FROM RAW WATER INTAKE TO DISTRIBUTION NETWORK: THE JOURNEY OF DBP CONTROL

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1 FROM RAW WATER INTAKE TO DISTRIBUTION NETWORK: THE JOURNEY OF DBP CONTROL Jane Xiaojie Gan, E.I.T., Porter Rivers, P.E. URS, 101 Research Drive, Columbia, SC ABSTRACT The Chester Metropolitan District (CMD) is considered a Schedule 3 system for compliance with Stage 2 Disinfectants and Disinfection Byproducts Rule (Stage 2). The compliance monitoring will begin in October The CMD system withdraws raw water from the Catawba River. Chlorine dioxide is added at the raw water pump station, chlorine is added pre and post filtration, and ammonia is added post clearwell to produce a chloramine residual in the distribution system. This treatment strategy has allowed CMD to maintain historical disinfection byproduct (DBP) levels below the MCLs set by the Stage 2 rule. However, several episodes of high DBP levels have been observed over the last two years. In addition, the CMD system showed significantly higher DBP levels in 2011 than two other water systems withdrawing raw water from the Catawba River. To address this issue, an evaluation of the CMD Water Treatment Plant (WTP) was conducted to determine possible causes as well as to identify optimization techniques to minimize DBP formation. The evaluation included a review of raw water characteristics, treatment process performance, disinfection strategies, and historical DBP levels. A special monitoring program was also conducted during the study to gain more insight into performance of the WTP. The DBP levels from 2005 to 2012 at CMD were carefully reviewed. Surprisingly no increasing trend of DBP over time was observed. Instead, two isolated episodes of high DBPs occurred in February and August of One episode was explained by the annual burnout practice when the WTP switched the residual from chloramine to chlorine in the distribution system. During the second episode, the DBP levels approached MCLs at the POE. This would indicate that the high DBP levels were formed within the plant. The study found several areas where CMD can make improvements to minimize DBP formation. These include: Increase monitoring to establish a better understanding of fluctuations of raw water quality and impact on DBP formation Improve chlorination efficiency in the clearwell to minimize the DBP formation in the WTP Optimize the chloramine formation at point of entry Optimize the distribution system to reduce the water age This presentation will discuss implementation of these potential strategies to minimize DBP formation in each stage of water treatment from raw water intake to distribution network. KEYWORDS Disinfection Byproducts, Stage 2 Disinfectants and Disinfection Byproduct Rule INTRODUCTION Due to concerns regarding recent high levels of disinfection by-products (DBPs), the Chester Metropolitan District (CMD) tasked URS with an evaluation to identify potential causes of these high levels and recommend techniques to minimize DBP formation. The goal of the evaluation is to assist CMD with compliance for Stage 2 of the Disinfectants and Disinfection By-Products Rule (Stage 2). The CMD system is considered a Schedule 3 system which means that Stage 2 compliance monitoring will begin in October 2013.

2 METHODOLOGY DBPs form as a result of complex reactions between precursors (natural organic matter) in source water and oxidants (primarily chlorine) used during water treatment. The levels of DBPs formed in a particular system depend on many factors including raw water quality, removal of natural organic matter, disinfection strategy, and detention time (water age) in the distribution system. In order to minimize the DBP levels, CMD needs to optimize treatment as well as minimize water age in the distribution system. To accomplish the goals of this study, three major tasks were accomplished. Develop performance baseline. By knowing when the system is at higher risk for DBP formation, operators can adjust the treatment process as necessary to reduce DBP formation. The goal of this task is to identify water quality trends and correlations between parameters (i.e., raw water quality, finished water quality, weather, etc) that could lead to the formation of DBPs. This data will be compared to the finished water results collected by SC Department of Health & Environmental Control (DHEC) for DBP compliance (and other DBP data, if available). Review engineering process. This task will investigate existing treatment strategies and operating procedures for particle and organics removal (coagulation, flocculation, sedimentation, filtration, and disinfection). The goal for this task will be to optimize each of these process strategies without causing negative impacts on water quality. Goals to be used for evaluating process performance will include standards used by SC DHEC s Area Wide Optimization Program (AWOP). Prepare summary and recommendations. Based upon the findings from the previous tasks, recommendations will be made to adjust process control and / or operational procedures (as necessary). Impacts of these changes should be monitored for at least two quarterly sampling events. Results and Discussion Raw water characteristics The CMD water treatment plant (WTP) uses the Catawba River as its sole raw water source. The monthly raw water total organic carbon (TOC) levels from January 2007 to December 2011 at CMD ranged from 1.5 to 11.3 mg/l, with an average of 4.3 mg/l. These levels are moderate to high compared with other surface water sources in South Carolina. The temporal variation of TOC in the raw water is considerable. The source water quality at CMD was compared with the data at upstream WTP A using the same source water. During the period from March 2010 to November 2011, the monthly raw water TOC (based upon monthly samples) showed a higher average (44.4%) and greater fluctuation for CMD. Since turbidity is sampled daily instead of monthly, it provides a better understanding of the raw water characteristics. The monthly average turbidity at the WTP A was below 20 NTU mostly, whereas it ranged from 20 to 40 NTU at CMD. Also the range of turbidity in each month was much wider at CMD. The WTP A pumps water from the Catawba River to a raw water reservoir prior to the treatment process. The reservoir allows sediment in the raw water to settle prior to treatment, which helps reduce the fluctuation in turbidity. But, because the majority of TOC is actually dissolved organic carbon (DOC), the presence of the reservoir does not explain the difference in TOC between the two plants. Typically, the DOC will remain in solution and pass through the reservoir into the treatment process. One potential explanation for the difference in organics loading between the two facilities is the presence of an industrial facility with an NPDES discharge permit located between the two WTPs (downstream of WTP A, upstream of CMD WTP). One method for verifying the impact of this facility would be to collect in-stream samples upstream of the discharge for the industrial facility and compare this with samples collected downstream. These samples should be monitored for TOC, DOC, and ultraviolet (UV 254 ).

3 Effect of rainfall on raw water quality. Heavy rainfall and seasonal events (i.e., turnover of reservoir) can increase the amount of contaminants in a raw water source. Based upon the data collected, a major storm event is typically followed by a spike in turbidity. Stormwater runoff (resulting from heavy rainfall) in the watershed (upstream of the intake) can carry sediment and organic matter into the river. In addition to increasing turbidity in the raw water, rainfall can also increases the load of TOC in surface water. However, the effect of rainfall on TOC loading is inconclusive due to a lack of daily TOC data from the SC DHEC monthly operating reports (MORs). Raw water quality during additional sampling. Additional sampling was conducted to further evaluate daily fluctuation of raw water TOC. During the period from April 14 th to May 31 st, 2012, TOC, UV 254, and turbidity were monitored simultaneously for both the raw and finished waters. Precipitation data during the same period were also obtained from weather channel. This monitoring program lasted for six weeks and captured two major rainfall events (> 0.4 inches). Data gathered during this monitoring program allowed for better comparison of the correlation between water quality parameters and rainfall. The raw water TOC (4 mg/l), turbidity (5-10 NTU), and UV 254 ( m -1 ) were relatively constant in dry weather. The light precipitation events (less than 0.4 inches) did not cause significant impact on raw water quality, while the two major rain events caused TOC, turbidity, and UV 254 increasing more than two times in raw water. The highest level of these water quality parameters occurred within one or two days after the rain events. The higher levels of TOC and UV 254 during heavier rainfall events can lead to increased formation of DBPs. It is recommended that CMD continue monitoring the effect of rainfall on raw water quality in the future. Since it is impractical to measure TOC on a daily basis, testing UV 254 is a good alternative for monitoring precursor levels. The impact on trends for turbidity, TOC, and UV 254 resulting from stormwater runoff are similar. CMD staff should monitor UV 254 on a daily basis to evaluate potential for DBP formation. Since major rainfall events adversely impact the raw water quality, CMD should review current operating procedures for making process adjustments in response to changing raw water quality conditions. Efforts should be made to standardize response procedures for all shifts. If necessary, CMD staff should consider doing more frequent jar tests during these events. Treatment process In addition to evaluating the impact of raw water on DBP precursors, the effectiveness of the existing treatment processes for DBP removal was reviewed. The CMD WTP employs a conventional surface water treatment process illustrated in Figure 1. Ferric chloride is used as the coagulant. Sodium hydroxide (caustic soda) is used for ph adjustment. Chlorine dioxide (ClO 2 ) is added at the raw water pump station for peroxidation. Chlorine is fed on top of the filters for manganese (Mn) control and prior to the clearwell for disinfection (CT). Ammonia is added following the clearwell to provide chloramines as a secondary disinfectant in the distribution system. TOC Removal. A bench scale process analysis conducted in 2001 recommended that CMD switch from alum to ferric chloride as the primary coagulant to increase TOC removal and reduce the potential for DBP formation. Subsequent testing by CMD confirmed that the optimum dosage of ferric is approximately 40 mg/l (under normal conditions). The process analysis also indicated that a flocculation ph of 5.3 is optimal for TOC removal using ferric chloride. This value is used by CMD staff for process control. Based upon review of MORs submitted to SC DHEC, the WTP is effective at removing TOC. The treated water TOC remained stable around 2 mg/l despite fluctuated between 1.5 to 11.3 mg/l in raw water, and CMD meets the required percentage TOC removal (typically 35 % or 45 %). The WTP appears to be operating in an enhanced coagulation mode, implying that precursor removal has been optimized.

4 Figure 1. WTP Schematic Turbidity Removal. Based upon discussions with CMD staff, the optimum ph for turbidity removal in the sedimentation basins is between 5.5 and 5.6. Although the coagulation conditions don t appear to be optimized for particle removal (as measured by settled water turbidity), the WTP is still able to meet the filter performance criteria for SC DHEC s Area Wide Optimization Program (AWOP). As noted in a recent SC DHEC sanitary survey (2010), the two-stage flocculators are equipped with variable frequency drives (VFDs), but CMD staff have the VFDs set at the maximum speed. A previous comprehensive performance evaluation conducted by SC DHEC indicated that the mixing energy in the flocculators was not optimum. It is recommended that CMD conduct a series of jar tests to simulate different mixing energies in the flocculation basins. If jar test results show that increasing mixing energy could significantly improve the settled water turbidity, CMD should consider upgrading the current flocculators to provide the staff with additional flexibility in optimizing prefiltration treatment. This could improve performance for both TOC and particle (turbidity) removal. Treatment performance during additional sampling. Data collected during the raw water quality evaluation (April 14 th to May 31 st 2012) was used to monitor performance through the treatment plant (as measured in the filter effluent). The percentage removals of TOC and UV 254 during this period are compared with the raw water turbidity. Despite the variations in raw water quality, the removal efficiency for TOC (53-75 %) and UV 254 (77-92 %) were consistently high. During the period, higher coagulant dosages were applied during spikes of turbidity during May, but overall lower coagulant dosages were used in May than April. Based upon discussions with operations staff, the current coagulation control is based upon operator s experience and some jar testing. A charge analyzer is also used to help dial in the proper coagulant dosage.

5 As previously discussed, more frequent jar tests during rainfall events could help CMD better determine the coagulant dosage in response to changes in raw water quality. It also may be possible to adjust the optimum ph for precursor removal to favor particle removal (from 5.3 to 5.6), but CMD will need to build a data base to evaluate the impact of making this adjustment. The database should include daily monitoring of WTP performance for UV 254 removal to determine impact during DBP sample events. There is potential for improvement in removal of turbidity and organics during rainfall events. By switching to UV 254 as the coagulant control parameter, CMD should be able to maintain adequate precursor control and improve particle removal. Disinfection Strategy CMD uses a combination of chlorine dioxide, chlorine, and chloramines as oxidants for the purpose of manganese (Mn) control and disinfection. The locations of oxidant addition and their dosages are shown in Figure 1. As part of the overall evaluation for DBP control, CMD s disinfection strategy was reviewed and evaluated. Chlorine Dioxide. Chlorine dioxide is a strong oxidant that can react with many substances in water such as organic matter and manganese. The byproduct of oxidation with chlorine dioxide is chlorite (ClO 2 - ). Chlorite is a regulated DBP with an MCL of 1.0 mg/l. CMD is required to monitor chlorite on a daily basis at the point of entry (POE) to the distribution system. The primary use for chlorine dioxide in the CMD WTP is to oxidize Mn. In addition, ClO 2 fed in the pretreatment processes helps reduce the oxidant demand for chlorine later in the treatment process (top of filter). Under current operation, the initial ClO 2 dose is approximately 1 mg/l at the raw water. The dosage for ClO 2 is manually adjusted, based upon the daily monitoring of chlorite residual measured in the finished water. The dosage is reduced if a high chlorite residual is detected at the POE. The operation goal for CMD staff is to maintain the chlorine dioxide dosage such that chlorite residuals do not exceed 0.7 mg/l. The level of manganese is not monitored in raw water on a daily basis. The daily dosages of chlorine dioxide and chlorite residual at the P.O.E. ranged from 0.6 to 1.60 mg/l, and 0 to 0.84 mg/l, respectively. The correlation between initial chlorine dioxide dosage and chlorite at the POE is somewhat weak. The lack of correlation between the ClO 2 dosage and the chlorite residual could imply that the level of chlorite residual is not determined only by the chlorine dioxide dosage. Instead, it could also be dependent upon the oxidant demand in the raw water. These results are consistent with the fluctuating water quality experienced by the CMD WTP. As the raw water quality changes, the demand for chlorine dioxide varies. Consequently, the chlorite level cannot be predicted accurately based only on chlorine dioxide dosage. Daily monitoring of UV 254 and Mn could help to optimize the ClO 2 dosage. Another tool that may help CMD estimate / control chlorine dioxide dosage is oxidation reduction potential (ORP). ORP is a measurement of oxidant demand in the water. Combined with measurements of ph, this could help operators make adjustments in the ClO 2 dosage to meet the oxidant demand on a daily basis. This may also require automation and / or remote control for the chlorine dioxide feed equipment. There are two potential control strategies that can be considered for DBP control utilizing ClO 2. These include the following: Optimize use of ClO 2 at the existing feed point. The first option is to increase chlorine dioxide level at raw water. Chlorine dioxide has been shown to oxidize a portion of natural organic matters, and consequently reduce the formation of DBPs. However, the effect of ClO 2 might be limited when applied in raw water, because substances including turbidity and manganese in raw water can compete with organics for chlorine dioxide. So ClO 2 dosage needs to be carefully evaluated for organics removal and chlorite compliance in this option.

6 Relocate ClO 2 feed to top of filters. The second option is to reduce the chlorine dioxide dosage in the raw water to a maintenance level, and to add chlorine dioxide in the settled water instead of chlorine for manganese removal in the filters. Chlorine will be added to clearwell to achieve the required CT credit. This approach is used by the Charleston Water System s Hanahan WTP. Using ClO 2 offers a realistic solution to CMD s manganese problem through direct oxidation and later sequestering and filtration of insoluble products (Gates, 1998). It is recommended that additional evaluation for manganese removal using ClO 2 be conducted to develop a responsive control strategy. Feed location, dosage, and dosage adjustment need to be determined in bench- and pilot-scale testing. This approach can lower the overall dosage of disinfectant and reduce the contact time of water with chlorine. As a result, DBP formation in the system will be reduced. Chlorine. Chlorine is responsible for a majority of the THM/HAA formation in the CMD system. Primary factors in the formation of the DBPs include chlorine concentration, contact Therefore, optimizing chlorination is key to minimizing DBP levels. Currently, chlorine is initially added on top of the filters for manganese control and after the filters (prior to the clearwell) for disinfection credit (i.e., CT). The top of filter (TOF) dosage is recorded as pounds of chlorine per day. Typical setting is approximately 80 lb/day. At a 4.0 MGD production rate, this is equal to a dosage of 2.4 mg/l Cl 2. Control of the TOF dosage is based upon chlorine residual measured at the bottom of the filters (BOF). Online monitoring devices are not installed. Chlorine residual is measured at the bottom of each individual filter by grab sample. The goal is to maintain a minimum residual of 0.4 mg/l with a maximum of mg/l. For disinfection, chlorine is added at the chemical injection tee located between the filters and clearwell. Typical dosage at this feed point ranges between 2.3 and 4.7 mg/l with an average of 3.4 mg/l (based upon MORs for 2011). Control for the dosage is based on chlorine residual measured leaving the clearwell. The chlorine feed has a compound control loop using both flow pacing and residual levels to adjust the dosage. The operational goal for chlorination is to have a residual between 2.2 and 2.8 mg/l free chlorine leaving the clearwell. This is based upon maintaining CT credit as well as maintaining the residual in the distribution system. Data from the MORs (2011) indicate residual levels of free chlorine leaving the clearwell range from 1.2 to 3.0 mg/l with an average of 2.5 mg/l. The efficiency of disinfection with chlorine is impacted by both temperature and ph. Chlorine disinfection is more effective under higher temperature and higher ph values. In addition, residuals tend to dissipate quicker under warmer water conditions. Figure 2 compares the seasonal variation in CT credit achieved at the CMD WTP (August 2011 versus February 2011), and the corresponding chlorine residuals and contact time. The CMD WTP has a higher level of residual in the clearwell during the warmer summer months. The chlorine concentration in the clearwell is based upon the residual at distant locations in the distribution system. Since these locations have difficulty in maintaining a residual, the operators cannot decrease the residual in the clearwell. Based upon discussions with CMD staff, the higher clearwell residual during the summer is typically required to maintain residuals in the distribution system. While the higher levels of CT in the summer mean that the CMD system is well protected from water borne contamination, the higher residual levels increase the risk of DBP violation. The problem is more of a concern in summer than in winter, because higher temperatures also tend to increase the DBP formation rate. One of the two DBP spikes for CMD occurred in August 2011 under these conditions.

7 CT Calc/CT (table) (a) Day August 2011 Chlorine Concentraion (mg/l) Figure 2. (a) CT Achievement from TOF through Clearwell with Chlorination and (b) the corresponding Chlorine Residual in Clearwell (b) Day August 2011, Contact Time = 45 min February 2011, Contact Time = 45 min To address this issue, the best approach is to reduce / minimize the concentration of free chlorine and / or contact time of free chlorine with the DBP precursors (i.e., TOC), especially during the summer months. Options for achieving this objective include the following: Replace the chlorine at top of the filters with chlorine dioxide (as previously discussed). Install baffles in the clearwell to minimize short circuiting and potentially reduce water age in the clearwell. Develop a distribution system optimization program (i.e., Partnership for Safe Water) to help turn over water in storage tanks, reduce water ages, and maintain disinfectant residuals in the system. Chloramines (Ammonia Addition). CMD adds ammonia after the clearwell to form chloramines as a secondary disinfectant to maintain a residual in the distribution system. The advantage of using chloramines is that they are not as reactive as chlorine which allows them to sustain longer in the distribution system. In addition, they do not form as many THMs and HAAs (compared to free chlorine). The ammoniator at the WTP is control based upon flow and residual. The ammonia dosage is set based on a chlorine to ammonia-nitrogen ratio (Cl:NH 3 -N) between 3:1 and 5:1. The ratio is adjusted by operators as necessary. CMD recently initiated a practice of measuring monochloramine, total chlorine, free ammonia, and ph at POE in an effort to maximize production of monochloramine and minimize the amount of free ammonia entering the distribution system. The goal is to maintain the monochloramine concentration above 90 % of the total residual and keep the free ammonia to less than 0.2 mg/l. These efforts will increase the effectiveness of the chloramine disinfectant (monochloramine is the strongest of the chloramines species). More than 90% of the free chlorine is converted to monochloramine after ammonia addition in all samples collected at CMD and a trace amount of free ammonia is maintained to maximize the monochloramine formation. Maintaining the low levels of free ammonia also helps minimize the potential for biofilm growth and nitrification.

8 DBP Formation During the time of the evaluation, the CMD water system was operating under the Stage 1 criteria for compliance with DBP MCLs. A review of historical DBP data for CMD indicates that the system was in compliance with the Stage 1 criteria for the period from 2005 through Two DBP sample events (February 2011; August 2011) during the data period ( ) had quarterly values that exceeded the MCL, but these values did not cause the RAA to exceed the MCL. To evaluate the higher DBP events that occurred in 2011, TTHM / HAA5 were plotted for each Stage 1 sample site (Figure 3). TTHM, ug/l 110 DBP- 1 DBP DBP- 3 DBP TTHM MCL Jan- 05 Jan- 06 Jan- 07 Jan- 08 Dec- 08 Dec- 09 Dec- 10 Dec- 11 (a) HAA 5, ug/l 80 DBP- 1 DBP DBP- 3 DBP- 4 HAA MCL (b) 0 Jan- 05 Jan- 06 Jan- 07 Jan- 08 Dec- 08 Dec- 09 Dec- 10 Dec- 11 Figure 3. Quarterly (a) TTHM and (b) HAA5 values for Stage 1 DBP Sites ( ) February As indicated in Figure 3, the THM levels for DBP Site 2 (103 µg/l) were much higher than the other Stage 1 sample sites (< 70 µg/l). The HAA levels were higher than average, but did not exceed the MCL. Based on the MOR for February 2011, the plant was practicing secondary disinfection with free chlorine (part of system burnout practice) until two days before the DBP samples were collected by SC DHEC. In the initial distribution system evaluation (IDSE) Sampling Plan for CMD, the water age for DBP Site 2 is estimated at greater than 5 days. Therefore, the DBP sample for Site 2 would have been collected while the system was disinfecting with free chlorine instead of chloramines. This would account for the high levels of DBPs in the distribution system. Based upon the TTHM results, the water age at DBP Site 2 appears to have been higher than the other sites at the time of sampling. To avoid this type of excursion, it is recommended that CMD increase the coordination and communication between the WTP and the crews that are flushing the distribution system so that the system burnout is not occurring within two weeks of the sample date for DBPs. CMD WTP staff should

9 stay in contact with SC DHEC personnel to maintain an updated calendar of when DBP samples will be collected in the system. August For the August 2011 occurrence, several sites exceeded the MCLs for either THMs or HAAs. The MCL for THMs was exceeded at DBP Site 3 (86.2 µg/l) while the MCL for HAAs was exceeded at DBP Site 1 and DBP Site 2 (76.8 µg/l and 65.4µg/L, respectively). For this episode, the cause is not as clear as with the February episode. It appears that the higher levels may have been formed by an unfortunate combination of events. First, the potential to form DBPs during the warmer months is higher due to the faster formation rate with higher water temperatures. August is typically the warmest month of the year. The average temperature was C for that month in Second, a review of the 2011 MORs indicates that the disinfection levels were higher than average during the period just prior to the collection of the DBP compliance sample by SC DHEC on August 18, Third, both color and turbidity increased in the raw water to above average levels during the period from August which means that during that period, the potential for elevated levels of precursors was high. The estimated water age at DBP Site 1 and 2 is greater than 7 days under average demand conditions. During August, the demand is typically higher than average so it is feasible that the water age at the sample sites was lower. If the water age at the sample sites coincided with the combination of high precursor levels and higher than average disinfectant dosage in warm weather, this would explain the DBP levels that exceeded the MCL for the quarterly values. To avoid this type of DBP excursion, CMD should increase monitoring of DBP precursors (i.e., UV 254 ) in the raw water on a daily basis so that staff knows when the system is at risk of higher formation. During this period, efforts should be made to increase preoxidation (i.e., chlorine dioxide) and minimize chlorine dosage, if possible. For Stage 2 of the DBP Rule, the compliance calculation for the MCL will switch from the running annual average (RAA) format to a locational running annual average (LRAA). This means that CMD will have to meet the RAA for each sample site rather than have the values averaged for the entire system. Evaluation of the Stage 1 data using the criteria for Stage 2 indicated that none of the samples exceeded the LRAA MCL set by the Stage 2 DBP rule. However, as indicated by the results from August 2011, the system is at risk of exceeding the quarterly levels. CMD should take steps to minimize this risk. To provide a point of reference, the quarterly average DBP levels for CMD were compared with those for two other water treatment plants that withdraw from the Catawba River. Review of the data indicates that the other systems using the same water source also experienced a similar spike during the August 2011 period, but the February 2011 episode was unique to CMD. This would support the observation that the August 2011 event was a function of raw water quality conditions. CONCLUSIONS No increasing trend of DBP levels over time was observed. Instead, two isolated episodes (February & August 2011) of high DBPs occurred in the system. One episode was explained by the annual burnout practice (February 2011) being conducted at the same time DBP samples were collected by SC DHEC. The other episode (August 2011) was attributed to increased precursor levels in the raw water (caused by heavy rainfall) during optimum conditions for DBP formation (i.e., warm weather and higher chlorine concentrations). These conditions occurred just prior to the sample collection by SC DHEC. CMD should continue to monitor information relative to DBP precursors as frequently as feasible. Rainfall events can adversely impact the raw water quality by increasing turbidity and organics. Operators need to pay special attention to treatment during these events. Collect river water upstream and downstream of the industrial discharge can help CMD evaluate the potential impact of the wastewater stream on raw water quality.

10 The CMD WTP is meeting the SC DHEC s requirements for TOC removal but the WTP may not be fully optimized for TOC removal. More jar tests during rainfall events could help CMD better determine the coagulant dosage in response to changes in raw water quality. CMD should evaluate the possibility of change chlorine dioxide dosage based upon oxidant demand in the raw water. This should allow for maximizing the impact of chlorine dioxide without exceeding the MCL for chlorite in the finished water. It is recommended that CMD conduct a DBP profile through the WTP at settled water, bottom of the filters, and end of the clearwell. The results will answer if relocating of chlorine dioxide on top of the filters instead of free chlorine helps reduce the formation of DBPs. Increasing coordination/communication between treatment staff and distribution staff is important. This is especially critical during the burnout period. CMD treatment staff should always know when SC DHEC is scheduled to sample for DBP compliance and communicate this with distribution staff to appropriately coordinate timing of the burnout. The burnout should not be conducted within two weeks prior to the scheduled sampling events. It is also recommended to the distribution staff to develop a distribution system optimization program to help turn over water in Sstorage tanks, reduce water ages, and maintain disinfectant residuals in the system. Therefore, the operators do not need to increase chlorine doses in the clearwell in summer to maintain residuals in the distribution system, and reduce the risk of DBP spikes. ACKNOWLEDGEMENT Laboratory and operating data were provided by Mr. Jak Krehan, retired Water Treatment Superintendent and Mr. David Sloan Water Treatment Plant Superintendent, for the Chester Metropolitan District. REFERENCES Gates Don The Chlorine Dioxide Handbook, American Water Works Association, Denver, CO.