Establishing minimum free chlorine residual ...

© IWA Publishing 2014 491

Water Practice & Technology Vol 9 No 4 doi: 10.2166/wpt.2014.055

Establishing minimum free chlorine residual concentration for microbial control in a municipal drinking water distribution system Jennie L. Randa, Graham A. Gagnonb and Alisha Knowlesc a Corresponding author. Ivan Curry School of Engineering, Acadia University, Wolfville, NS, Canada B4P 2R6 E-mail: [email protected] b

Department of Civil and Resource Engineering, Dalhousie University, Halifax, NS, Canada B3H 4R2

c

Water Quality, Halifax Water, Halifax, NS, Canada B3K 5M1

Abstract Distribution system data from a Nova Scotia municipal drinking water supply was collected over four years, including free chlorine residual concentration, heterotrophic plate count (HPC) bacteria, and temperature. These data were analyzed for occurrences of HPC bacteria greater than 500 colony forming units (CFU)/mL. The municipality was interested in determining if secondary chlorination practices were sufficient in maintaining microbial health in their distribution system. Coliform data were non-detect (total coliforms and Escherichia coli) in the distribution system over this period and thus heterotrophic bacteria were used to assess microbial health. Results were compared to similar data collected from pilot-scale studies that had been carried out using the same municipal water as the source. Analysis showed that a similar trend was observed between pilot- and full-scale samples. Full-scale data analysis revealed that the minimum disinfection requirement of 0.2 mg/L did not consistently control occurrences of heterotrophic bacteria from being greater than 500 CFU/mL. By comparison, maintaining a concentration of 0.3 mg/L or above, particularly in warm-weather systems, maintained the number of heterotrophic bacteria at below 500 CFU/mL. Fortunately the majority of samples collected in the full-scale distribution system (.89%) had a free chlorine residual concentration of greater than 0.30 mg/L. While it is recognized that this system had 100% compliance for E. coli, the goal of this work will help utilities understand how to utilize microbial data to inform operational disinfection targets for their distribution system. Key words: Chlorine, Disinfection Residual, Distribution System, Heterotrophic Bacteria, Municipal Water Supply, Regulations

INTRODUCTION Microbial contamination in drinking water distribution systems can occur as a result of a system breech or microbial regrowth. Once in the distribution system, cells attach to pipe walls to form a biofilm, which can cause bio-corrosion of the pipes, harboring of microbial pathogens, demand of disinfectant residual, noncompliance with drinking water guidelines, and undesirable aesthetic changes in the drinking water (van der Kooij et al. 1999; Chandy & Angles 2001; Lehtola et al. 2004; Helmi et al. 2008; Moritz et al. 2010; Wang et al. 2012). Other repercussions associated with microbial activity in the distribution system include episodes of coliform growth (LeChevallier et al. 1996; Zacheus et al. 2001), an increased likelihood of pathogens in the water and waterborne disease (Payment 1997; Lehtola et al. 2007), and increased occurrence of nitrification (Wilczak et al. 1996; Lipponen et al. 2002; Pintar & Slawson 2003). Secondary or residual disinfection is the process of applying a disinfectant that provides a residual concentration in the distribution system (Haas 1999). The three main disinfectants used for secondary

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disinfection are chlorine, chloramines, and chlorine dioxide. Each of these has advantages and disadvantages, and the type of disinfectant used, as well as the residual required, will depend upon the finished water quality and pipe characteristics. At the same time, suppliers must minimize disinfection byproduct formation, as well as control any unwanted taste and odor episodes that may be associated with secondary disinfection. There have been several studies evaluating control of microbial regrowth in drinking water distribution systems. Control depends on the bio-stability of the water, which is determined by the concentrations of residual protection and substrate in the system. It has been shown that relying on substrate removal to minimize regrowth requires an assimilable organic carbon (AOC) concentration ranging from 10 to 100 ug/L as acetate-C (LeChevallier et al. 1996; Srinivasan & Harrington 2007). Many systems achieve bio-stability through combining AOC reduction with secondary disinfection. In North America, maintaining a disinfectant residual in the distribution system has been long practiced and is a critical treatment requirement for ensuring water safety. In particular, secondary disinfection has been conducted on the premise that maintaining an adequate disinfectant residual in the distribution system can minimize regrowth of indicator or pathogenic microorganisms, detect microbial intrusion more rapidly than microbial monitoring alone, and reduce the risk of general contamination as a result of a distribution system breech (Haas 1999). But what is considered an adequate residual for each disinfectant strategy? Across North America disinfectant residual guidelines range from specified minimum/maximum residual concentrations to ‘detectable’ presence of a residual. The disinfection target for water utilities in Nova Scotia, Canada, is to maintain a minimum of 0.2 mg/L free chlorine residual in the distribution system, as per the Nova Scotia Treatment Standards for Municipal Drinking Water Systems (NSE 2012). This project assessed four years of data collected from several Halifax Water distribution system sampling points, as well as bench-scale experiments conducted using Halifax Water as the source (i.e., pipe loop studies) to evaluate the effectiveness of maintaining a 0.2 mg/L free chlorine residual in achieving a microbiological target of maintaining heterotrophic bacteria levels to below 500 CFU/mL.

METHODS AND MATERIALS Halifax water plant description

Halifax Water services over 310,000 people in the greater Halifax area in Nova Scotia. The J D Kline Water Supply Plant (JDKWSP) is one of two major plants operated by Halifax Water that has a design capacity of 220 ML/day and treats raw surface water from Pockwock Lake in Halifax. In this paper, full scale sampling and pipe loop distribution systems were operated with water from JDKWSP. This source water is characterized by having a low pH (5.6), and low turbidity (0.3 NTU) and alkalinity (,1.0 mg/L as CaCO3). The treatment is done by direct filtration with free chlorine for final disinfection. A full description of the plant has been provided elsewhere (Vadasarukkai et al. 2011; Knowles et al. 2012). Briefly, in terms of distribution system operations, water leaving the plant typically will have a pH of 7.4, phosphate dose of 0.8 mg/L, and chlorine residual of between 0.8 and 1.0 mg/L. The plant is required to provide a chlorine residual of at least 0.2 mg/L across the system. Halifax water distribution system sampling

Data analyzed for this study covered a sampling period of four years in the full-scale distribution system, and were sorted into small- and large-scale sub-systems. The two larger water systems had an average daily flow of 95 and 45 ML/d, with a combined service population exceeding 300,000. Each of the small-scale systems had service populations of less than 100 people with variable flow

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rates based on residential demand. Samples were collected approximately weekly at 14 points in large full-scale distribution systems and 5 points in small-scale systems, and were analyzed for free chlorine residual (Cl2), temperature, and heterotrophic plate counts (HPCs). Description of pipe loops

Four pipe loops were operated and maintained at JDKWSP, and received treated and disinfected water from the plant without any further chemical additions. A schematic of the pipe loop is shown in Figure 1. The pipe loops consisted of seven main components: the test section (composed of 100 mm diameter pipe), the recirculation pump, the support frame, the return section, the transition section, the feed pump and the influent/effluent sample ports. A detailed description of the setup and operation of the pipe loop pilot system was reported by Gagnon et al. (2008). The pipe loops consisted of new, cement-mortar lined, ductile iron (DI) section(s), polyvinyl-chloride (PVC) or local material (unlined cast-iron, CI), which were normally operated at 6 and 24 hour HRT, respectively. Each pipe loop contained a single test section, consisting of a 1.5-m (5-ft) length of 100 mm (4-inch) diameter pipe. The total volume of each pipe loop was approximately 32 L (8.5 US gal), which corresponds to a flow rate of approximately 88 mL/min for a 6 h HRT and 22 mL/min for a 24 h HRT.

Figure 1 | Schematic of the recirculating flow pipe loop. (1) test section, (2) recirculation pump, (3) support frame, (4) return section, (5) transition section. The feed pump and influent/effluent sample ports are not shown.

Microbial and chemical analysis

Suspended heterotrophic bacteria samples were collected in sterile, 50 mL, disposable plastic tubes (Corning Inc., Acton, MA), or 100 mL IDEXX bottles, each containing 10% w/v sodium thiosulfate to quench disinfectant residual. Pipe loop samples were collected from the influent stream, and also as effluent at 6 and 24 h in the DI pipe loops, and after 24 h only in the PVC and local material (cast iron [CI]) pipe loops. Suspended bacteria samples collected as described above were enumerated with HPCs. The process involved a standard spread plate technique as described in Standard Methods for the Examination of Water and Wastewater (21st edition) on R2A agar (Difco Laboratories). Sterile glass test tubes containing 9 mL phosphate buffer solution were used in series to obtain dilutions from 10!1 to 10!5, depending on concentration. Dilutions were used to target a microbial yield of 30 to 300 colonies per plate per 1 mL of sample. Duplicate plates were spread for each dilution and generally 2 to 3 dilutions were plated for each sample for adequate quality assurance. All equipment used was sterilized and the work was completed on a clean surface near a flame to prevent contamination. Plates were incubated at room temperature upside down in the dark for 7 days, after which time colonies were counted. Free and total chlorine were measured in each study using the DBP colorimetric method (10 mL sample size) and a spectrophotometer (HACH DR/890, HACH DR/2000, HACH DR/4000). The

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HACH DR/890 was used for field collection analysis in both the small and large systems. Split samples for the chlorine measurements were measured by the HACH DR/4000 and the HACH DR/890 in Halifax Water’s laboratory to ensure consistency in data. Chlorine measurements taken during the pipe loop study were conducted using the DR/2000. It was recognized that the instrumentation across models may have slightly different calibrations, accordingly the chlorine data for this paper was analyzed using incremental ranges rather than absolute values.

RESULTS Pipe loops experimental data

Pilot-scale experiments were carried out by Dalhousie researchers using pipe loops with chlorine as the final disinfectant and Halifax Water as the source. Effluent samples were collected on a regular basis and analyzed for free chlorine residual and HPC bacteria. An average temperature of 7.1 °C with a standard deviation +3.9 °C was observed in the effluent water during pipe loop experiments. Data from this pilot study were reviewed and combined into Figure 2, which shows the relationship between chlorine residual concentration and HPC bacteria content. Data from the cement-lined DI and PVC pipe loops were combined, while data from the pipe loop consisting of local CI material were kept separate. Camper et al. (2003) demonstrated that CI pipes, in the presence of chlorine residual, resulted in the highest number of heterotrophic bacteria when compared to cement-lined DI and PVC based material.

Figure 2 | Pipe Loop HPC Bacteria for Varying Free Chlorine Residual Concentrations (N ¼ 62).

The United States Environmental Protection Agency (USEPA) Surface Water Treatment Rule (SWTR), (1989), states that there must be a detectable level of disinfectant residual or that HPC bacteria concentrations should be less than 500 CFU/mL in 95% of samples taken from drinking water distribution systems each month for two consecutive months. Canadian Drinking Water Guidelines do not set a maximum level for HPC bacteria, but recommend that concentrations should be kept below baseline levels and remain constant over time. When levels exceed baseline limits an inspection of the system should be conducted. Figure 2 indicates how often a baseline level of 500 CFU/mL was exceeded with varying free chlorine residual concentrations in the PVC and DI pipe loops, and the CI pipe loops (N ¼ 62). It is evident from the

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figure that more than 70% of the time the number of heterotrophic bacteria exceeded 500 CFU/ mL when the free Cl2 concentration was between 0.20 and 0.30 mg/L in the DI/PVC systems, suggesting that 0.20 mg/L was insufficient to control bacterial regrowth. The percentage of exceedances decreased as the free chlorine concentration increased. In particular, when the chlorine residual was between 0.31 and 0.50 mg/L, the percentage of occurrences where bacteria exceeded 500 CFU/mL fell drastically to just below 10%. It should be noted that only one data point was collected for a Cl2 residual of 0.51–1.00 mg/L, and no data were collected for a Cl2 residual greater than 1.0 mg/L. All of the data points collected from the CI pipe loop were in the 0–0.10 mg/L free Cl2 residual range, and HPCs exceeded 500 CFU/mL around 90% of the time. Halifax water distribution system data

Halifax Water supplied HPC bacteria concentration and free chlorine residual data from 19 sampling locations (14 large-scale and 5 small-scale systems) over four years (2008–2011). These data were analyzed according to distribution system size (large and small) and water temperature at sample collection time (#15 °C and .15 °C). Figure 3 shows heterotrophic bacteria versus free Cl2 concentration in large systems during winter (temperature #15 °C) and summer months (.15 °C). It can be seen that when the temperature is below 15 °C, HPC bacteria levels are below 500 CFU/mL the majority of the time, except when the Cl2 residual falls below 0.10 mg/L. The data show that for system temperatures above 15 °C, increasing the Cl2 residual leads to decreasing occurrences of HPC bacteria concentrations over 500 CFU/mL. At a residual of 0.21–0.30 mg/L, this level is exceeded approximately 25% of the time, while above 0.30 mg/L it is exceeded much less than 10% of the time. The majority of data collected over the four-year period showed free Cl2 above 0.30 mg/L in both the cold months (82%) and warm months (75%) in the large systems.

Figure 3 | Halifax Water HPC Bacteria for Varying Free Chlorine Residual Concentrations in Large Systems (N ¼ 2,265).

Figure 4 shows similar data for small systems, in both the colder and warmer seasons. As in the large systems, when the system temperature is below 15 °C occurrences of HPC bacteria concentrations greater than 500 CFU/mL are not common, except when the free chlorine

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Figure 4 | Halifax Water HPC Bacteria for Varying Free Chlorine Residual Concentrations in Small Systems (N ¼ 562).

residual is less than 0.10 mg/L. Results from the warmer weather are similar to large system trends, where increasing Cl2 residual leads to decreasing occurrences of HPC bacteria limit exceedances. Limited data were collected in the 0.21–0.30 mg/L residual ranges (5 points), but all showed levels below 500 CFU/mL. Once again, the majority of data collected in both cold (89%) and warm (91%) weather related to Cl2 residuals greater than 0.30 mg/L in the small systems. As noted, the majority of close to 3,000 data points collected over the four-year period from Halifax Water sampling sites fell in a higher chlorine residual range. To more clearly illustrate the data distribution, Figure 5 shows the percentage distribution of chlorine residuals. It can be seen that over 80% of the samples collected had a free chlorine residual of more than 0.30 mg/L.

Figure 5 | Distribution of Free Chlorine Residual Concentrations in Full-scale Halifax Water (Large and Small) Systems (N ¼ 2,827).

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DISCUSSION Results from the two types of studies indicate that chlorine residual and temperature both have an impact on maintaining control of heterotrophic bacteria in drinking water distribution systems. For Halifax Water, the data show that a free chlorine residual of 0.20 mg/L, particularly in warmer temperatures, will not ensure that heterotrophic bacteria levels are consistently maintained below 500 CFU/mL. Data from the pipe loops study consistently fell into the category of having low or non-detectable disinfectant residual (0–0.10 mg/L), and HPC levels that exceeded 500 CFU/mL the majority of the time, which would approach non-conformance with USEPA SWTR. In the data from the full-scale system, the number of data points that show non-detectable or low free chlorine residual is below 5%, but occurrences above 500 CFU/mL are observed, particularly in warm weather. There is a definite trend that as disinfectant residual increases, HPC levels decrease, and maintaining a free chlorine residual over 0.30 mg/L lowers occurrences exceeding 500 CFU/mL to less than 10% in both warm and cold temperatures in large systems. The question may be posed as to the importance of these results, and in general, the importance of maintaining HPC levels below 500 CFU/mL, and a disinfectant residual that achieves this goal. Allen et al. (2004) did an extensive literature review on the topic of controlling HPCs, and noted that healthbased standards cannot be established for HPC bacteria due to a lack of evidence to support them. The authors also acknowledged the variance in methodology to detect HPC bacteria, and the possibility that HPC levels greater than 500 CFU/mL may interfere with coliform/Escherichia coli analysis. Their final point was that HPC monitoring is a useful tool for determining the overall health of the system in terms of efficacy of treatment processes and changes in microbial water quality. Nescerecka et al. (2014) studied the biological stability of a full-scale chlorinated distribution system in Latvia, and evaluated adenosine tri-phosphate (ATP) and flow cytometric (FCM) analyses in comparison with HPCs. They also found increased bacteria levels in areas of decreased chlorine residual, particularly at sample points a distance away from the treatment plant or in ‘mixing zones’ where two water sources meet. The authors attributed the biological instability to high concentrations of organics that consumed free chlorine, and noted that biological stability can be achieved through nutrient limitation without additional disinfection residual. The organic content is low in Halifax Water and the current study establishes the baseline residual concentration required in addition to nutrient limitation to control HPCs. In addition, the Latvia study found no correlation between either FCM and HPCs, or ATP and HPCs, but did find good correlation between ATP and FCM. The authors indicate that measuring HPCs is not useful for assessing microbiological quality of drinking water. Halifax Water is considering the use of ATP for biomass measurement in their distribution system and additional ATP data will be collected from the distribution system in 2014. Analysis will be conducted to determine if previous relations for critical concentrations hold up with ATP analysis, and whether the data show a similar trend to those collected in relation to HPC content. Hrudey (2011) encourages water utilities to ‘know your own system’ through the water safety plan approach. In presenting international water approaches to supplying safe water, Hrudey notes that water safety plans, such as those adopted by New Zealand (public health risk management plans), and Australia (Australia Drinking Water Guidelines), allow suppliers to be preventive rather than reactive to water quality issues. This approach relies less on meeting numerical targets and more on achieving the optimum performance for a given system. In order to achieve optimum performance in producing high quality water, the operators must know the potential threats and their ability to deal with them, and identify ways in which to respond and improve. Hrudey (2009) also prepared a substantial review of public health risks associated with chlorination DBPs, in which the author discusses the importance of risk management, particularly synthesizing disinfection to prevent microbial infections. In establishing a system-specific goal rather than aiming for numeric guidelines, a utility has a

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better understanding of risk trade-offs, and will have a better chance of identifying issues that will trigger responses. Considering a ‘know your system’ approach for Halifax Water, results from the current study indicate a free chlorine residual of 0.30 mg/L is the optimum concentration for controlling HPCs, particularly in warmer months. This value could be used as a seasonal goal, rather than simply achieving compliance with the regulated numeric value of 0.20 mg/L, allowing the utility to optimize performance. Data show that 0.20 mg/L is a reasonable target and 0.30 mg/L is an optimum goal, however, this value is likely site-specific and other systems could establish similar goals that may vary in value. Establishing, and further investigating this site-specific goal, (i.e., evaluation of resulting DBPs given the goal residual concentration) will allow the utility to assess risks within the system, as Hrudey suggests. Taking the ‘know your system’ approach a step further could include identifying the type and distribution of bacterial communities in drinking water systems, and the factors that affect them. Pinto et al. (2012) determined that bacteria colonized in filters were more likely to persist in the distribution system than bacteria in the source water. They indicate that manipulation of the filter biofilm community could influence downstream microbial communities, (i.e., ensuring population with innocuous bacteria), and create centralized risk management for the distribution system through filter operation. Williams et al. (2004) observed that α-proteobacteria dominated in systems where chlorine residual was maintained, whereas β-proteobacteria occurred in water with lower Cl2 residual, suggesting that chlorine concentration is a determining factor in bacterial populations. Site-specific risk management plans for drinking water systems could attempt to address both quantity (i.e., HPC, ATP) and types of bacteria, in addition to understanding how treatment practices affect bacterial communities, in order to optimize treatment alternatives and reduce the risk of regrowth of pathogenic organisms in the distribution system. Although HPC bacteria may not be a direct threat to public health, being informed as water suppliers about the typical levels in a given system, and having an understanding of what changes in those levels may indicate, may be more valuable than having a simple numeric goal. Knowing typical HPC values in the distribution system may enable utilities to develop a framework for chlorination. However, mining the data is critical in order to establish optimum treatment and performance targets. As previously mentioned there are no maximum acceptable levels set by federal government for HPC bacteria in North America (except in the case of non-detectable residual (USEPA 1989)). European Guidelines are similar in that they do not specify numerical limits but recommend no abnormal changes in HPC levels. However, some countries differ slightly in their guidelines; Germany and Australia have regulations that set the limit in disinfected systems at 100 CFU/mL (WHO 2003) using a 48 h incubation time under specific temperature and substrate conditions. In considering these stricter regulations, Halifax Water data were reviewed for occurrences above 100 CFU/mL in relation to free chlorine residual concentration in large and small-scale systems, shown in Figures 6 and 7. The trends observed are similar to those shown in Figures 3 and 4 in that an increasing disinfectant concentration leads to decreased occurrences above the set level, in this case 100 CFU/mL. However this finer resolution scale showed that maintaining occurrences below 100 CFU/mL was more difficult, particularly in warmer conditions. Although this is not a standard that needs to be met by Halifax Water, this analysis sets a framework for increasing knowledge and ability to recognize changes in their system. Setting treatment targets to maintain occurrences below 100 CFU/mL could represent a utility goal, which would then inform optimization of disinfection practices. As shown in Figure 5, Halifax Water generally maintains a free chlorine residual concentration in their distribution systems above 0.30 mg/L (over 80% of samples collected). This is greater than the required concentration of 0.20 mg/L but allows the system to achieve improved control of microbial activity.

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Figure 6 | Halifax Water HPC Bacteria for Varying Free Chlorine Residual Concentrations in Large Systems (N ¼ 2,265).

Figure 7 | Halifax Water HPC Bacteria for Varying Free Chlorine Residual Concentrations in Small Systems (N ¼ 562).

CONCLUSIONS This paper incorporates an analysis of free chlorine and HPC bacteria data in a full-scale distribution system collected over four years, in conjunction with pipe loop data using the same water source. It has been shown that maintaining a free chlorine concentration of 0.30 mg/L or above substantially reduced occurrences of HPC bacteria exceeding 500 CFU/mL. This residual is higher than the guideline set at 0.20 mg/L free chlorine, but the data can be used in a ‘know your system’ approach in supplying high quality water to optimize performance. This is a system-specific number, and utilities may see value in establishing similar goal values to improve water quality rather than relying on

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compliance with numerical guidelines. Occurrences of HPC bacteria exceeding 100, rather than 500, CFU/mL were more difficult to control, but the utility may choose this as a treatment goal and use it to develop a disinfection framework.

ACKNOWLEDGEMENTS The authors would like to acknowledge and extend thanks for the financial support provided through the Foulis Chair Program at Acadia University and the NSERC/Halifax Water Industrial Research Chair in Water Quality & Treatment at Dalhousie University. Funding partners in this Industrial Research Chair program are NSERC, Halifax Water, LuminUltra, Cape Breton Regional Municipality Water Department, and CBCL Ltd. The authors are also grateful to Halifax Water staff for their assistance in sample collection.

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