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Thickening and Dewatering OptimizationMatt Van Horne, PE – Hazen and Sawyer

July 14, 2016

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Agenda

• Equipment Specific Modifications

• Chemicals

• Feeding

• Cleaning and Maintenance

• Pretreatment

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Optimization

Approach

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Dewatering and Thickening Optimization is a…

Balancing Act

Solids Capture

Throughput

Polymer Dosage

Cake Solids

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What is the Goal of Optimization?

• Key Performance Indicators

• Solids content output

• Solids capture

• Polymer dose

• Energy consumption

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What Can We Do To Achieve These Goals?

• Vary different inputs

• Equipment parameters

• Hydraulic and solids loading rates

• Polymer characteristics

• Washwater

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Equipment Specific

Modifications

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Belt Presses and Gravity Thickeners

• Belt speed

• Belt tension

• Chicane configuration

• Washwater pressure

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Belt Press and Gravity Thickener – Belt

Speed

• Faster speed

• More throughput

• Shorter retention time in gravity and pressure zones

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Belt Press – Belt Tension

• Higher tension

• More force applied in pressure zone

• More force applied to belt

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Belt Filter Press and Gravity Thickener –

Chicane Configuration

• Balance turning of sludge with water drainage

time

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Centrifuges

• Differential speed

• Torque

• Pond levels

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Centrifuge – Differential Speed

• Conveyor moving faster than bowl

• Increased differential speed

• Solids move through quicker

• Reduced cake solids

• Increased capture

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Centrifuge - Torque

• Use torque measurement to estimate cake

dryness

Source: Andritz

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Centrifuge – Pond Levels

• Increased pond level

• More room for liquid clarification

• Likely reduced cake solids

• Possibility for increased solids capture

Shallow Levels

Deep Levels

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Gravity Thickener

• Elutriation water

• Solids removal rate

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Gravity Thickener – Elutriation Water

• More water = lower hydraulic retention time

• Lower retention time = less chance for septicity

• Temperatures increase → fermentation

increases → gas is produced → sludge floats

• Reduce residence time by increased hydraulic

load to reduce gas production potential

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Gravity Thickener – Solids Removal Rate

• Pumping rates

faster than sludge

“refill” rates

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Solids Pumping

• Clarifier blanket level controls

• TSS meters

Chemicals

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What Types of Chemicals

• Iron salts

• Ferric chloride

• Coagulates colloids

• Lime

• Combine with iron salts for filter press applications

• Increase resistance to filtration

• Polymers

• Organic flocculants

• Fix destabilized particles on the long monomer chains

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Most Applications Use Polymers (Flocculants)

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Typical Polymer Doses for Dewatering

0

5

10

15

20

25

30

Ce

ntr

ifu

ge

Be

lt P

ress

Scre

w P

ress

Ce

ntr

ifu

ge

Be

lt P

ress

Scre

w P

ress

Ce

ntr

ifu

ge

Be

lt P

ress

Scre

w P

ress

Waste Activated Aerobic Digested Anaerobic Digested

Po

lym

er

Do

sage

, lb

/to

n

Equipment / Sludge Type

Polymer Dosage, lb/ton

EPA 832-F-00-053, September 2000

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Polymer Types

• Polymer is used for sludge conditioning and to

enhance settling, thickening, and dewatering

• Electronic charge

• Charge density

• Molecular weight

• Molecular structure

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Electronic Charge

• Anionic (negative) to attract mineral particles

• Cationic (positive) to attract organic particles

• Best determined via lab/bench testing

Source: SNF/Floerger

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Charge Density

• More biological sludge requires more positive

charge required

Source: SNF/Floerger

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Molecular Weight

• Molecular weight = length of chain

• Longer chain is more able to handle high shear

forces (i.e. centrifuge)

• Low to medium weight promotes good gravity

drainage (i.e. GBT, RDT, BFP)

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Molecular Structure

• Linear

• Low shear resistance

• Branched

• Cross linked

• High shear resistance

• Blocked charges

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Emulsion Polymer

• Milky/cloudy liquid totes

• Higher concentration of active

polymer

• Shorter self life than dry

polymer

• Usually 25% to 60% active

polymer

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From Chemical Unloading

Polymer Storage

Tank

Dilution Water

Polymer

Mixing

Unit

Direct Feed Polymer System

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From Chemical Unloading

Polymer

Mixing UnitPolymer Storage

Tank

Mix/Age Tank

M

Dilution Water

Mix-Age Polymer Feed System

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Dry Polymer

• Pellet or flake provided in large

bulk bags

• Lower concentration of active

polymer

• Longer shelf life than emulsion

polymer

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Dry Polymer

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Polymer Conclusions

• Polymer aging is important for dewatering

• Polymer dosing location can be important for

thickening

• SCADA can provide real time polymer

performance tracking

• Feed rates

• Solids concentrations

• Close mass balance

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Feeding

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Sludge is Inherently Variable

• But thickening and dewatering processes like

consistent characteristics…

• Influent changes

• Biological process variations

• Digestion process variations

• Seasonal fluctuations

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Control Feeding Equipment

• Manage pumping times and intervals

• Monitor solids content – ratholing

• Improve control over upstream processes (if

possible)

• Sludge storage and blending

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Consistent Feeding Has Multiple Benefits

• Predictability of thickening/dewatering

performance

• Consistent polymer dosing

• Reduces equipment adjustments

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Feed Distribution Can Impact Performance

Credit: BDP

Cleaning and

Maintenance

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Poor Cleaning Can Reduce Capture

Credit: BDP

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Uneven Belt Stretching

Credit: BDP

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Clogged Spray Nozzle

Credit: BDP

Pretreatment

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An Emerging Approach

• Likely are many options similar to those used for

digestion pretreatment

• One manufacturer recently working in the North

American market

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Orege SLG Approach

• Compressed air injection into sludge

• Short retention time

• Allows release of free water from sludge

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Orege SLG

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Initial Installation – Allentown, PA

• Goals

• Improve cake solids – 15.3% baseline

• Increase BFP throughput – 100 gpm baseline

• Decrease polymer dose – 60 lbs/dry ton baseline

• Initial Results

• Cake solids – 18.6%

• BFP throughout – 150 gpm

• Polymer dose – 45 lbs/dry ton

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Case Study

RDT Optimization

F. Wayne Hill WRF, Gwinnett County, GA

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PS and WAS Holding Tank

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Rotary Drum Thickeners

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Rotary Drum Thickener

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Thickened Sludge Hopper & Pump

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Optimization Approach

• Understand performance

• Long term historical

• Variability on different time scales

• Field sampling

• Full scale parameter adjustment

• Tracking key performance indicators

• Develop “real life” guidance and approaches

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RDT Analysis – Historical Data

• Average solids and hydraulic loading rates

• Sludge feed to digesters

• Field sampling

• Conclusions

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WAS Load

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WAS + PS Load

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Average RDT Solids Loading Rate

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Average RDT Hydraulic Loading Rate

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Calculated Thickened Sludge Loading to Digesters

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Thickened Sludge Flow to Digesters

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Digester SRT

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Feb 1-14, 2014

Individual RDT Thickened Sludge Flow

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Oct 14-21, 2014

Individual RDT Thickened Sludge Flow

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Oct 14 – 21, 2014

Thickened Sludge flow to digesters

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Oct 2014 – PS and WAS Loading

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RDT Data Update Conclusions

• Periods of:

• Good Thickening and High Digester SRTs

• Sporadic Thickening and Lower Digester SRTs

• Variable Individual RDT Performance

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RDT Field Sampling

• Measured TSL concentration of operating RDTs

• Including new drum of RDT 4 (same day as install)

• Varied polymer dose on one RDT

• Measured WAS, PSL, and TSL concentrations

• Field Observations:

• RDTs had different polymer doses & different TSL

concentrations

• TSL concentration can be optimized by varying polymer

dose

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Field Sampling Results

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Field Sampling Results

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Diurnal Sampling Results

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RDT Field Sampling

• Still have variable RDT performance

• Use Thickened Sludge Flow as quick

assessment of individual thickening performance

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RDT Analysis – Key Conclusions

• Target thickened sludge concentration of 6.5%

(Max 8%)

• Necessary for maintaining digester capacity

• Variable performance across different RDTs

• Likely due to varying polymer doses

• Target uniform polymer dose

• Average % capture >99.5%

• Average filtrate TSS range = 100-200 mg/L TSS

Wrap Up

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What Did We Learn?

• Equipment adjustments

• Polymer optimization

• Feeding controls

• Cleaning and maintenance

• Pretreatment potential

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Questions and Discussion

Matt Van Horne, P.E.

mvanhorne@hazenandsawyer.com

703-267-2738