Strategies for Using Tank Aeration for Stage 2 DBP Compliance

Transcription

1 Strategies for Using Tank Aeration for Stage 2 DBP Compliance David S. Briley, P.E. Hazen and Sawyer Tiffanie Hawley Cape Fear Public Utility Authority Erik Rosenfeldt, Ph.D, P.E. Hazen and Sawyer Allison Reinert Hazen and Sawyer Water JAM 2010

2 Drivers for Tank Aeration Challenges of Stage 2 DBP Rule Shifts in compliance sites Increasing bromide levels in some water sources Cost-effective Stage 2 DBP Compliance options NC AWWA Water JAMWEA Most options focus on removal of DBP precursors Limit DBP formation (i.e. chloramines) Aeration removes formed THMs PAX TRS System Medora SolarBee

3 Brominated DBP Challenges Locational Running Annual Average (ppb) Raw Bromide RAA Echo THM LRAA Crosswinds LRAA Echo HAA LRAA 0 0 Dec-02 Sep-05 Jun-08 Feb-11 Nov Raw Water Bromide RAA (mg/l)

4 Brominated DBP Challenges Bromide in Source Waters provide challenges for THMs TTHM Concentration (ug/l), Chloroform (%) Kinetics Time (h) TTHM, 100 ug/l Br- TTHM, 300 ug/l Br- % Chloroform, 100 ug/l Br- % Chloroform, 300 ug/l Br- Mass Chloroform (CHCl 3 ): Bromodichloromethane(CHCl 2 Br): Dibromochloromethane(CHClBr 2 ): Bromoform(CHBr 3 ): CHBr 3 /CHCl 3 = 2.1

5 Several Approaches for Aeration y out y in y out y in Q in C in Diffusers Q out C out Air Compressor Q C in Q R P C Q C Diffused Bubble Aeration (air-in-water) Fixed Spray Aeration (water-in-air) y out y in Q C in Q R P C Floating Spray Aeration (water-in-air) Q C

6 Aeration Henry s Law Compound For Comparison, HAAs: x 10-7 HOCl: Henry s Law Constant at 20 o C Chloroform Bromodichloromethane Dibromchloromethane Bromoform OCl - : 5.3 x NH 2 Cl: NHCl 3 : 8.6x10-6

7 Aeration Techniques: Diffused Bubble Removal of THMs at Several Air:Water Ratios % THM Removal Air to Water Ratio Adapted from: Brooke and Collins, JAWWA, 2011

8 Aeration Techniques: Spray Aeration Reduction of DBPs with Spray Aeration Percent THM reduction Apparent Air:Water Ratio Adapted from: Brook and Collins, JAWWA 2011

9 Holistic Approach to Aeration NC AWWA Water JAMWEA Review of historical THM data to establish reduction goals. 2. Assessment of THM formation kinetics in the distribution system. 3. Evaluate where aeration is required 4. Establish design criteria through aeration performance modeling 5. Evaluation of tank influence areas using a calibrated distribution system hydraulic model.

10 Quarterly THM Results at Stage 2 Sites TTHMs (ppb)

11 TTHM Reduction Goals Goal of 80% of MCL on LRAA basis Provides buffer for: Variable water quality (DOC, bromide) Water demands and water age Location Max THM LRAA (ppb) THM Reduction To Achieve Goal of 64 ppb (%) B % B % B % B % B % B % B %

12 Aeration Performance Modeling Impacting factors: Recycle rate No. and design of nozzles or aerators Ventilation of head space Distance from nozzle to max water surface Hydraulic detention time Temperature Process Modeling Establish design criteria Assisted by Dr. Robin Collins, UnivNew Hampshire TTHM, ppb Time from Initial Conditions, hr 13% 18% 27% 33% 41%

13 Where to Aerate? Clearwell Aeration Larger aeration system and higher capital and O&M costs Provides TTHM reduction for entire system Effectiveness depends on THM formation in WTP Tank Aeration in Distribution System Smaller aeration system and lower capital and O&M costs THM levels usually higher due to higher water age Provides TTHM reduction for a targeted area

14 Prelim Spray Aeration Results (4 MG Clearwell) Based on 60% Recycle - ~25% THM reduction THMS (PPB) TIME (HOURS)

15 Assessment of Clearwell Aeration Sweeney WTP has two clearwells in series. Concept is allow THM formation in 12 MG clearwell and then aerate in 4 MG clearwell 4 MG NC AWWA Water JAMWEA MG Clearwell

16 Modeling of 4 Million Gallon Clearwell Aeration Aeration at the WTP could still result in some high THMs in higher water age areas LRAA (ppb)

17 Water JAM 2010 Water Age Simulation Water age is low (3-4 days max in most areas) 24 automatic flushers in system help reduce water age in localized areas Even with clearwell aeration, predict higher TTHMs in higher water age areas

18 NC AWWA Water JAMWEA Tank Influences in Distribution System To assess aeration performance in elevated tanks, need to understand hydraulic patterns Used calibrated hydraulic model to determine influence on each elevated tank Factored in tank influence (i.e. how much water in a certain area went through the tank)

19 Tank Influences in Distribution System

20 17 th Street and Dawson Street Tank Aeration Example calculation for evaluating tank aeration performance Location of Aeration System (Tank) Percent THM Reduction at the Tank Average Impact on the Stage 2 Monitoring Location (%) Modeled Reduction at Site with Tank Aeration (%) 17 th Street 40% 26% (BO7) 10.4% (BO7) Dawson Street 40% 18% (BO4) 7.2% (BO4)

21 Combined 4 MG Clearwell and Tank Aeration

22 Aeration System Optimization Plan to assess further evaluate TTHM formation and reduction trends with aeration to assess seasonal operation and scaling back CFPUA is collecting baseline data using an online THM analyzer (Foundation Instruments)

23 Preliminary Costs Aeration System CFPUA is proceeding with spray aeration installation Elevated tanks -Summer MG clearwell Fall 2015 Spray Aeration Capital Cost Annual O&M Costs 2 Elevated Storage Tanks $610,000 $30,000 4 MG Clearwell $2,100,000 $95,000 Total Aeration Cost $2,710,000 $125,000

24 Summary Multiple aspects should be considered before installing mixing and/or aeration: Effectiveness for THM Reduction: Modeling of THM Reduction Available and Helpful Consider speciation of TTHM in selecting an aeration system Impact of WQ Improvement must be considered At Stage 2 sites Area of influence Improvement possible? NC AWWA Water JAMWEA Must weigh localized treatment vs. full treatment Cost implications and efficiencies Obligations for improved WQ to entire system Both localize and full treatment? Remember: Only effective for THMs

25 Questions??? David S. Briley, P.E. Hazen and Sawyer, P.C.