Aeration Strategies for TTHM Control

Transcription

1 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

2 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

3 Regulatory Drivers Stage 2 D/DBP Rule Compliance year: 2012 MCL same as Stage 1 DBPR mg/l TTHM (80 μg/l) 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

4 DBP Control Options Existing Facilities Optimization Treatment Plant Distribution System Make the most of what you ve got! New Facilities Implementation Treatment Plant Distribution System Remote DBP Control Evaluate using lifecycle costs!

5 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

6 Remote DBP Control Works in isolated areas of high DBPs WTP Low DBP High DBP Implement Remote DBP Control

7 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 rechlorination 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 O 2 43,000 Carbon Dioxide CO 2 1,510 Chlorine Cl Chloroform CHCl Bromodichloromethane Dibromochloromethane CHCl 2 Br 118 CHClBr 2 47 Bromoform CHBr 3 35 Ammonia NH

8 Aeration Strategies In-reservoir Aeration Strategies Bubble Aeration Spray Aeration Surface Aeration External Aeration Strategies Tray / Packed Tower Spray / Bubble Vessel

9 Bubble Aeration Technology Assessment 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 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

10 Bubble Aeration Model Estimates Chloroform Bromodichloromethane Dibromochloromethane Bromoform TTHM Annual Energy Cost % Removal 90% 80% 70% 60% 50% 40% 30% $- $10,000 $20,000 $30,000 Assumptions: TTHM = 102 ug/l CHCl 3 = 48.5 ug/l CHCl 2 Br = 32.0 ug/l CHClBr 2 = 19.2 ug/l CHBr 3 = 2.3 ug/l Daily Outflow = 0.68 MGD Res. Retention Time = 29 hr Water Depth = 4 m Blower Efficiency = 40% Electricity Cost = $0.08/kWh 20% 10% 0% A/W Ratio

11 Spray Aeration Technology Assessment 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

12 Surface 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/m 3 ) Considerations: Equipment must be supported in place NSF approved equipment Limited application in drinking water Reservoir access to each aeration unit

13 Surface Aeration Model Estimates Chloroform Bromodichloromethane Dibromochloromethane Bromoform TTHM Annual Energy Cost $0 $5,000 $10,000 $15,000 $20,000 $25, % 90% 80% 70% % Removal 60% 50% 40% 30% 20% 10% Assumptions: TTHM = 102 ug/l CHCl 3 = 48.5 ug/l CHCl 2 Br = 32.0 ug/l CHClBr 2 = 19.2 ug/l CHBr 3 = 2.3 ug/l Daily Outflow = 0.68 MGD Res. Retention Time = 29 hr Aerator Motor Efficiency = 40% Electricity Cost = $0.08/kWh 0% Power Input (W/m 3 )

14 Tray Tower Aeration Technology Assessment 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

15 Aeration Strategy Evaluation Tools ASAP Aeration Modeling Aeration System Analysis Program (ASAP) Bubble Surface Spray aeration spreadsheet tool Based on AWWA Water Quality Treatment Handbook 5 th edition Tray tower TTHM reduction Based on WaterRF Project No 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

16 DBP Formation Potential Factors: USEPA WTP Model Form DBP A TOC UVA *( Cl *( Br a b c ( T 20) ( ph 7.5) = ( * ) 2 α β DBP = TTHM or HAA5 (μg/l) TOC = Treated Water TOC (mg/l) UVA = Treated Water UVA (1/cm) Cl 2 = 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 ) *( ) *( ) *( t) (Solarik et al., 1999) d

17 Arizona Case Studies City of Tempe, AZ City of Phoenix, AZ City of Mesa, AZ

18 City of Tempe, AZ Evaluation: Surface water treatment plant 12 MG reservoir Methods evaluated Groundwater Blending Coagulation / Enhanced Coagulation Aeration Bubble (modeling & bench A/W=15) Spray (modeling & bench testing) Surface (modeling) TTHM Reformation (modeling & SDS tests)

19 Baseline SDS Test ph: Temperature (ºC): Chlorine Residual (mg/l): ph and Temperature Chlorine Residual Aeration followed by rechlorination Elapsed Time (hr) 0

20 Baseline & Bubble Aeration SDS Test Baseline SDS Post Aeration Rechlorination SDS TTHM (ug/l) TTHM Removal: 48.6 ug/l 21.8 ug/l = 26.8 ug/l = 55% Removal Model Prediction ~ 56% Removal Aeration followed by rechlorination Elapsed Time (hr)

21 City of Tempe, AZ 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 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 postaeration limited DBP control effectiveness

22 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 A/W ~ 6) Spray (modeling) Surface (modeling) External Methods (testing & vendor coordination)

23 City of Phoenix, AZ 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 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

24 City of Mesa, AZ Evaluation: Multiple distribution system reservoirs including 0.5 & 0.1 MG shown Aeration methods evaluated Bubble A/W ~ 6) Spray (modeling) Surface (modeling)

25 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

26 Impact on Chlorine Residual Predominant chlorine species in water around neutral ph are HOCl and OCl - Species are less volatile than Cl 2 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

27 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

28 Contact Information Chad Seidel, Ph.D., P.E. Damon S. Williams Associates, L.L.C Market Street, Suite 475 Denver, CO Phone: (303) Stephen Acquafredda, P.E. Damon S. Williams Associates, L.L.C East Camelback, Suite 700 Phoenix, AZ Phone: (602)