In-Reservoir TTHM Surface Aeration: A year of results, experience and lessons learned

Transcription

1 In-Reservoir TTHM Surface Aeration: A year of results, experience and lessons learned Jeanne M. Jensen, P.E. Steve Acquafredda, P.E. Jacobs Engineering Group, Inc. February 2, 2012

2 Presentation Outline Introduction TTHM Surface Aeration Strategies Design and permitting considerations Surface Aeration Full Scale Implementation Phoenix, AZ: 7.5hp aerator in a 2MG reservoir Mesa, AZ: 1hp aerator in a 0.25MG reservoir Lessons Learned Conclusions

3 Regulatory Drivers Drinking Water: Stage 2 DBPR MCL same as Stage 1 DBPR 80 µg/l TTHM 60 µg/l HAA5 Compliance based on Locational Running Annual Average (LRAA) Monitor sites with highest DBP concentrations Wastewater: THM discharge compliance Surface discharge limits Aquifer Protection Permits for groundwater injection

4 DBP Control Options Treatment Plant Enhanced Coagulation GAC Adsorption PAC Adsorption MIEX Process Advanced Oxidation River Bank Filtration Chloramination Distribution System Reduce water age Blending with lower TOC/DBP water Remote DBP control GAC/BAC Aeration

5 DBP Control: Distribution Systems Effective in isolated areas of high DBPs WTP Low DBP High DBP Implement Remote DBP Control

6 TTHM Aeration Strategies THMs can be removed by air stripping Efficiency depends on volatility (Henry s Constant) TTHM reformation after re-chlorination should be evaluated Aeration does not remove HAAs THM Species Henry s Constant (m 3 atm mol -1, 20 C) Chloroform (3.0 ± 0.1) x 10-3 Bromodichloromethane (1.6 ± 0.2) x 10-3 Chlorodibromomethane (8.7 ± 0.2) x 10-4 Bromoform (4.3 ± 0.3) x 10-4

7 TTHM Aeration Strategies In-Reservoir Aeration Strategies Bubble Spray Surface External Aeration Strategies Tray / Packed Tower Liqui-Cel Membrane Contactor Spray / Bubble Vessel

8 Design and Permitting Strategies Identify contributing sources to TTHMchallenged sites Apply aeration models (e.g. ASAP TM ) Determine wire-to-water or air-to-water ratio needed to achieve needed TTHM reduction Compare available aeration strategies Consider local VOC air emission requirements Early discussion with permitting agencies Novel application requires more interaction to inform agency Can reduce re-submittals and clarification responses

9 TTHM Aeration Strategy Evaluation Tools ASAP Aeration Modeling Aeration System Analysis Program (ASAP) Bubble, Surface, Packed Tower Spray aeration calculations Based on AWWA Water Quality Treatment Handbook 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

10 Applied Aeration 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 consider impact on chlorine residual

11 Aerator Batch Test Chlorine Residual Impacts Chlorine Residual (mg/l) Test conducted at end-of-line reservoir where chlorine demand had been previously met within distribution system Aerator Run Hours

12 Phoenix, AZ: 7.5-hp aerator in a 2 MG reservoir

13 Case Study: City of Phoenix, AZ Evaluation criteria: TTHM reduction, lifecycle cost, constructability, ease of operation, impact on operation, required time out of service, mixing Aeration methods evaluated Bubble, Spray, Surface, External Methods Surface aeration capital and O&M costs lower than for other strategies (25% lower lifecycle cost) Non-cost surface aeration advantages Capability to access and maintain equipment without entering reservoir

14 Case Study: City of Phoenix, AZ Distribution system downstream sample site Surface aeration reservoir test site Fills from Zone 2S Drains to Zone 2S Pump station to Zone 3S Flow through reservoir varies throughout the day

15 Case Study: City of Phoenix, AZ Concentration (µg/l) Before start of aeration After start of aeration TTHMs Distribution System Sample Site 0750 HAAs Distribution System Sample Site 0750 Jun-1 Jun-6 Jun-11 Jun-16 Jun-21 Jun-26 Jul-1 Jul-6 After start of aeration 23% avg. TTHM reduction achieved Model estimate of 29% reduction at 1.3 MGD

16 Case Study: City of Phoenix, AZ Concentration (mg/l) Before start aeration TOC Distribution System Sample Site 0750" Cl2 Distribution System Sample Site 0750" After start of aeration 0.0 Jun-1 Jun-6 Jun-11 Jun-16 Jun-21 Jun-26 Jul-1 Jul-6 TOC was relatively constant over test period TTHM variability partially attributed to chlorine residual

17 Case Study: City of Phoenix, AZ

18 Comparing model estimates with test results Reservoir residence time fluctuated between about 5 to 25 hours due to variations in: Water storage level Flow rate through reservoir Extent of mixing Aerators float at water surface Reservoir inlet and outlet located near bottom Variable air flow and turnover within headspace

19 Headspace Ventilation Air turnover dependent upon Water storage level Air filter clogging Design air flow rate: 700 scfm Clean filter air flow rate: 1,055 scfm Air flow rate after haboob 7/5/2011: 519 scfm Air filter selection for protection & clogging prevention Air supply fan with filter Air Filter MERV Rating Controlled Particulate Size 1 to 4 > 10 µm 5 to 8 3 to 10 µm 9 to 12 1 to 3 µm 13 to to 1 µm 17 to 20 < 0.3 µm Source: HPAC Engineering, February 2006

20 Further Study Impact of variable flow rates, storage level, and residence time on TTHM reduction Monitoring parameters to assess reservoir mixing (temperature, chlorine residual) Optimization of filter selection

21 Mesa, AZ: 1-hp aerator in a 0.25 MG reservoir

22 Case Study: City of Mesa, AZ Target reservoir 40% TTHM reduction Aeration methods evaluated Bubble, Spray, Surface Surface aeration capital and O&M costs lower than for other strategies Reduced maintenance for very remote facility Achieved high TTHM reduction 1-hp aerator installed Sizing analysis indicated 0.5-hp aerator needed 400 scfm air supply Approx. 78% of flow to be aerated in larger of 2 onsite reservoirs

23 Case Study: City of Mesa, AZ /14 6/16 6/20 6/22 6/24 6/28 6/30 7/5 7/7 7/11 7/18 7/22 7/27 8/1 8/5 8/10 8/22 9/7 9/14 9/19 9/23 9/28 10/3 10/7 10/12 10/17 10/21 10/26 10/31 TTHM (ug/l) HLR Aeration OOS Period Off Inlet TTHM HLR2 Top TTHM HLR2 Bottom TTHM 3249 DBPR N Compliance 91st St Site 4225 DBPR N Compliance Pinnacle Ridge Site Hydrant

24 Case Study: City of Mesa, AZ Test observations Similar TTHM results for top and bottom of reservoir indicator of good mixing Mesa has been able to optimize fill and drain times Maximize aerator contact with water

25 Lessons Learned Access hatches Material construction to avoid dissimilar metals Intrusion alarm Common Hatch? Dedicated Hatch? Size vs. Weight Reservoir Modifications Coating condition Penetration placement Structural reinforcement Cathodic protection

26 Lessons Learned Truck or Crane access Can access on-site with Davit Crane Boom truck arm distances limited Maintenance Schedule Semi-annual inspection & greasing Wear bars Fresh Air Supply Filtered MCESD Standard Haboobs!

27 Conclusions & Further Evaluation Surface Aeration is an effective TTHM reduction approach Site specific technology evaluation recommended Further Evaluation Extent of mixing Refine model estimates for dynamic reservoir flow conditions Optimization of air filter size for protection and minimal plugging

28 Acknowledgements We would like to thank our teaming partners for their important contributions to this work! City of Phoenix, AZ City of Mesa, AZ City of Scottsdale, AZ City of Tempe, AZ Aqua-Aerobic Systems, Inc.

29 Questions Jeanne M. Jensen, P.E. Phone: (602) Steve Acquafredda, P.E. Phone: (602) Chad Seidel, Ph.D., P.E. Phone: (303)