Clarification (Settling): Theory and Applications

Plant Operations SeminarThursday, July 22, 2010

DeKalb, Illinois

Krishna Pagilla, Ph.D., P.E.Illinois Institute of Technology

Chicago, Illinoispagilla@iit.edu

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