Aeration Strategies for TTHM Control

Chad Seidel, Ph.D., P.E. Stephen Acquafredda, P.E.

Damon S. Williams Associates, LLC AZWA Water Treatment Committee Seminar Series

Water System Optimization Tuesday, February 23, 2010

Presentation Outline • Background

– Stage 2 Disinfection Byproducts Rule • DBP Reduction Alternatives

• Aeration Strategies • Aeration Strategy Evaluation Tools

– Modeling – TTHM Reformation – Bench / Pilot Scale Testing

• Case Studies – Tempe, Phoenix, Mesa

Regulatory Drivers • Stage 2 D/DBP Rule

– Compliance year: 2012 – MCL same as Stage 1 DBPR

• 0.080 mg/L TTHM (80 μg/L) • 0.060 mg/L HAA5 (60 μg/L)

– Compliance based on Locational Running Annual Average (LRAA)

• RAA for Stage 1 D/DBP Rule – Monitor sites with highest DBP concentrations

• Sites selected through Initial Distribution System Evaluation (IDSE)

– Operational Evaluation

DBP Control Options

• Existing Facilities Optimization – Treatment Plant – Distribution System

• New Facilities Implementation

– Treatment Plant – Distribution System – Remote DBP Control

Make the most of what you’ve got!

Evaluate using lifecycle costs!

DBP Control Options

• Treatment Plant – Enhanced Coagulation – GAC Adsorption – PAC Adsorption – MIEX Process – Advanced Oxidation

• Ozone • Chlorine Dioxide • Permanganate • UV (with peroxide or

catalyst) – River Bank Filtration – Chloramination

• Distribution System – Reduce water age – Blending with lower

TOC/DBP water – Remote DBP control

• GAC/BAC • Aeration

WTP

Low DBP

High DBP

Implement Remote DBP Control

• Works in isolated areas of high DBPs

Remote DBP Control

Aeration for TTHM Removal • TTHMs can be removed by

aeration/air stripping • Efficiency depends on

volatility (Henry’s Constant), which increases with temperature

• TTHM reformation after re-chlorination a concern

• Well documented process, but limited implementation for this purpose

• Aeration does not remove HAAs

Compound Formula

Henry’s Constant

(atm, 20°C)

Oxygen O2 43,000

Carbon Dioxide CO2 1,510

Chlorine Cl2 585

Chloroform CHCl3 480

Bromodichloro-methane CHCl2Br 118

Dibromochloro-methane CHClBr2 47

Bromoform CHBr3 35

Ammonia NH3 0.76

Aeration Strategies • In-reservoir Aeration Strategies

– Bubble Aeration

– Spray Aeration

– Surface Aeration

• External Aeration Strategies – Tray / Packed Tower

– Spray / Bubble Vessel

• TTHM reduction and energy use dependent upon A/W ratio

• Considerations: – Blower passes air through distributor

• Distributor installed within reservoir • Liner penetrations require repair

– Orifice / nozzle clogging – Blower maintenance – Intake air filter change out

Bubble Aeration Technology Assessment

“The poor economy of the diffused aeration process for THM removal can be explained by the rapid saturation of air bubbles with THMs as the bubbles rise through the column.” Roberts and Levy, JAWWA 1985

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0 5 10 15 20

A/W Ratio

% R

emov

al

$- $10,000 $20,000 $30,000Annual Energy Cost

Chloroform Bromodichloromethane DibromochloromethaneBromoform TTHM

Assumptions:TTHM = 102 ug/LCHCl3 = 48.5 ug/LCHCl2Br = 32.0 ug/LCHClBr2 = 19.2 ug/LCHBr3 = 2.3 ug/L

Daily Outflow = 0.68 MGDRes. Retention Time = 29 hrWater Depth = 4 mBlower Efficiency = 40%Electricity Cost = $0.08/kWh

Bubble Aeration Model Estimates

• Pump flow from reservoir outlet through hub & lateral system

• Maximum flow evaluation – Spray area for each nozzle / orifice – Flow rate per nozzle / orifice

• Considerations: – Can use reservoir inflow pressure, if available,

for potential energy savings – Structural support of piping – Distributor system installed within reservoir – Can reservoir be shutdown / drained? – Liner penetrations require repair – Orifice / nozzle clogging

Spray Aeration Technology Assessment

• Surface aerators float at water surface • Maximum flow evaluation

– Determine area of influence per unit – TTHM reduction dependent upon unit

power input (W/m3) • Considerations:

– Equipment must be supported in place – NSF approved equipment

• Limited application in drinking water – Reservoir access to each aeration unit

Surface Aeration Technology Assessment

Surface Aeration Model Estimates

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Power Input (W/m3)

% R

emov

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$0 $5,000 $10,000 $15,000 $20,000 $25,000

Annual Energy Cost

Chloroform Bromodichloromethane DibromochloromethaneBromoform TTHM

Assumptions:TTHM = 102 ug/LCHCl3 = 48.5 ug/LCHCl2Br = 32.0 ug/LCHClBr2 = 19.2 ug/LCHBr3 = 2.3 ug/L

Daily Outflow = 0.68 MGDRes. Retention Time = 29 hrAerator Motor Efficiency = 40%Electricity Cost = $0.08/kWh

• Tray tower flow rates up to around 1,000 gpm

• Considerations: – Additional footprint required – Minimal service & equipment interruption

for installation & maintenance – Maintenance may include removal of

scaling from trays on regular intervals – Automation required to control flow rate to

tray tower system

Tray Tower Aeration Technology Assessment

Aeration Strategy Evaluation Tools

• Aeration Modeling – Aeration System Analysis Program (ASAP)

• Bubble • Surface

– Spray aeration spreadsheet tool • Based on AWWA Water Quality Treatment

Handbook 5th edition

– Tray tower TTHM reduction • Based on WaterRF Project No. 3103 Localized

Treatment of Disinfection By-Products, Las Vegas Valley Water District, South Central Connecticut Regional Water Authority, and City of Phoenix Water Services Department, 2009

ASAP

DBP Formation Potential Factors: USEPA WTP Model Form

– DBP = TTHM or HAA5 (μg/L) – TOC = Treated Water TOC (mg/L) – UVA = Treated Water UVA (1/cm) – Cl2 = Applied Chlorine Dose (mg/L) – Br- = Treated Water Bromide (μg/L) – T = Temperature (°C) – pH = Treated Water pH – t = Reaction Time or Water Age (hours) – A = Base Coefficient – a,b,c,d = Exponent Coefficients – α = Temperature Coefficient – β = pH Adjustment Coefficient

Simulated Distribution System (SDS) tests used to

validate model predictions

dpHTcba tBrClUVATOCADBP )(*)(*)(*)(*)(*)*( )5.7()20(2

−−−= βα

(Solarik et al., 1999)

Arizona Case Studies • City of Tempe, AZ

• City of Phoenix, AZ

• City of Mesa, AZ

City of Tempe, AZ

• Evaluation: Surface water treatment plant 12 MG reservoir

• Methods evaluated – Groundwater Blending – Coagulation / Enhanced Coagulation – Aeration

• Bubble (modeling & bench testing @ A/W=15) • Spray (modeling & bench testing) • Surface (modeling) • TTHM Reformation (modeling & SDS tests)

Baseline SDS Test

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pH: Temperature (ºC): Chlorine Residual (mg/L):

Aeration followed by rechlorination

Baseline & Bubble Aeration SDS Test

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Elapsed Time (hr)

TTHM

(ug/

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Baseline SDS Post Aeration Rechlorination SDS

TTHM Removal: 48.6 ug/L – 21.8 ug/L = 26.8 ug/L = 55% Removal Model Prediction ~ 56% Removal

Aeration followed by rechlorination

City of Tempe, AZ

• Bubble aeration capital and O&M costs higher than spray or surface aeration – Bubble aeration achieved greatest TTHM reduction

given physical constraints • Long residence time in distribution system post-

aeration limited DBP control effectiveness

Parameter Target Bubble Spray Surface

Average Flow rate, mgd 24 Reservoir Max Level, ft 18 % TTHM Reduction 35% 23% 8% 10%

Energy Use (kWh/yr) N/A 7,884K 1,144K 1,769K

City of Phoenix, AZ • Evaluation: Distribution system 2 MG

reservoir – Criteria: TTHM reduction, lifecycle cost,

constructability, ease of operation, impact on operation, required time out of service, mixing

• Aeration methods evaluated – Bubble (modeling @ A/W ~ 6) – Spray (modeling) – Surface (modeling) – External Methods (testing & vendor coordination)

City of Phoenix, AZ

• Surface aeration capital and O&M costs lower than for other strategies

• Non-cost surface aeration advantages – Equipment maintenance with minimal impact on

reservoir operation

Parameter Tray Bubble Spray Surface

Average Flow rate, mgd

0.7

Reservoir Max Level, ft 25

% TTHM Reduction 30%

Energy Use (kWh/yr) 210K 196K 299K 49K

City of Mesa, AZ • Evaluation: Multiple distribution system

reservoirs including 0.5 & 0.1 MG shown

• Aeration methods evaluated – Bubble (modeling @ A/W ~ 6) – Spray (modeling) – Surface (modeling)

City of Mesa, AZ

Parameter Target Bubble Spray Surface Average Flow rate, mgd 0.5 Reservoir Max Level, ft 10 % TTHM Reduction 15% 55% 22% 60% Energy Use (kWh/yr) N/A 98K 65K 49K

• Surface and spray aeration had low capital and O&M costs

• Spray aeration becomes competitive when target TTHM reduction is low

Impact on Chlorine Residual

• Predominant chlorine species in water around neutral pH are HOCl and OCl- – Species are less volatile than Cl2

• Literature and recent testing have mixed results – Some show no chlorine residual loss – Some show up to 40% chlorine residual loss

• All aeration strategies should include chlorine residual maintenance facilities

Conclusions • In-reservoir aeration is an efficient strategy to

reduce TTHMs in terms of capital and annual operating costs – Target high TTHM water at remote storage

facilities • Modeling shows surface aeration has the

lowest capital & operating cost – Limited application in drinking water

• NSF certification required for equipment – Application not appropriate for some reservoirs

• Use available literature and models to compare aeration strategies and determine best solution

Contact Information • Chad Seidel, Ph.D., P.E.

Damon S. Williams Associates, L.L.C. 1624 Market Street, Suite 475 Denver, CO 80202 Phone: (303) 989-2205 cseidel@dswa.net

• Stephen Acquafredda, P.E. Damon S. Williams Associates, L.L.C. 2355 East Camelback, Suite 700 Phoenix, AZ 85016 Phone: (602) 265-5400 acqua@dswa.net