RECLAIMED WATER DISINFECTION ALTERNATIVES TO AVOID NDMA AND THM FORMATION

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1 RECLAIMED WATER DISINFECTION ALTERNATIVES TO AVOID NDMA AND THM FORMATION Shiaw-Jy Huitric, Jeff Kuo, Michael Creel, Chi-Chung Tang, Dave Snyder, Robert Horvath, James Stahl Sanitation Districts of Los Angeles County ABSTRACT The Sanitation Districts of Los Angeles County (Districts) operate seven tertiary water reclamation plants (WRPs) with a combined treatment capacity over 200 million gallons per day (MGD). Chloramination is used at these WRPs for effluent disinfection. It was recently discovered that chloramination results in the formation of N-nitrosodimethylamine (NDMA), a compound with high carcinogenic potency. NDMA is formed when chloramines react with dimethylamine (DMA) in the water. Sources of DMA included polymer used as a settling aid in the activated sludge process. To minimize NDMA formation from the chloramination disinfection process, the Districts evaluated two alternatives adapted from the existing practice. These alternatives were breakpoint chlorination while using the existing DMA containing polymer and chloramination while using emulsion polymers that do not contain DMA. Both laboratory and full-scale tests were conducted to evaluate disinfection efficacy and formation of NDMA and trihalomethanes (THM) with these alternatives. Breakpoint chlorination effectively inactivated total coliform and generated insignificant amounts of NDMA. However, it generated higher levels of THM than chloramination. THM generation was directly related to chlorine residual and contact time. The results suggested that breakpoint chlorination could effectively inactivate total coliform while meeting the drinking water standard. Chloramination with the use of emulsion polymers to enhance mixed liquor settling produced much lower NDMA levels than when Mannich polymer was used. However, emulsion polymers were less effective than the Mannich polymer as a settling aid. Depending on water reuse applications, these two approaches are viable options to manage effluent NDMA and THM concentrations. KEYWORDS Chloramination, breakpoint chlorination, NDMA, THM, Mannich polymer, emulsion polymer INTRODUCTION The Sanitation Districts of Los Angeles County (Districts) operate seven tertiary water reclamation plants (WRPs), with a combined treatment capacity over 200 million gallons per day 4397

2 (MGD), in the Los Angeles County. Approximately 65 MGD of reclaimed effluent is reused for a variety of applications including groundwater replenishment, landscape and agricultural irrigation, wildlife habitat maintenance, and industrial process water supply. Among these various applications, groundwater replenishment is most significant and represents approximately half of the total reuse amount. Reclaimed water used for groundwater replenishment and applications described above is required to meet the drinking water standards set by the California Department of Health Services (DHS) in the California Code of Regulations Title 22 (California Title 22). Typical processes employed at the Districts tertiary WRPs include primary sedimentation, activated sludge with biological nitrogen removal, secondary clarification, media filtration, disinfection with chloramination, and dechlorination (see Figure 1). Sludge produced from the WRP is usually transported to a regional plant for treatment and disposal. A Mannich type cationic polymer is often added to the return activated sludge or to the mixed liquor entering the secondary clarifiers to enhance sludge settling and for foam control. Mannich polymer has a polyacrylamide/methyl amine formulation with dimethylamine (DMA) and formaldehyde added to activate the Mannich process and to increase the chain length. The polymer is delivered as a solution flocculent and is diluted with chlorinated final effluent before use. Depending on needs, the polymer dose may vary from approximately 0.4 to 2 mg/l. Figure 1 Typical Treatment Processes at the Districts Tertiary Water Reclamation Plant 4398

3 Chloramination is used for disinfection because chloramines produce lower levels of trihalomethanes (THM) than free chlorine (Kuo et al., 2003). Low levels, 0.5 to 1.5 mg/l, of ammonia are added to fully nitrified secondary effluent to form chloramines. For unrestricted reuse of reclaimed wastewater, the California Title 22 requires a seven-day median of less than 2.2 MPN/0.1 L for total coliform. Chloramination has consistently met this requirement at all Districts WRPs. Typical total THM levels in effluent are lower than 20 μg/l. The California drinking water standard for total THM is 80 μg/l. While chloramination effectively controls the formation of THM, it was recently discovered that chloramines are precursors to nitrosamines, a group of organic compounds with high carcinogenic potency. The nitrosamine receiving the greatest attention is N- nitrosodimethylamine (NDMA). The California DHS has set a notification level of 10 nanograms per liter (ng/l) for NDMA (for drinking water, this level requires notification of governing bodies; the response level is 20 times higher). NDMA is formed during the disinfection process when chloramines react with DMA in the water (Mitch and Sedlak, 2002; Choi and Valentine, 2002; Schreiber and Mitch, 2005). DMA can be of both domestic and industrial origins, and is present in raw sewage (Mitch and Sedlak, 2004; Sedlak et al., 2005). Although biodegradable in the activated sludge process, low levels of DMA remain in the secondary effluent. Additional DMA is added to secondary effluent when Mannich polymer is used to enhance settling and to control foaming. A study conducted by the Districts confirmed NDMA formation from the chloramination process at their WRPs (Huitric et al., 2005). Although there is no federal or California drinking water standard for NDMA at present, the Districts decided to proactively pursue prevention of NDMA formation from the existing disinfection process. Since 2004, the Districts have evaluated three alternatives to minimize NDMA formation at their WRPs. These alternatives include using ultraviolet (UV) irradiation for disinfection, breakpoint chlorination with existing Mannich polymer, and chloramination with DMA free polymers. Results from UV testing and its effect on NDMA were previously presented (Jalali et al., 2005). This article focuses on the Districts efforts on the other two alternatives to minimize NDMA formation. OBJECTIVES The objective of this study was to evaluate disinfection alternatives that minimize NDMA and THM formation. These alternatives, unlike UV, would not require significant change of the existing disinfection practice and infrastructure. If successfully demonstrated, these alternatives may be implemented as interim disinfection measures to allow the Districts to evaluate the performance of UV which will be installed at one of the Districts WRPs to replace chloramination. Two disinfection alternatives were identified for study. The first alternative is breakpoint chlorination, or the use of free chlorine for disinfection. Because the Districts WRPs practice biological nitrogen removal, breakpoint chlorination can be readily implemented by simply shutting off ammonia addition to the secondary effluent. No change is necessary on the use of Mannich polymer to enhance mixed liquor settling. The objectives of this testing were (1) to 4399

4 determine the disinfection efficacy of breakpoint chlorination; (2) to evaluate NDMA and THM formation; and (3) to gain operational experience with this practice. The second disinfection alternative investigated was to continue chloramination, but using alternative polymers that do not contain DMA, one of the NDMA precursors. The objectives of this testing were: (1) to determine the effect of alternative polymers on NDMA formation during chloramination; (2) to evaluate the mixed liquor settling performance when the alternative polymers were used; and (3) to gain operational experience with the alternative polymers. METHODOLOGIES Breakpoint Chlorination The Districts first conducted laboratory testing using fully nitrified secondary effluent samples from one of the WRPs to evaluate the effect of free chlorine and chloramines on NDMA formation. Different amount of chloramines and free chlorine (in the form of sodium hypochlorite), with and without Mannich polymer, were added to these samples. Following two hours of chlorine contact, the chlorine residual of each sample was measured. The samples were then dechlorinated and analyzed for NDMA. The laboratory experiments were repeated on five different occasions. Based on the positive results from using free chlorine in the laboratory testing, the Districts determined full-scale testing was warranted to verify the laboratory study findings. Full-scale breakpoint chlorination was implemented at two tertiary WRPs, the San Jose Creek West Water Reclamation Plant (SJCWWRP) and the Whittier Narrows Water Reclamation Plant (WNWRP), between April and October The SJCWWRP treats an average flow rate of approximately 30 MGD. Daily flow rate fluctuates between 10 and 40 MGD. The step-feed activated sludge process is used for biological nitrogen removal. The WNWRP treats an average daily flow rate of approximately 8 MGD. Flow rate treated at this plant is relatively constant. The plant employs the modified Ludzack-Ettinger process to remove nitrogen. Both plants produce secondary effluent that is essentially free of ammonia nitrogen (<1 mg N/L). Mannich polymer is used at both plants for foam control and to enhance mixed liquor settling. Polymer is usually added to the mixed liquor effluent channel. Testing of breakpoint chlorination involved suspension of ammonia addition to the secondary effluent and adding chlorine both before and after the media filters. Chlorine dose was controlled by two chlorine residual set points, one before the filters and the other after. The chlorine residual set point before the filters (named pre-chlorination set point) controlled chlorine concentration of the filter influent, while the chlorine residual set point after the filters (named post-chlorination set point) controlled chlorine concentration of the filter effluent, or influent to the chlorine contact tanks. The chlorine residual set points were used to control biofouling on filter media and to maintain chlorine residual levels between 3 and 4 mg/l in the chlorine contact tank effluent, or final effluent. Past experience, based on chloramination, indicated that these chlorine residual levels provided effective disinfection based on the total coliform concentration monitoring results. In addition, these chlorine residual levels were required to meet the 4400

5 reclaimed water disinfection requirement specified in California Title 22 regulations. These requirements specify a minimum CT (chlorine residual concentration multiplied by modal contact time in the chlorine contact tanks) value of 450 mg-minute/l. During the study, secondary effluent and final effluent grab samples were collected at least twice a day at these plants. The secondary effluent samples were analyzed for NDMA, and the final effluent samples were analyzed for NDMA, total coliform, and THM. Final effluent samples were usually collected approximately three hours after the secondary effluent samples to account for the hydraulic retention time in the filters and chlorine contact tanks. This allowed the NDMA concentrations of the same slug of water to be compared. Due to diurnal flow fluctuation experienced at the SJCWWRP, the Districts conducted a one-day hourly sampling event of the final effluent. These samples were analyzed to determine THM fluctuation during the day. The Districts also sampled an outfall location on the same day to compare total THM concentrations in the final effluent and at the outfall location. The outfall location is approximately 150 feet downstream of the final effluent sampling location. There is a 15-foot hydraulic drop of the flow at this location where the plant effluent is discharged offsite. The purpose of sampling the outfall location was to determine the effect of the hydraulic drop on the THM concentrations and the actual total THM concentrations in the plant discharge. During full-scale breakpoint chlorination testing at WNWRP, the Districts also conducted a laboratory study to determine the effect of CT value on THM formation and disinfection efficacy. Filtered effluent samples were collected and chlorine residual concentrations were measured at pre-determined contact times. The samples were then dechlorinated and measured for THM, total coliform, and, in some cases, indigenous coliphage. This exercise was repeated five times to cover a CT range between 40 and 400 mg-minute/l. Chloramination with Alternative Polymers The Districts began this evaluation with laboratory testing of emulsion polymers. Emulsion polymers were tested because (1) they do not contain the NDMA precursor DMA as an additive; (2) they can be applied using the existing polymer feed system; and (3) jar test results showed that these polymers enhance mixed liquor settling. A total of six emulsion polymers (three from SFN Polydyne and three from Degussa) were selected for laboratory NDMA formation potential evaluation. All emulsion polymers tested contained high molecular weight (-10 7 g/mole) and high cationic charges. A polymer concentration of 1.5 mg/l (based on active ingredient) was used in all laboratory tests. Deionized (D.I.) water and permeate from a membrane bioreactor (MBR) pilot plant treating primary effluent at the WNWRP were used as testing matrix. The MBR permeate sample was used in lieu of plant secondary effluent sample because it did not contain any Mannich polymer. To assess the effect of polymers on NDMA formation, the Districts laboratories conducted NDMA formation potential analysis following established procedures (Mitch et al., 2003). The formation potential analysis involved using relatively high dose of chlorine and long contact time to convert the organic NDMA precursors to products. In the laboratory study, chlorine was 4401

6 dosed at approximately 40 mg/l, and the contact time was set to be 3 hours. Chlorination was applied in three different forms: pre-formed chloramines, ammonia followed by sodium hypochlorite, and free chlorine. The samples were dechlorinated following the three-hour contact time, and analyzed for NDMA. The experiment was conducted five times. Full-scale testing of alternative polymers was conducted at WNWRP and Long Beach Water Reclamation Plant (LBWRP) following laboratory testing. The LBWRP treats an average daily flow rate of 20 MGD (daily flow rate ranges from 10 to 30 MGD). The plant employs the stepfeed activated sludge process for biological nitrogen removal; secondary effluent ammonia concentration is usually below 1 mg-n/l. A total of three emulsion polymers were tested at these two facilities. During the study, each emulsion polymer was used for a minimum of 10 days. Polymer doses ranged from 0.4 to 0.8 mg/l at WNWRP, and from approximately 1.5 to 2 mg/l at LBWRP. These doses were determined based on jar test results and from past experience when Mannich polymer was used. For NDMA analysis, both grab and 24-hour time composite secondary effluent and chlorinated final effluent samples were collected from these facilities. In addition, the mixed liquor settling performance was evaluated when emulsion polymers were used. The secondary effluent quality was visually inspected and compared with that when Mannich polymer was used. Filter performance, in terms of backwash frequency, was also noted to determine any effect due to change in secondary effluent water quality. Sample Collection and Analysis Glass amber jugs and glass clear vials were used for the collection of NDMA and THMs samples, respectively. Sterilized plastic containers were used for total coliform samples. Dechlorination of the NDMA samples was performed directly in the sample containers using sodium thiosulfate. Because of the variation in chlorine residual concentrations in the final effluent, the samples for THM analysis were first quantitatively dechlorinated and then poured into the sample vials. The quantitative dechlorination was necessary because over dechlorination would result in damage in the analytical instrument. All laboratory analyses were conducted by the Districts San Jose Creek Water Quality Laboratories (SJCWQL) which are certified by California DHS for the analyses performed in this study except for NDMA. Currently, there is no federal or California approved analytical method to analyze NDMA concentrations to nanogram per liter range. The SJCWQL analyzes NDMA using liquid-liquid extraction followed by chemical ionization isotope dilution gas chromatography/mass spectrometry. This method meets performance-based guidelines established by the California DHS. The reporting limit for NDMA for secondary and final effluent samples is 2 ng/l. EPA method 8260 was used for THM analysis. Four halogenated compounds, chloroform, bromoform, bromodichloromethane, and dibromochloromethane, were analyzed and reported individually. The reporting limit for each THM is 2 μg/l. Total THM concentration is the sum of the concentrations of the four THM. 4402

7 Standard Method 9222 B, standard total coliform membrane filter (MF) procedure, was used for total coliform analysis. The MF method was chosen because membrane filter technique is highly reproducible and usually yields numerical results more rapidly than the multiple-tube fermentation procedure (American Public Health Association, 1998). The MF method is appropriate for chlorinated filtered effluent samples collected in this study because the sample turbidity value is typically below 1 NTU. The outcome of this method is a direct count of coliform colonies and is comparable to the statistic probability produced by the multiple-tube fermentation procedure. The detection limit is 1 colony forming unit (CFU)/0.1 L. For indigenous coliphage, the Districts SJCWQL adapted the analytical procedures described in EPA Method 1601 (USEPA, 2001) for male-specific and somatic coliphage. Method 1601 was validated as a qualitative, presence-absence method. However, the method allowed for adaptation to quantitatively determine coliphage concentrations in an MPN format. The MPN format is the protocol implemented at the Districts SJCWQL. RESULTS AND DISCUSSION Breakpoint Chlorination Table 1 summarizes the results of laboratory tests comparing NDMA formation from free chlorine, chloramines (ammonia plus chlorine), and pre-formed chloramines. The results confirmed that a significant amount of NDMA was formed when chloramines were present with the Mannich polymer. Free chlorine appeared to have an insignificant effect on NDMA formation when there was no Mannich polymer in the secondary effluent. When Mannich polymer was present, NDMA formation from free chlorine was observed, but the amount was much less than that from chloramines (pre-formed chloramines or ammonia plus chlorine). These preliminary results prompted the Districts to embark on full-scale breakpoint chlorination testing, first at SJCWWRP (April to May and July to August, 2005) and then at WNWRP (October to November, 2005). Table 2 summarizes chlorine residual set points, average chlorine dosage, and daily sampling time during the full-scale studies conducted at these plants. When the study first started at SJCWWRP, chlorine residual was set at relatively high levels (9 mg/l before the filters and 7 mg/l after) to ensure that total coliform concentration in the chlorinated final effluent would be in compliance with the permit requirement for reuse applications. These chlorine residual set points were lowered as data indicated that the amount of chlorine added was sufficient for disinfection purposes, as evidenced by the lack of total coliform detection (<2 CFU/0.1 L). At both plants, it was found that a pre-chlorination residual set point between 2.0 and 2.5 mg/l, and a post-chlorination residual set point between 4.0 and 4.5 mg/l were adequate for keeping the total coliform levels in check. Lower chlorine residual set point values resulted in lower chlorine demand and reduced chlorine usage, as indicated in Table 2. Breakpoint chlorination at SJCWWRP used approximately 30% more chlorine than when chloramination was practiced. At WNWRP, the chlorine dosage increased about 50%. Throughout the entire study at both SJCWWRP and WNWRP, total coliform concentration in the final effluent was maintained at levels below the limit specified in California Title 22, for unrestricted reuse. 4403

8 Table 1 NDMA Formation from Free Chlorine, Chloramines, and Pre-formed Chloramines, with and without Mannich Polymer Addition Sample Description NDMA Concentration (ng/l) Nitrified Secondary Effluent Chloramines: 2 mg/l NH mg/l Cl (250) 15 (480) 21 (190) Chloramines: 2 mg/l NH mg/l Cl 2 76 (17,000) 18 (860) Pre-formed Chloramines: 5 mg/l (3,700) 56 (890) 36 (540) Pre-formed Chloramines: 10 mg/l 8.6 (1,500) 95 (>10,000) 380 (>10,000) 120 (580) 77 (3,100) Free Chlorine: 5 mg/l (460) 15 (230) 19 (74) Free Chlorine: 10 mg/l 8.5 (4,500) 20 (850) Note: All samples had chlorine contact time of 2 hours. Values in parentheses represent samples dosed with 1.5 mg/l of Mannich polymer. Table 2 Full-scale Breakpoint Chlorination Study: Operating Conditions and Sampling Time SJCWWRP SJCWWRP WNWRP Study Period April May, 2005 July August, 2005 Oct. Nov., 2005 Pre-chlorination Set Point (mg Cl 2 /L) Post-chlorination Set Point (mg Cl 2 /L) Average Chlorine Dosage (mg Cl 2 /L) Secondary Effluent Sampling Time 6:30 a.m. & 9:30 a.m. 6:30 a.m. & 9:30 a.m. 7:30 a.m. & 9:30 a.m. Final Effluent Sampling Time 9:30 a.m. & 12:30 p.m. 9:30 a.m. & 12:30 p.m. 10:30 a.m. & 12:30 p.m. Figure 2 depicts the NDMA concentrations in the secondary effluent samples and the corresponding final effluent samples collected at SJCWWRP and WNWRP during the breakpoint chlorination study. The majority of the points are scattered around the 1:1 slope line, indicating that breakpoint chlorination has insignificant effect on NDMA formation despite the continued use of Mannich polymer at both plants during the study. In contrast, typical NDMA concentrations in the effluent at WNWRP and SJCWWRP are approximately 400 and 1,500 ng/l, respectively, after chloramination. 4404

9 Figure 2 - NDMA Concentration in Secondary Effluent and Chlorinated Final Effluent Samples During Breakpoint Chlorination Study Final Effluent NDMA Concentration (ng/l) Secondary Effluent NDMA Concentration (ng/l) Whittier Narrows WRP San Jose Creek West WRP The main concern with breakpoint chlorination is the generation of THM. Figure 3 shows the total THM levels in the two final effluent samples collected each day at SJCWWRP. The samples collected at 9:30 a.m. typically had higher total THM levels than the samples collected at 12:30 p.m. This is because of the diurnal flow pattern experienced at this plant. Samples collected at 9:30 a.m. represented flow that entered the chlorine contact tanks at approximately 4 a.m. Flow rate was at its lowest value during these early morning hours. The chlorine contact time for this flow was about five hours making the corresponding CT value greater than 900 mgminute/l. The CT values for samples collected at 12:30 p.m., on the other hand, were approximately 480 mg-minute/l. As expected, larger CT values resulted in higher THM levels. Figure 4 shows the total THM results from the 24-hour sampling event during which hourly final effluent and outfall samples were collected. Except for the morning hours, when the discharged final effluent had a long contact time (approximately five hours) with chlorine, total THM levels in the effluent were all below the drinking water standard of 80 μg/l. At the outfall location, the total THM levels were, on the average 15%, lower than the levels in the final effluent leaving the chlorine contact tanks, and were all below the drinking water standard. Figure 5 presents the total THM results in the final effluent samples from the WNWRP study. There was not a pronounced difference in total THM concentrations in the samples collected at 10:30 a.m. and those collected at 12:30 p.m. This is because the flow rate was relatively constant at this plant. When the study began, the plant operated two chlorine contact tanks. Average contact time was over 200 minutes, and the corresponding CT values were in the 700 to 4405

10 Figure 3 - Total THM Concentration - SJCWWRP Breakpoint Chlorination 140 Total THM Concentration (ug/l) Drinking Water Standard Days 9:30 a.m. 12:30 p.m. Figure 4 - SJCWWRP Breakpoint Chlorination -Total THM Levels in Final Effluent and Outfall Total THM Concentration (ug/l) Drinking Water Standard 6:30 8:30 10:30 12:30 14:30 16:30 Hours 18:30 20:30 22:30 0:30 2:30 4:30 Final Effluent Outfall 4406

11 900 mg-minute/l ranges. These high CT values resulted in high total THM concentrations, 100 to 130 μg/l, in the final effluent. Recognizing that these high CT values might be overly protective, based on the lack of total coliform detection, the Districts took one of the chlorine contact tanks out of service in the middle of the study. The total THM concentrations subsequently decreased to the 80 to 100 μg/l range, while total coliform concentrations remained below the Title 22 limit for unrestricted reuse. Figure 5 - Total THM Concentration - WNWRP Breakpoint Chlorination Total THM Concentration (ug/l) Drinking Water Standard Days 10:30 a.m. 12:30 p.m. The effect of CT value on THM formation was investigated in a laboratory study. Postchlorinated filtered effluent samples were collected and chlorine residuals were measured at predetermined contact times in the laboratory. The samples were then dechlorinated and analyzed for THM, total and fecal coliform, and, in some cases, indigenous coliphage. None of the samples had confirmed total coliform detection above the California Title 22 limit, and fecal coliform and indigenous coliphage were all below 1 CFU/0.1 L and 2 MPN/0.1 L, respectively. Figure 6 plots the total THM concentration against the CT value for both the field and laboratory samples. The data clearly showed that total THM concentration increased with CT value, and that field and laboratory results were compatible. For this WRP, total THM concentrations would be below 80 μg/l if CT values were below approximately 300 mg-minute/l. Laboratory study indicated that effective disinfection, based on coliform data, could be achieved with breakpoint chlorination at CT values as low as 40 mg-minute/l. Current California regulations on wastewater disinfection require a minimum CT value of 450 mg-minute/l. This requirement was developed based on using chloramines, rather than free chlorine, for disinfection. The results from the current study suggested that this requirement could be overly stringent for breakpoint chlorination, and would produce more THM. 4407

12 Breakpoint chlorination could offer equivalent protection of public health to chloramination, at lower CT values than 450 mg-minute/l, because free chlorine is more effective than chloramines for pathogen inactivation. At lower CT values, breakpoint chlorination would produce less THM and potentially capable of meeting the drinking water standard for total THM. Figure 6 - WNWRP Breakpoint Chlorination - Total THM Concentration vs. CT Values Total THM Concentration (ug/l) Drinking Water Standard CT (mg-minute/l) Field Laboratory Chloramination with Alternative Polymers Results from the laboratory NDMA formation potential study using different polymers are summarized in Table 3. As mentioned previously, DMA is an essential ingredient in Mannich polymer formulation. Consistent with previous findings (Mitch and Sedlak, 2004), the highest NDMA formation potential occurred when chloramines and DMA (in this study, Mannich polymer) were both present in the samples. On the other hand, emulsion polymers appeared to have generated insignificant amount of NDMA. NDMA formation potential from different emulsion polymers was similar. Based on the encouraging laboratory results, the Districts implemented full-scale testing at WNWRP from December 2005 to January 2006, and at LBWRP from February to March Three emulsion polymers, one supplied by Degussa and two supplied by SFN Polydyne were tested. Polymer doses ranged from 0.4 to 0.8 mg/l at WNWRP, and 1.5 to 2.0 mg/l at LBWRP. More than 120 sets of secondary and corresponding chlorinated final effluent samples were collected and analyzed for NDMA. Table 4 summarizes the median NDMA concentrations in the samples and the median increase in NDMA concentrations (NDMA formation) at WNWRP. Historical monitoring results, when Mannich polymer was used at 0.2 and 0.4 mg/l, are also listed for comparison. As shown in 4408

13 Table 4, NDMA levels increased when no polymer was used. This is believed to be due to chloramines reacting with DMA remaining in the secondary effluent. Use of Mannich polymer resulted in NDMA formation as more NDMA was formed with higher dose. Use of emulsion polymers also resulted in NDMA formation, but the extent was much less than that caused by the Mannich polymer at the same polymer dosage. NDMA formation appeared little changed with higher dose of emulsion polymer. The amount of NDMA increase from using the Degussa #1 emulsion polymer (70 ng/l) was about the same as that when no polymer was used (60 ng/l). Table 3 NDMA Formation Potential from Mannich and Emulsion Polymers D.I. Water NDMA Formation Potential (ng/l) No Polymer Mannich Polymer Emulsion (SFN Polydyne ) Polymers (SFN Polydyne ) Emulsion Polymers (Degussa) Free Chlorine <4 < 4 Pre-formed Chloramines - 96, , < 4 10 MBR Free Chlorine < < Permeate Pre-formed Chloramines , , Ammonia/Hypochlorite ,000 1,800, Table 4 -Median NDMA Concentrations and Formation from Polymers WNWRP Secondary Effluent (ng/l) Chloraminated Final Effluent (ng/l) NDMA Formation (ng/l) No Polymer Mannich Polymer, 0.2 mg/l Mannich Polymer, 0.4 mg/l SFN Polydyne Emulsion Polymer # 1, mg/l SFN Polydyne Emulsion Polymer # 1, mg/l Degussa Emulsion Polymer # 1, 0.4 mg/l Table 5 summarizes the results from the LBWRP testing. At this plant, typical polymer dose is between 1.5 and 2 mg/l. Table 5 indicates that Mannich polymer resulted in NDMA concentration increase over 1,000 ng/l, while the two emulsion polymers resulted in much smaller increase (approximately one fourth of the amount generated by the Mannich polymer). 4409

14 Table 5 - Median NDMA Concentrations and Formation from Polymers LBWRP Mannich Polymer mg/l Degussa Emulsion Polymer # mg/l SFN Polydyne Emulsion Polymer # mg/l Secondary Effluent (ng/l) Chloraminated Final Effluent (ng/l) NDMA Formation (ng/l) 235 1,850 1, Figure 7 is a graphical summary (Box-and-Whisker plot) of NDMA formation (differences in NDMA concentrations between chloraminated final effluent and secondary effluent samples) data. As shown in the figure, the use of emulsion polymers resulted in much lower NDMA formation at LBWRP. The same conclusion can be drawn at WNWRP, but the effect was less dramatic. This could be attributable to the lower polymer dose used at WNWRP. Among the emulsion polymers, there appeared to be little difference in NDMA formation potential. Figure 7 NDMA Formation from Different Types of Polymers 3000 NDMA Formation (ng/l) Degussa 1 (WNWRP) Polydyne 1 (WNWRP) Mannich (WNWRP) Degussa 1 (LBWRP) Polydyne 2 (LBWRP) Mannich (LBWRP) Although emulsion polymers helped to significantly reduce the NDMA levels in the chloraminated effluent, they performed less effectively as a settling aid at LBWRP. Even though the optimal polymer dosage was determined from jar tests, and the jar test results showed little difference in settling performance between the Mannich and emulsion polymers, this was not the case in full-scale testing. Operational issues such as pin floc formation and floc carry-over were observed during full-scale emulsion polymer testing at LBWRP. The degraded settling performance resulted in higher head loss build-up in the media filters and more frequent filter 4410

15 backwash. This problem was not observed at WNWRP, which does not rely heavily on using polymer to enhance mixed liquor settling. SUMMARY AND CONCLUSIONS In this study, the Districts investigated two alternative modifications from the existing chloramination practice to minimize NDMA formation. Breakpoint chlorination could be readily implemented at the Districts WRP which practice biological nitrogen removal. Breakpoint chlorination effectively inactivated total coliform and did not generate much NDMA. However, it generated higher levels of THM than chloramination. THM generation was directly related to the CT value. Laboratory studies showed that at CT values lower than 300 mg-minute/l, breakpoint chlorination would effectively inactivate total coliform while keeping the total THM levels below the drinking water standard. Chloramination while using emulsion polymers resulted in significant NDMA reduction from the current levels achieved with Mannich polymer. THM would not be a concern with this approach. The effectiveness of emulsion polymers as a settling aid was a concern; further evaluation and optimization is necessary. Depending on water reuse applications, these two alternatives may serve as acceptable solutions for managing effluent NDMA and THM concentrations. ACKNOWLEDGEMENT The authors are grateful for the contribution made by the following Districts coworkers during this study: Maria Pang, Dwayne Fischer, Paul Prestia, Brian Villacorta, and Ramon Gonzalez. REFERENCES American Public Health Association (1998) Standard Methods for the Examination of Water and Wastewater, 20 th ed.. Washington, DC. Choi, J. H.; Valentine, R. L. (2002) Formation of N-nitrosodimethylamine (NDMA) from Reactions of Monochloramine: a New Disinfection By-product, Water Res., 36, Huitric, S-J.; Kuo J., Tang C-C.; Creel M., Horvath, R.W.; Stahl, J.F. (2005) Fate of NDMA in Tertiary Water Reclamation Plants, Proc. Technology nd Joint Specialty Conference for Sustainable Management of Water Quality Systems for the 21st Century, San Francisco, California, August Jalali, Y.; Huitric, S-J.; Kuo, J.; Tang, C-C.; Garcia, A.; Thompson, S.; Horvath, R. W.; Stahl, J. F. (2005) A Large-Scale UV Pilot-Plant Study: Tertiary Effluent Disinfection and Effect on NDMA and Cyanide, Proc. WEFTEC th Annual Technical Exhibition and Conference, Washington, D.C., October 29 November

16 Kuo, J.; Stahl, J.; Burton, D.; El Jack, Z.; Horvath, R.; and Tang, C. (2003) Chloramination of N/DN Effluent - Meeting the Ammonia, Coliform And THM Limits, Proc. WEFTEC 2003 Water Environment Federation, Los Angeles, CA, October Mitch, W. A.; Sedlak, D. L. (2002) Factors Affecting the Formation of NDMA During Chlorination, Environ. Sci. Technol., 36, Mitch, W. A.; Gerecke, A.C.; Sedlak, D.L. (2003) A N-Nitrosodimethylamine (NDMA) Precursor Analysis for Chlorination of Water and Wastewater, Water Res., 37, Mitch, W. A.; Sedlak, D. L. (2004) Characterization and Fate of NDMA Precursors in Municipal Wastewater Treatment Plans, Environ. Sci. Technol., 38, Schreiber, I. M.; Mitch, W.A. (2005) Influence of the Order of Reagent Addition on NDMA formation during Chloramination, Environ. Sci. Technol., 39, Sedlak, D. L.; Deeb, R. A.; Hawley, E. L.; Mitch, W. A.; Durbin, T. D.; Mowbray, S.; Carr, S. (2005) Sources and Fate of Nitrosodimethylamine and its Precursors in Municipal Wastewater Treatment Plants, Water Environ. Res., 77, United States Environmental Protection Agency (USEPA) (2001), Method 1601: Male-specific (F + ) and Somatic Coliphage in Water by Two-step Enrichment Procedure, Washington, D.C. 4412