DISINFECTION BYPRODUCT COMPLIANCE TECHNICAL MEMORANDUM

Transcription

1 DISINFECTION BYPRODUCT COMPLIANCE TECHNICAL MEMORANDUM David M. Murbach No. C54419 Exp. 12/31/13 Prepared for: Amador Water Agency June 1, 2012 Prepared by: Reviewed by: Kevin Kennedy, PE and Dave Murbach, PE Karl Brustad, PE PURPOSE This Technical Memorandum (TM) was prepared as part of the Buckhorn Water Treatment Plant (WTP) Disinfection Byproducts / Backwash Disposal Project (Project), which has the goal to identify a cost effective strategy to: 1. Assure compliance with the Stage 2 Disinfection Byproduct (DBP) Rule 2. Develop a spent backwash water management plan These two issues are interrelated. The Amador Water Agency (Agency) must comply with US EPA s Stage 2 Disinfection Byproduct Rule (Stage 2 DBP Rule) starting January 1, 2013, although testing for compliance will not begin until October 1, 2013 per the California Department of Public Health (CDPH). The methods used to reduce the formation of haloacetic acids (HAAs) and total trihalomethanes (TTHMs) require modification of the treatment process at the Buckhorn WTP. These modifications potentially impact the water quality of the WTP s spent backwash water. The quality of the spent backwash water, in turn, impacts the options for managing the treatment and disposal of the spent backwash water. This TM presents the key findings and recommendations associated with disinfection byproduct compliance for the Buckhorn WTP distribution system. The Backwash Water Management TM presents the key findings and recommendations related to spent backwash water management for the Buckhorn WTP. 1

2 BACKGROUND The Stage 2 DBP Rule compliance requires meeting the maximum contaminant level (MCL) for both total trihalomethanes (TTHMs) and haloacetic acids (HAA5) at each individual sample location in the Buckhorn WTP distribution system, including the Agency s retail systems. The MCLs for the Stage 2 DBP Rule are as follows: TTHM 80 ug/l running annual average of 4 quarterly samples HAA5 60 ug/l running annual average of 4 quarterly samples The Agency currently meets the requirements of the Stage 1 DBP Rule, which requires compliance for the overall system average, not for each individual location. Previous Studies Agency staff have previously evaluated several methods for DBP compliance. These studies focused on removing the total organic carbon (TOC), the precursors for DBP formation, and reducing the initial chlorine dose. Table 1 presents a summary of the methods considered. Table 1. DBP Compliance Options Previously Considered By Agency Staff DBP Compliance Option Previous Findings Comments Aluminum Chlorohydrate (ACH) coagulation followed by membrane filtration Granular Activated Carbon (GAC) filtration Removes 25% to 60% of the raw water TOC, depending on the dose Reduces TTHM by 35% to 50%, depending on location Reduces HAA5 by 0% to 50%, depending upon location ACH introduces aluminum into the backwash water, creating issues with the current discharge permit Requires solids removal from the backwash water with high capital and operating costs Removes up to 80% of the raw water TOC, depending on the contact time High GAC capital and replacement costs Conclusion of Backwash Water Management TM to remove solids from spent backwash water regardless of chemical addition makes ACH use a viable option Alternative solids removal processes can reduce the capital and operating costs associates with solids removal from the backwash water Eliminated from consideration due to high capital and operating costs 2

3 DBP Compliance Option Previous Findings Comments Add Clearwell Baffling to reduce disinfection CT, reducing initial chlorine dose Relocate ph Adjustment location to reduce disinfection CT, reducing initial chlorine dose Ultraviolet Light (UV) for Giardia inactivation only to reduce the initial chlorine Increases tank baffling factor from 0.1 to 0.3 Reduces average chlorine residual required from 1.25 mg/l to 0.35 mg/l for CT Requires the addition of chlorine booster station in the distribution system to maintain chlorine residual Reduced initial chlorine dose reduces the potential for DBP formation Reduces ph used in determining the required CT from 7.8 to 7.0 Reduces average chlorine residual required from 1.25 mg/l to 0.90 mg/l for CT Reduced initial chlorine dose reduces the potential for DBP formation Reduces chlorine use for virus inactivation and distribution system residual only Reduces average chlorine residual required from 1.25 mg/l to 0.3 mg/l for CT Requires the addition of chlorine booster station in the distribution system to maintain chlorine residual High UV operating costs Option remains viable Option remains viable Eliminated from consideration due to high operating costs EVALUATION OF EXISTING SYSTEM Formation of DBPs is dependent on several factors including the TOC concentration before chlorine is first added to the water. In general, the greater the time in the distribution system, chlorine concentration, water temperature, and/or TOC concentration, the greater the formation of DBPs. In addition, DBP formation can be influenced by the organic material present in the distribution system. Typical DBP control strategies address removing TOC prior to disinfection, reducing chlorine concentrations, and reducing the water age in the distribution system. Discussions regarding the historical DBP formation, TOC concentrations and removal, chlorine dosing, and distribution system water age are present in the following sections. 3

4 Historical DBP Formation The Agency currently monitors 4 locations within the Buckhorn WTP distribution system on a quarterly basis for Stage 1 DBP Rule compliance. Figures 1 and 2 present the TTHM and HAA5 data since January 2009 for the following locations (See Figure 3 for location in the distribution system): Site Pioneer Creek Road Site Gy-Tam Lane Site Meadowbrook Drive Site Sugar Pine Drive Figure 1 shows that running annual averages (RAAs) for Sites 2, 3, and 4 would be out of compliance with the Stage 2 DBP Rule for TTHMs, with all three site peaking their running annual average above 90 ug/l. Figure 2 shows that RAA for Site 3 would be out of compliance with the Stage 2 DBP Rule for HAA5, with the site peaking its running annual average at 80 ug/l. Figure 1. Buckhorn WTP TTHM Concentrations and Running Annual Averages (RAA) 4

5 Figure 2. Buckhorn WTP HAA5 Concentrations and Running Annual Averages (RAA) Figure 3. Sample Site Locations in the Buckhorn WTP Distribution System 5

6 Historical TOC Concentrations Table 2 summarizes the raw and treated water TOC concentrations at the Buckhorn WTP. Note that these data do not include the TOC concentrations taken during the ACH and GAC pilot studies. Table 2 shows that the average raw water TOC concentration is 1.4 mg/l, which is considered low. For comparison, the Stage 1 DBP Rule requires TOC removal levels for facilities with raw water TOC concentrations greater than 2.0 mg/l. However, the TOC removal at the Buckhorn WTP is minimal. Table 2. Buckhorn WTP TOC Concentrations and Removal Date TOC Concentration TOC Removal Raw Water Treated Water (%) (mg/l) (mg/l) 1/8/ % 1/13/ % 1/20/ % 2/19/ % 7/8/ % Average % Historical Chlorine Dosage and Disinfection Parameters The Buckhorn WTP doses chlorine in the form of sodium hypochlorite solution for disinfection after membrane filtration. The chlorine dose at the Buckhorn WTP is determined by two factors: (1) the need to provide the required disinfection CT to the water and (2) the need to maintain a chlorine residual throughout the distribution system The required CT values are based on the chlorine residual level, water temperature, ph, disinfectant contact time before the first customer. The level of CT provided is calculated from the product of chlorine residual and contact time. Figure 4 presents a summary of the temperature and ph historical data for the Buckhorn WTP. Colder water temperatures increase the required CT and all other things being equal, the chlorine residual level must rise to meet this increased CT. Figure _ shows that the water temperature varies between 6 degc and 15 degc throughout the year. Higher ph levels require increase CT values. As previous studies have identified, the Buckhorn WTP adjusts the finished water ph before the disinfection clearwell. Figure 4 shows that relocating the ph adjustment point until after the clearwell would lower the ph level used for calculating CT requirements from about 7.8 to

7 Figure 4. Buckhorn WTP Historical Temperature and ph Levels Figure 5 presents a summary of the historical chlorine residual levels along with the amount of CT required and the amount of CT actually provided. Figure 5 shows that the chlorine residual level prior to May 2010 was about 1.0 mg/l. Since June 2010, the chlorine residual level has been maintained at about 1.3 mg/l. Figure 5 also shows the additional CT provided by the existing disinfection system which on average provides about 2 times the required CT level. 7

8 Figure 5. Buckhorn WTP Historical Chlorine Residual and CT Levels Distribution System and Water Age Increased water age in the distribution system leads to increased levels of DBPs. Water age is a function of the storage available in the distribution system, the configuration of the individual tank inlets and outlets and the water demands throughout the year. The total storage volume provided in 26 storage tanks is nearly 3,000,000 gallons throughout the entire Buckhorn WTP distribution system. The Buckhorn WTP production rate ranges between 0.5 and 1.5 million gallons per day (mgd). Theoretically this means the average water age in the system ranges between 2 and 6 days. The storage tanks closest to the Buckhorn WTP should have shorter water age, but the storage tanks at the farthest ends of the distribution system could have water age significantly longer. The configuration of each tank s inlet and outlet piping can also lead to increased water age. Tanks with a single pipe that act as both inlet and outlet provide the greatest water age. Similarly, tanks with inlet and outlet pipes that are adjacent to each other create significant water age. Minimum water age is provided where inlet and outlet pipes are located on opposite sides of the tank or where tanks are baffled. In addition to water age, the tank materials can impact the formation of disinfection byproducts. Steel and floating cover reservoirs allow for higher water temperature in the storage tanks than a 8

9 concrete or wooden tank. This increased water temperature increases the formation of DBPs. Also, redwood tanks were shown by Calaveras County Water District to increase the formation of DBPs by mg/l, presumably due to the contact between the organic material in the tank wall and the chlorine. Table 3 summarizes the storage tanks in the Buckhorn WTP distribution system. Figure 6 presents the distribution of tanks with adjacent inlet/outlet and those constructed from redwood. Table 3. Summary of Buckhorn WTP Distribution System Storage Tanks Tank Storage Tank Capacity Storage Tank Material Configuration of Inlet/Outlet Jackson Pines 177,621 Hypalon/Steel Separate, wall/floor Pine Acres 1 253,661 Steel Separate, wall/wall Pine Acres A 68,700 Redwood Separate, wall/floor Pine Acres B 68,700 Redwood Separate, wall/floor Pine Grove 1 320, Pine Grove 2 included with Pine Grove Pine Grove 3 included with Pine Grove Ranch House 427,631 Hypalon/Steel Separate, wall/wall Sunset Heights 28,184 Steel Adjacent, floor/floor A 425,705 Steel Adjacent, wall/wall B 187,697 Steel Separate, wall/floor C 89,838 Redwood Adjacent, floor/floor D 100,407 Steel Separate, floor/floor Mace Meadows 2 36,252 Redwood -- Alpine 1 49,323 Steel Adjacent, wall/wall Alpine 2 129,367 Steel Adjacent, wall/floor CAWP 62,147 Steel -- CSA Franks 46,610 Steel Adjacent, wall/wall Mace Meadows 3 44,038 Redwood -- Mace Meadows 4 100,407 Steel -- Mace Meadows 5 100,408 Steel -- Madrone 53,997 Steel Separate, wall/wall McKenzie 38,841 Steel Adjacent, wall/wall Rabb Park 34,103 Steel -- Ridgeway Pines 1 44,984 Redwood -- Ridgeway Pines 2 48,313 Steel -- 9

10 Figure 6. Buckhorn WTP Distribution System Storage Tank Material and Inlet/Outlet Configuration DBP REDUCTION ALTERNATIVES Typical control strategies address removing TOC prior to disinfection, reducing chlorine concentrations, and reducing the water age in the distribution system. The discussion below presents an overview of the various alternatives available to the Agency along with their applicability to the Buckhorn WTP distribution system. TOC Removal Strategies Table 4 presents the three main TOC removal strategies: pre-oxidation, coagulation / filtration, and adsorption. Pre-oxidation can be achieved using ozone or chlorine dioxide to improve the TOC removal efficiency at the membrane filters. The coagulation / filtration strategy relies on coagulants such as alum, polymer, ACH or ferric chloride to form a floc that can be subsequently removed by the membrane filters. Finally, adsorption of TOC onto carbon can be achieved using a separate GAC filter or by adding powdered activated carbon (PAC) to the water for removal at the membrane filters. Table 4. Applicability of TOC Removal Strategies to the Buckhorn WTP Oxidation: - Ozone TOC Removal Strategy - Chlorine Dioxide Applicability to the Buckhorn WTP - Not compatible with either the Pall or Memcor membranes currently used in the treatment process - Generated on-site requiring the handling of sodium chlorite, sodium hypochlorite, and sulfuric acid; dose is limited under the DBP Rule due to formation of chlorite upon reaction with TOC; potentially applicable at the Buckhorn WTP, but would require pilot testing 10

11 TOC Removal Strategy Coagulation/Filtration: - Alum - ACH - Polymer - Ferric Chloride Adsorption: - GAC - PAC Applicability to the Buckhorn WTP - Contributes aluminum to the backwash sludge; generates more solids than ACH - Contributes aluminum to the backwash sludge (priority pollutant) - Not compatible with the membranes currently used in the treatment process - Compatible with the current treatment process, but not the currently backwash recovery skid membranes; contributes iron (not a priority pollutant) to the backwash sludge - Adds a new process to the current treatment train; GAC filter size determines TOC removal; previous studies by Agency have shown this option to be too costly - Increases solid content in backwash water; dose determines level of TOC removal; not compatible with current treatment process membranes Chlorine Dose Reduction Strategies The chlorine dose at the Buckhorn WTP is set by two factors: (1) the need to provide the required disinfection CT to the water and (2) the need to maintain a chlorine residual throughout the distribution system. The required CT values are based on water temperature, ph, disinfectant contact time before the first customer, and chlorine residual level entering the distribution system. The level of CT provided is calculated from the product of chlorine residual and contact time. Altering the level of CT required is one way to reduce the use of chlorine. Another would be to provide an alternative disinfectant to reduce or eliminate the use of chlorine. Table 5 presents a summary of the potential chlorine dose reduction strategies. Table 5. Applicability of Chlorine Dose Reduction Strategies for the Buckhorn WTP Chlorine Dose Reduction Strategy Reduce Required CT: - Increase Baffling in Clearwell - Relocate ph adjustment Applicability to the Buckhorn WTP - Previous Agency studies show significant drop in chlorine dose for CT, but requires booster chlorination stations in distribution system - Previous Agency studies show slight drop in chlorine dose for CT Alternative Disinfectants: - Ozone - Chloramine - UV - High capital and operating costs for power; still requires chlorine for distribution system residual - Generated on-site from sodium hypochlorite and ammonium hydroxide; requires significant public education, training and monitoring to avoid nitrification in the distribution system - High capital and operating costs for power; previous studies by the Agency has shown this option to be too costly Water Age Increased water age leads to increased levels of DBPs. In addition, prolonged contact with organic material in the distribution system leads to increased DBP formation. Table 6 presents a summary of potential water age reductions strategies. 11

12 Table 6. Applicability of Water Age Reduction Strategies for the Buckhorn WTP Water Age Reduction Strategy Reduce Tank Storage Time: - Separate Tank Inlets/Outlets - Provide Tank Mixing - Minimize Tanks Operating Level Applicability to the Buckhorn WTP - Potential to reduce the water age in a tank by a factor of 3; cost is dependent on tank and site constraints - Potential to reduce the water age in a tank by at least a factor of 3; cost is typically less than piping modifications, but is dependent on tank access - Minimal cost; requires coordination with fire flow storage Reduce Organic Material: - Eliminate or Line Redwood Tanks - Distribution System Flushing - Tank Cleaning - High capital cost, but significant reduction in DBPs in the area served by redwood storage tanks - Increases operating costs; difficult to assess DBP impact - Increases operating costs; difficult to assess DBP impact EVALUATION OF DBP REDUCTION ALTERNATIVES Based on the information presented in Tables 4-6, the following DBP reduction alternatives are recommended for consideration at the Buckhorn WTP to achieve compliance with the Stage 2 DBP Rule in 2013: TOC Removal : o Chlorine Dioxide Oxidation o ACH Coagulation o Ferric Chloride Coagulation Reduce Required CT: o Clearwell Baffling o Relocate ph Adjustment Reduce Water Age: o Tank Mixing In addition to consideration of the alternatives above, there are several alternatives that should be incorporated into the on-going planning for the Buckhorn WTP: Eliminate or Line Redwood Tanks based on the testing done by CCWD, redwood tanks can contribute a significant increase in DBPs (10-20 ug/l); providing a hypalon liner or replacing these tanks is a significant capital cost that should be incorporated into the Agency s long range planning. Distribution System Flushing and Storage Tank Cleaning while it is difficult to assess the impact on DBP formation for these alternatives, it is good practice to keep the distribution system clean; contact between sediment and chlorine has the potential to form additional DBPs; the Agency currently has a flushing program in place. Minimize Storage Tank Operating Levels the distribution system should be modeled to determine if excess storage is provided in any of the system s pressure zones; reduced storage volumes should be considered on a seasonal basis to reduce water age and DBP formation if sufficient fire storage is available. 12

13 TOC Removal Chlorine Dioxide Chlorine dioxide can be used to oxidize and remove TOC. Expected TOC removal from chlorine dioxide usage typically ranges between 20 and 50%, depending on the initial TOC level. For the relatively low TOC levels present in the Buckhorn WTP raw water, the TOC removal should be closer to 20%. Chlorine dioxide is formed on-site from sodium chlorite, hydrochloric acid, and sodium hypochlorite solutions. All of these chemicals must be treated as hazardous materials. The chlorine dioxide solution is used shortly after generation. For the TOC levels at the Buckhorn WTP, the anticipated dosage of chlorine dioxide is 1.0 to 1.5 mg/l. Based on the Buckhorn WTP capacity of 2 mgd, the chlorine dioxide generation equipment must provide a minimum of 25 lb/day of chlorine dioxide. The Buckhorn WTP Pall membrane filtration equipment is compatible with the use of chlorine dioxide. Additionally, the membranes in the backwash recovery skid are also compatible with chlorine dioxide. The capital cost of an installed chlorine dioxide generation system is approximately $180,000. The annual operating costs, assuming a 1.0 mgd average WTP flow and a dose of 1.0 mg/l, is approximately $30,000. The use of chlorine dioxide for TOC removal is dependent on the water quality being treated. Pilot testing of chlorine dioxide is recommended to determine the TOC removal effectiveness, equipment size and operating costs. TOC Removal Aluminum Chlorohydrate (ACH) ACH can be used as a coagulant to remove TOC in the membrane filtration process. The Agency successfully tested this DBP removal strategy at the Buckhorn WTP in the summer of During the extended portion of the pilot, when the ACH dose was set at 4.5 mg/l, the TOC reduction was 25%. The impact on DBP formation was also analyzed at three locations (Sample Sites 2, 3 & 4 as shown in Figure 3). Tables 7 and 8 present the DBP reduction results for the ACH pilot study. Table 7. TTHM Reduction During ACH Pilot Study Date Sample Site 2 Sample Site 3 Sample Site 4 TTHM (ug/l) Removal (1) TTHM (ug/l) Removal (1) TTHM (ug/l) Removal (1) 8/9/ % 50 47% 38 48% 8/11/ % 69 27% 67 8% 8/31/ % 48 49% 43 41% 9/20/ % 42 56% 37 49% Average 71 32% 52 45% 46 37% Notes: (1) Removal based on TTHM levels present in the system before and after the ACH pilot study. 13

14 Table 8. HAA5 Reduction During ACH Pilot Study Sample Site 2 Sample Site 3 Sample Site 4 Date HAA5 (ug/l) Removal (1) HAA5 (ug/l) Removal (1) HAA5 (ug/l) Removal (1) 8/9/ % 36 45% 33 40% 8/11/ % 39 40% 31 44% 8/31/ % 32 51% 25 55% 9/20/ % 37 43% 32 42% Average 20 29% 36 45% 30 45% Notes: (1) Removal based on HAA5 levels present in the system before and after the ACH pilot study. The Buckhorn WTP Pall membrane filtration equipment is compatible with the use of ACH. Additionally, the membranes in the backwash recovery skid are also compatible with ACH. The addition of ACH adds aluminum to the sludge produced at the Buckhorn WTP. The majority of the aluminum will remain in the sludge, but a small amount will be dissolved aluminum in the filtrate of the backwash recovery skid. During the ACH pilot study, dissolved aluminum levels were monitored. Table 9 presents the results of this aluminum testing. Note that the treated water levels of dissolved aluminum were not detected meaning that all of the aluminum added as ACH is removed by the membranes. Table 9. Dissolved Aluminum Levels During ACH Pilot Study Date Dissolved Aluminum Concentration ACH Dose Backwash Water Treated Water (mg/l) (ug/l) (ug/l) 6/2/ /8/ < 50 6/16/ /22/ < 50 6/29/ /8/ /14/ /23/ Aluminum has a primary MCL of 1,000 ug/l and a secondary MCL of 200 ug/l. Aluminum is also considered a priority pollutant by the RWQCB and could impact some of the options for reuse of the treated backwash water, if the levels exceed the secondary MCL. The capital cost of an installed ACH storage and system is approximately $25,000. The annual operating costs, assuming a 1.0 mgd average WTP flow and a dose of 4.5 mg/l, is approximately $20,000. TOC Removal Ferric Chloride Ferric chloride can also be used as a coagulant to remove TOC in the membrane filtration process. Expected TOC removal from ferric chloride should be similar to other coagulants, such as ACH. Ferric chloride solution would be delivered in drums, totes or bulk tanker truck to the Buckhorn WTP. Ferric chloride, due to its low ph, must be treated as a hazardous material. Ferric chloride 14

15 doses are typically 2-3 times greater than ACH when used as a coagulant. For the TOC levels at the Buckhorn WTP, the anticipated dosage of ferric chloride is 10 to 15 mg/l. Based on the Buckhorn WTP capacity of 2 mgd, the maximum ferric chloride use is anticipated to be 60 gallons per day, while the current average use at 1.0 mgd would be about 20 gallons per day. The Buckhorn WTP Pall membrane filtration equipment is compatible with the use of ferric chloride. However, the membranes in the backwash recovery skid are not compatible with ferric chloride and would need to be replaced at a cost of $20,000. The addition of ferric chloride adds iron to the sludge produced at the Buckhorn WTP. The majority of the iron will remain in the sludge, but a small amount will be dissolved iron in the filtrate of the backwash recovery skid. Iron has a secondary MCL of 300 ug/l, but is not considered a priority pollutant by the RWQCB and should not impact any option for reuse of the treated backwash water. Assuming ferric chloride would be fed from totes, the capital cost of an installed ferric chloride storage and feed system is approximately $25,000. The total cost for implementing ferric chloride coagulation is $45,000, including the cost to upgrade the backwash recovery skid membranes. The annual operating costs, assuming a 1.0 mgd average WTP flow and a dose of 10 mg/l, is approximately $25,000. The use of ferric chloride for TOC removal is dependent on the water quality being treated. Pilot testing of ferric chloride is recommended to determine the TOC removal effectiveness, equipment size and operating costs. This pilot testing could be done as jar testing in the laboratory to reduce costs. Reduce Required CT Clearwell Baffling Clearwell baffling has been studied previously by the Agency. Providing baffling in the clearwell would increase the contact time for disinfection and allow the chlorine dose to be reduced. The conclusions of the previous work were that the required chlorine residual could be dropped from 1.25 to 0.35 mg/l. It was also noted that providing a lower chlorine residual at the Buckhorn WTP would require up to 7 chlorine booster stations throughout the distribution system to maintain a residual level. The cost of the baffle installation was estimated at $160,000 and the cost of the chlorine booster stations was estimated at $90,000, plus another $75,000 to provide power to three of the sites. In addition, operational costs for operating and maintaining the chlorine booster stations were estimated by the Agency to be $20,000 per year. The impact on the formation of disinfection byproducts was estimated in a laboratory test. This test showed TTHM levels dropping from 88 ug/l to 36 ug/l and HAA5 levels dropping from 120 ug/l to 30 ug/l. The construction sequencing to implement this alternative needs to be considered. Since the one clearwell needs to stay in service at all to provide disinfection, the baffles would have to be installed during a period of low water demand to not impact the WTP s operation and assure CT requirements are met. 15

16 It should also be noted that Figure 5 shows that the CT provided is currently about 2 times the CT required. Simply providing a chlorine residual level closer to that required for actual CT compliance could reduce the chlorine residual level as low as 0.7 mg/l. This reduced level of chlorine residual could require chlorine booster stations, depending on the reduction in chlorine residual level, but any reduction in chlorine residual would provide a reduction in DBP formation. If the chlorine residual level were reduce slowly over time and the distribution system chlorine residual monitored, implementation of a reduced chlorine residual level would have no cost. Figure 5 shows that prior to June 2010, the chlorine residual level was closer to 1.0 mg/l than the 1.3 mg/l currently in use. Reduce Required CT Relocate ph Adjustment Relocation of the current ph adjustment point has also been previously studied. Relocating the ph adjustment point to the clearwell outlet, after disinfection is complete, lowers the ph used to determine the required CT from 7.8 to 7.0 on average. This reduces the required CT by about 30% and the chlorine residual level required from 1.25 mg/l to 0.9 mg/l. At this chlorine residual level, chlorine booster stations may be required but probably fewer than 7. The cost to implement this alternative was estimated to be $20,000. As stated previously, Figure 5 shows that the CT provided is currently about 2 times the CT required. Simply providing a chlorine residual level closer to that required for actual CT compliance could reduce the chlorine residual level to 0.7 mg/l. Reduce Water Age Tank Mixing Increased water age can significantly increase the formation of DBPs. Storage tanks in the distribution system with inlets and outlets in the same location have poor mixing, which leads to increased water age. Table 3 shows at least 7 of the tanks in the Buckhorn WTP distribution system have inlets and outlets that are adjacent to each other. Tank A is one of the largest tanks in the distribution. Tank A also has adjacent wall mounted inlets and outlets. The Agency performed testing in August 2009 that shows the DBP formation in Tank A. Table 10 presents the results of this testing, which show significant DBP formation in Tank A one of the first tanks in the distribution system. The Tank Specifications spreadsheet provided by the Agency shows that 12 storage tanks are downstream of Tank A. This means that the increased DBP formation in Tank A impacts a major portion of the distribution system. Table 10. DBPs Formed in Tank A and Tank B DBP Levels (ug/l) DBPs Formed (ug/l) DBP Buckhorn Tank A Tank B Buckhorn WTP Outlet Outlet WTP Tank A Tank B TTHM HAA

17 Reducing the water age in the distribution system can be accomplished by separating the inlet and outlet piping. This can be accomplished in several ways, depending on the configuration of the storage tank: Reroute Piping Outside Tank requires new pipe penetration and may not be feasible for tanks with existing floor connections Reroute Piping Inside Tank extends inlet or outlet connection to opposite side of tank to separate the tank s inlet and outlet connections Prefabricated Tank Mixing System - extends inlet or outlet connection to opposite side of tank and adds nozzles to promote mixing and reduce water age Add Baffles Inside Tank separates the inlet and outlet connection to remove areas of stagnant water that increase the water age The costs of the inside tank options range from about $10,000 to $50,000 for a 50 foot diameter steel tank. Alternatives Evaluation Summary Table 11 presents a summary of the alternatives along with scores to prioritize implementation of the alternatives. Note that the alternative to reduce CT Provided levels was added to the list of alternatives. Table 11. Scoring of DBP Reduction Alternatives DBP Capital Cost Operating Cost Reduction (1-5, 5 = (1-5, 5 = Alternative Lowest) Lowest) Chlorine 2 1 Dioxide ($180,000) ($30,000/yr) 4 3 ACH ($25,000) ($20,000/yr) 3 2 Ferric Chloride ($45,000) ($25,000/yr) Clearwell 1 3 Baffling ($160,000) ($20,000/yr) ph Adjustment 4 5 Location Reduce CT Provided Tank Mixing ($20,000) 5 ($0) 3 ($175,000) ($0/yr) 5 ($0/yr) 5 ($0/yr) DBP Reduction (1-5, 5 = Greatest) Complexity (1-5, 5 = Simplest) Total Score

18 PROJECT CONCLUSIONS AND RECOMMENDATIONS Figures 1 and 2 show that compliance with the Stage 2 DBP Rule will be difficult to achieve without making modifications to the Buckhorn WTP and its distribution system. Six alternatives were evaluated for DBP removal effectiveness, capital and operating costs, and complexity of the installation and operation. To assure compliance with the Stage 2 DBP Rule, multiple methods for DBP reduction are recommended (based on the highest scoring alternatives in Table 11): TOC Removal using ACH ferric chloride has slightly higher operating and capital costs and would require pilot testing, but could be an alternative to ACH if need to address RWQCB discharge issues Reduce Chlorine Dose using reduced CT Provided the chlorine residual should be minimized to the level needed to maintain residual throughout the distribution system without adding chlorine booster stations; the overall chlorine dose should be reduced with the use of ACH and the removal of TOC, which creates chlorine demand Reduce Water Age separating Tank Inlet/Outlet Piping starting with Tank A to determine the impact on the distribution system 18