 45%
Owls Head 47% 22%
Owls Head 52% 23%
Port Richmond 66% 35%
Rockaway 67% 38%
Tallman Island East Battery 57% 29%
Tallman Island West Battery 50% 33%
Wards Island North Battery
Wards Island South Battery 66% 36%
30%
40%
50%
60%
70%
80%
90%
100%
10 12 14 16 18 20 22 24 26 28 30
Flow
TSS
Rem
oval
, %Archie Elledge WWTPPrimary Clarifier PerformanceTSS Removal vs. Flow
0.0%
10.0%
20.0%
30.0%
40.0%
50.0%
60.0%
70.0%
80.0%
90.0%
10 12 14 16 18 20 22 24 26 28 30
Flow
BO
D R
emov
al, %
Archie Elledge WWTPPrimary Clarifier PerformanceBOD Removal vs. Flow
Improving Performance
City of Lincoln, NETheresa Street Wastewater Treatment Facility
Primary Clarifier Performance Comparison
54%
56%
58%
60%
62%
64%
66%
68%
70%
PC1 PC2 PC3 PC4 PC5 PC6
Clarifier
TSS
Rem
oval
Pre-Retrofit
Post-Retrofit
Retrofit TSS Comparison
City of Lincoln, NETheresa Street Wastewater Treatement Facility
Primary Clarifier BOD Removal Peformance
0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
50%
PC1 PC2 PC3 PC4 PC5 PC6
Clarifier
BO
D R
emov
al
Pre-Retrofit
Post-Retrofit
Retrofit BOD Comparison
-
500
1,000
1,500
2,000
2,500
3,000
3,500
0% 10% 20% 30% 40% 50% 60% 70% 80%
Primary TSS Performance
MLS
S, m
g/l
380
390
400
410
420
430
440
450
OU
R, m
g O
/l/hr
MLSSOUR
Effect of Primary TSS Performance on Activated Sludge MLSS and OUR
• Coagulant/flocculant addition destabilizes particles and/or strengthen flocs structure (Metal Salts).
• Increased conversion of “Nonsettlable” particles to “Settlable” particles?
• TSS and BOD removal increases.• TSS removal increases from 50% - 60% to 70% - 85%. • (20 mg/l FeCl3, 0.25 mg/L polymer)
Chemically Enhanced Primary Treatment
Vs = g(ρs-ρ)d2
18µ
Chemically enhanced primary treatment plus a weighting agent to increase particle settling velocity.
High Rate Primary Clarification
Vs = g(ρs-ρ)d2
18µ
3) Settleable
2) Nonsettleable
1) Soluble
Five Wastewater Components
4) CoagulantFerric or Polymer
5) Sand
3) Settleable
2) Nonsettleable
1) Soluble
Flocculation – Floc Incorporates Sand
4) CoagulantFerric or Polymer
5) Sand
Energy
Settleable
Soluble
Coagulant Incorporates NS into Flocs
Settling Velocity Distribution
Waterloo, Canada Settling Velocity Distribution
Weight Concentration* Settling
Class Fraction Velocity
% mg l-1 gpd ft-2
1 0.15 37.5 5,054
2 0.15 37.5 2,907
3 0.05 12.5 1,688
4 0.05 12.5 1,122
5 0.05 12.5 742
6 0.05 12.5 539
7 0.05 12.5 283
8 0.05 12.5 177
9 0.40 100.0 0
100% 250.0
*Assumed 250 mg/L TSS Concentration
After Alarie, et al. (1980)
SVD Effects Behavior in Clarifiers
Weight Concentration* Settling
Class Fraction Velocity
% mg l-1 gpd ft-2
1 0.15 37.5 5,054
2 0.15 37.5 2,907
3 0.05 12.5 1,688
4 0.05 12.5 1,122
5 0.05 12.5 742
6 0.05 12.5 539
7 0.05 12.5 283
8 0.05 12.5 177
9 0.40 100.0 0
100% 250.0
Assumed 250 mg/L TSS Concentration
THESE YOU WILL GET = 75 mg l-1
THESE YOU WILL NOT GET = 137.5 mg l-1
THESE YOU WILL GET = 37.5 mg l-1
• Best way to remove and recover organic matter from wastewater is by effective primary clarifiers
• A significant amount of flocculation occurs during testing for particle size or velocity distribution of influent solids
• Particle size distribution and nature changes with seasons, flows, and other influent conditions
• Enhance solids removal by adding chemical to some extent
• Performance varies from plant to plant
Summary
Secondary Clarifiers
• Activated Sludge Process and Clarifiers
• Purpose and Objectives of Secondary Clarifiers
• Clarifier Overloading and Consequences
• Enhance solids removal by adding chemical to some extent
• Performance varies from plant to plant
Secondary Clarifiers
MIXEDLIQUOR
AERATION TANK
CLARIFIER
RETURN ACTIVATED SLUDGE
ACTIVATED SLUDGE PROCESS
EFFLUENTINFLUENT
WASTEACTIVATEDSLUDGE
• MLSS Retention – Liquids released from system; solids remain.
• Secondary Effluent Production• Permits controlled system wasting and SRT
control• Mass distribution between aeration basin and
secondary clarifier is critical.• Minimizing secondary clarifier resident biomass should
be objective.• Which components of the MLSS present problems for
mass distribution (clarifier overload)?
Secondary Clarifier Purpose
Secondary Clarifier Design and Operational Objectives
• Minimize Total Solids Present in theSecondary Clarifier Through DesignAnd During All Operating Conditions
• Clarifier is for Solid-Liquid Separation Only
Consequences of Excessive Clarifier Solids
Interference with settling and scour
Hydraulic Consequences
Biochemical Consequences
VFA production
Decreased aeration basin nitrifiers
Secondary Phosphorus release
Denitrification
Secondary Clarifier Performance Influencing Factors
• Solids Loading Rate = MLSS x (Q + QR)/Surface Area• Hydraulic Loading Rate = Q/Surface Area• Sludge Removal – RAS Rates and RAS Concentration• Hydraulics of the Tank• Solids Distribution in the Activated Sludge Process• Solids Flux Analysis• Settleability of Sludge
• Sludge Volume Index or SVI• Bulking and Foaming Issues
Loading Rates and Recycle Rate
• Solids Loading Rate = 30 to 50 lb/square foot-day• Hydraulic Loading Rate = 600 gal/square foot-day• Recycle Ratio, R = X/(XR –X) = 30 to 100%• These values are good when SVI = 50 to 100 range• The goal is get the solids back into the aeration tank
without storing them in the clarifier. • Increase recycle ratio, R or Recycle Rate, QR during
storm events
Hydraulics of the TankIdealized Clarifier Hydraulics
Hydraulics of the Tank
– Extended Sampling/Stress Testing
– Computational Fluid Mechanics
– Acoustic Doppler Velocity Measurement
– Sludge Judge Measurement
Hydraulics of the TankComputational Fluid Dynamics
Measuring Solids Distribution
2. Sample Clarifier with Sludge Judge at Several Locations
3. Empty Entire Sludge Judge in Bucket and Mix
1. Aeration Mass = MLSS * Aeration Volume
4. Measure Average Clarifier Concentration (bucket contents)
6. Plot Daily as Mab and Mc or %ab and %c
5. Clarifier Mass = Average Clarifier Concentration * Clarifier Volume
City of Lincoln, NEWest Side Activated Sludge System Solids Distribution
0%
10%
20%
30%
40%
50%
60%
70%
80%
1/30
/200
1
2/6/
2001
2/13
/200
1
2/20
/200
1
2/27
/200
1
3/6/
2001
3/13
/200
1
3/20
/200
1
3/27
/200
1
4/3/
2001
4/10
/200
1
4/17
/200
1
4/24
/200
1
5/1/
2001
5/8/
2001
5/15
/200
1
5/22
/200
1
5/29
/200
1
6/5/
2001
6/12
/200
1
6/19
/200
1
6/26
/200
1
7/3/
2001
7/10
/200
1
7/17
/200
1
7/24
/200
1
7/31
/200
1
Date
Cla
rifie
r Mas
s, P
erce
nt
Cedar Rapids, Iowa Water Pollution Control FacilityCarbonaceous Activated Sludge System
Mass Distribution
-
200,000
400,000
600,000
800,000
1,000,000
1,200,000
1,400,000
1/1/
2001
2/1/
2001
3/1/
2001
4/1/
2001
5/1/
2001
6/1/
2001
7/1/
2001
8/1/
2001
9/1/
2001
10/1
/200
1
11/1
/200
1
12/1
/200
1
1/1/
2002
2/1/
2002
3/1/
2002
4/1/
2002
5/1/
2002
6/1/
2002
7/1/
2002
8/1/
2002
9/1/
2002
10/1
/200
2
11/1
/200
2
12/1
/200
2
1/1/
2003
2/1/
2003
3/1/
2003
4/1/
2003
5/1/
2003
6/1/
2003
7/1/
2003
8/1/
2003
Date
Syst
em M
ass,
lbs.
CAS Biological Reactors
CAS Clarif iers
Hopewell Regional Wastewater Treatment FacilitySystem Mass Distribution
0
200,000
400,000
600,000
800,000
1,000,000
1,200,000
1,400,000
1/1/2001
3/17/2001
5/31/2001
8/14/2001
10/28/2001
1/11/2002
3/27/2002
6/10/2002
8/24/2002
11/7/2002
1/21/2003
4/6/2003
6/20/2003
9/3/2003
11/17/2003
1/31/2004
4/15/2004
6/29/2004
9/12/2004
11/26/2004
2/9/2005
4/25/2005
7/9/2005
9/22/2005
12/6/2005
Mas
s in
Sec
onda
ry C
larif
iers
, lbs
Clarifier Mass Unox Mass Based on 2001-2005 data
1
2
0.0
1.0
2.0
3.0
4.0
5.0
6.0
- 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0
X, g/l
G, K
g/m
2 hr
Co CL CUC’L
Overflow Rate Operating Line
Underflow Rate Operating Lines
Aeration Basin – ClarifierSolids Transfer
Solids Flux Analysis
SVI – Batch Test May Not Represent Behavior in a Continuous Clarifier
0102030TIME:
min
Hindered Settling
CompressiveSettling
Activated Sludge Settleability
Pilot Clarifier Operation Demonstrated Secondary Clarifier Behavior
BACTERIA -FILAMENTS AND FLOCS•70 TO 80% OF ACTIVATED SLUDGE IS BACTERIA
•BACTERIA CONTAINS
FILAMENTS < 10% OF BACTERIA
FLOCS > 90% OF BACTERIA
•FILAMENTS ARE BACKBONE OF ACTIVATED SLUDGE(REBAR IN REINFORCED CONCRETE)
•FLOC FORMERS ARE BULK OF THE ACTIVATED SLUDGE(GRAVEL, SAND, AND CEMENT IN CONCRETE)
•FILAMENTS REINFORCE THE FLOCS TOGETHER AND MAKE THEM LARGER
IDEAL, BULKING, AND PIN POINT FLOC STRUCTURE
Bulking Control by Chlorination - KNOW WHEN TO STOP!
CHLORINATION TIME
83400/417000 = 0.2 days OR ABOUT 5 HOURS FOR EACH CHLORINE EXPOSURE
MIXEDLIQUOR
AERATION TANK
CLARIFIER
RAS
10 MGD
5 MG2000 mg/L83400 lb SOLIDS
5 MGD10000 mg/L417000 lb/day
WAS