International Biodeterioration & Biodegradation 102 (2015) 231e236
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International Biodeterioration & Biodegradation journal homepage: www.elsevier.com/locate/ibiod
Can microbes significantly accelerate chloramine decay without severe nitrification? Bhagya S. Herath, Arumugam Sathasivan*, Hoi Ian Lam School of Computing Engineering and Mathematics, University of Western Sydney, Kingswood, NSW, 2747, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 January 2015 Received in revised form 13 March 2015 Accepted 13 March 2015 Available online 5 May 2015
The ability of microbes to accelerate chloramine decay to the same degree as under severe nitrification, but without the signs of severe nitrification is reported. Traditionally, only nitrification is believed to microbiologically challenge the stability of chloramine. Chloraminated water containing high amount of natural organic matter (10e12 mg L1 of dissolved organic carbon (DOC)) was fed to four lab scale reactors connected in series. Each reactor had one day retention time with a total of four days in total. The decay coefficient was observed to be a maximum of 0.06 h1 without substantial changes in ammonia, nitrite or nitrate levels. Despite very low chloramine residuals, nitrite only increased to less than 0.012 mg-N L1, indicating a mildly nitrifying condition. Previously reported decay coefficient (0.001e0.006 h1) for the condition was an order less. Changing of the feed to a new water from the same source, but with a low DOC (of 4 mg L1) led to the onset of nitrification complying biostability. The maximum observed chloramine decay coefficient with severe nitrification was 0.085 h1. Therefore, microbes present under mildly nitrifying condition can be as destructive as that in severely nitrifying condition. For better control of chloramine, attention on microbes present under mild nitrification is needed. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Chloramine Chloramine decay Nitrification Heterotrophic bacteria Biostability Dissolved organic carbon DOC
Introduction Chloramine especially monochloramine, behind chlorine, is the second most popular disinfectant used in water supply systems. The typical purpose of chloramine is to provide a longer-lasting residual as the water moves through pipes to consumers. Chloramine is formed when ammonia and chlorine is combined. Subsequently chloramine is not as reactive as chlorine, it forms fewer regulated disinfection by-products such as the Trihalomethanes (THMs) and Halo acetic acids (HAAs) (Bond et al., 2011). The longer lasting nature provides enhanced protection against bacterial regrowth in systems with large storage tanks. Apart from that, ability to minimise taste and odour is an additional advantage (Zaitsev and Dror, 2013). Maintaining an effective chloramine residual throughout a distribution system can be a challenge at times due to accelerated chloramine decay occurring through chemical and microbial reactions (Sathasivan et al., 2005). Auto-decomposition and direct
* Corresponding author. E-mail addresses: [email protected] (B.S. Herath), A.Sathasivan@ uws.edu.au (A. Sathasivan), [email protected] (H.I. Lam). http://dx.doi.org/10.1016/j.ibiod.2015.03.018 0964-8305/© 2015 Elsevier Ltd. All rights reserved.
reaction with chloramine demanding compounds including natural organic matter (NOM) present in water distribution system contribute towards chemical decay of monochloramine (Vikesland et al., 2001). Usually, nitrification is associated with chloramine loss or difficulty in maintaining an adequate disinfectant residual (Wolfe et al., 1990; Cunliffe, 1991; Skadsen, 1993; Odell et al., 1996; Wilczak et al., 1996) and hence nitrification related parameters and disinfectant residual are measured to judge the stability of a disinfectant. Nitrification is the biological oxidation of ammonia with oxygen into nitrite followed by the oxidation of nitrite into nitrate. The first step is performed by ammonia-oxidizing bacteria (AOB) and the second by nitrite oxidizing bacteria (NOB). However, recent findings have shown that other species e e.g., Nitrosomonas oligotropha (Regan et al. 2002), nitrifying Archaea (Hoefel et al., 2011), and heterotrophic nitrifiers (Daum et al., 1998) can be present and contribute to nitrification. In a chloraminated system, the highest residual at which onset of nitrification occurs for a given free ammonia and temperature is reported to follow biostability concept (Sathasivan et al., 2008; Sarker and Sathasivan, 2011; Sarker et al., 2013). According to the biostability concept (Woolschlager et al., 2001) the regrowth of
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AOB can be prevented if the inactivation rate equals or exceeds the bacterial growth rate at each location within the distribution system (Sarker et al., 2013). A biostable residual concentration (BRC) is the chloramine residual below which potential for nitrification occurrence exists. The equation for BRC was implemented by Fleming et al. (2005) and further modified by Sathasivan et al. (2008) as follows:
BRC ¼
mm free ammonia N $ kd Ks þ free ammonia N
(1)
where mm is the maximum specific growth rate of AOB (d1); free ammonia_N ¼ ammonia (NH3eN) plus ammonium (NHþ 4 -N) concentrations (mg-N L1); Ks ¼ half saturation constant for AOB (mg-N L1); kd ¼ the rate constant for inactivation of AOB by disinfectant (L d1 mg-Cl1 2 ); BRC is measured as total chlorine concentration (mg Cl2 L1). Sathasivan et al. (2008) proposed the values for mm/kd ¼ 2 mg-Cl2 L1and Ks ¼ 0.18 mg-N L1 at 20 C. In addition to nitrification, Sathasivan et al. (2005, 2008) and later Bal Krishna et al. (2012) observed microbiologically assisted chloramine decay with a lower but a steady production of nitrite until the onset of nitrification. The former authors termed this behaviour as mild nitrification. Bal Krishna et al. (2013) identified the microbes present under mildly nitrifying conditions; Microbes include heterotrophic bacteria such as Solibacteres, Sphingomonas, Actinobacteria, Psuedomonas and Methylobacterium. These results confirmed the findings of Williams et al. (2004) who analyzed the heterotrophic bacterial community in a chloraminated system and reported the dominance of Alphaproteobacteria. Sequences aligned with Alphaproteobacteria including Afipia, Sphingomonas, Brevundimonas, Blastomonas, Hyphomicrobium, Methylocystis and Bradyrhizobium were reported in the systems. Nitrification normally occurs in reservoirs which are situated far from the water treatment plant during warmer months. On other times, i.e., majority of the time, water in reservoirs experience mild nitrification and it is mild nitrification that accelerates chloramine decay to bring chloramine residual below BRC before the onset of nitrification occurs (Sathasivan et al., 2008). Therefore, investigation of accelerated chloramine decay in mild nitrification stage is crucial. Factors affecting the mildly nitifying microbes are not known but logically it could include organic carbon, chloramine residual and temperature. Organic carbon is known to inhibit nitrification in wastewater treatment systems, but in chloraminated systems the role remains indecisive (Zhang et al., 2009). Others state that nitrifiers are strongly dominated by heterotrophs at a high organic carbon-to-ammonia ratio (Verhagen and Laanbroek, 1991; Ohashi et al., 1995). As a source of organic carbon, natural organic matter (NOM) may also encourage the growth of microbes present under mildly nitrifying conditions and suppress severe nitrification. The NOM, an integral part of water sources used for drinking water purposes, is derived mainly from decaying vegetation and consists of a complex mixture of organic compounds. Therefore, the amount, character and properties of NOM could differ significantly according to origin and biogeochemical cycles of the surrounding environments. The NOM, which is measured as dissolved organic carbon (DOC) also vary on the same location seasonally (Matilainen et al., 2010). Studying such variation may help understanding the dynamics. In this paper, an unusually high DOC (10e12 mg-C L1) observed in natural water and a low DOC (4 mg-C L1) experienced soon after a few months in the same source are tested for its ability to cause/ inhibit nitrification or to encourage microbes causing mild nitrification. The conformance with the bio-stability concept was also tested.
Materials and methods Analytical procedures Total chlorine (TCl), total ammoniacal nitrogen (TAN), nitrite and nitrate were measured immediately after collecting the samples. TCl is the total chlorine and TAN is the summation of NH3eN, NHþ 4 -N and nitrogen associated with chloramine. The Gallery (Thermo Scientific), a high precision chemistry automated analyzer, was adopted for measuring TAN, nitrite and NOx concentrations. It performs discrete, spectrophotometric analysis on optical multi-cell cuvette. Available ammonia reacts with hypochlorite ions generated by the alkaline hydrolysis of sodium dichloroisocyanurate to form monochloramine which reacts with salicylate ions in the presence of sodium nitroprusside at around pH 12.6 to form a blue compound. The compound is measured spectrophotometrically at 660 nm. Nitrite is measured by reaction with sulphanilamide and N-(1-naphthyl)-ethylenediamine dihydorchloride to form a highly coloured azo-dye, thus, the absorbance is measured spectrophotometrically at 540 nm or 520 nm (APHA et al., 1998). The determination of nitrate is by catalytically reducing the nitrate ions into nitrite ions (by nitrate reductive enzyme in the presence of reduced nicotinaminde dinucleotide), the total nitrite ions are then measured by sulphanilamide method as the NOx, and nitrate is obtained by deduction of nitrite from the NOx. The analyzer has the detection limit for TAN, nitratre and nitrite of 0.002 mg-N L1 and NOx has an error of 0.005 mg-N/L. Standard curves for TAN, nitrite and NOx were calibrated for the range 0.0e1.0 mg-N L1 using stock solutions of ammonium chloride, sodium nitrite and sodium nitrate, respectively. The experimental errors were 1.5% for TAN, nitrite and nitrate measurement. TCl residuals were measured by DPD colorimetric method using a HACH pocket colorimeter. It was assumed that more than 99% of the chloramine was present in the form of monochloramine given that pH was above 8.0 and the Cl to TAN mass ratio was approximately 4.0 or less (Valentine and Wilber, 1987). TCl measurement had an experimental error of ±0.03 mg-Cl2 L1. Supplementary details of the analytical methods can be found in Bal Krishna and Sathasivan (2010). Total organic carbon (TOC) was measured using Shimadzu Total Organic Carbon Analyser with an experimental error for TOC of ±5%. DOC was measured using the same analyser, however the sample was filtered through pre-washed 0.45 mm polycarbonate membrane filter papers. After placing the filter paper on the filtration apparatus, around 100 ml of Milli-Q water was passed through the filter device for minimising the contamination of DOC from filter paper into the sample. Free ammoniacal nitrogen was determined (Equation (2)) by assuming all total Cl2 was present as chloramine.
Free Ammonia N ¼ TAN
Total Cl2 5
(2)
where, free ammonia nitrogen concentration is a function of the TAN concentration and the chloramine concentration is measured as total Cl2. Total chloramine decay coefficient was calculated using Equation (3) as defined by Sathasivan et al. (2010).
krt ¼
1 Clin 1 q Clout
(3)
where, Clout and Clin are outlet and inlet chloramine (as TCl) residuals respectively, kRt is the total chloramine decay coefficient and Ɵ is the water retention time in the reactors.
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Portable pH meter (HACH 40d) was used to measure pH values and the measurement error was ±0.1. Stock chemical solutions preparation Stock solutions for all chemicals were prepared using analytical grade chemicals in Milli-Q water. Monochloramine solution was prepared using stock solutions of ammonium chloride (1 g-N L1) and sodium hypochlorite (1 g-Cl2 L1). 1 M sulfuric acid (H2SO4) and 1 M sodium hydroxide (NaOH) solutions were prepared for pH adjustment. Stock solutions of sodium nitrite (NaNO2) and sodium nitrate (NaNO3) for the later experiments were also prepared with the concentration of 1 g-N.L1. Feed water sample collection and feed preparation The water from Warragamba dam, 1.0 m3 in volume, was collected at the point before the disinfectant was added and was transported to University of Western Sydney and stored in High Density Poly-ethylene (HDPE) tank at room temperature. To eliminate contamination by indigenous microbes present in feed water, chlorine (from a stock solution of 1 g-Cl2 L1 sodium hypochlorite) was added to achieve a higher chlorine residual than required at least 3 h before ammonia (from a stock solution of 1 g-N L1 ammonium chloride) was dosed. If chlorine dropped below required level at the time of ammonia dosing, additional (usually a maximum of 0.3 mg-Cl2 L1) amount of chlorine was added. This was kept in 20 L tank (HDPE), as a feed water tank. Feed was prepared every 4 days. The DOC of feed water was 10e12 mg-C L1 and 4 ± 0.5 mg-C L1 during the first 52 days and the next 20 days of the experimental period, respectively. The pH in the feeding tank was adjusted to 8.0 ± 0.1 before feeding the reactors.
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volume and was made of HDPE closed with a lid (HDPE) and the feed water tank was also made of HDPE with a lid (HDPE). The system was facilitated with automatic water flow; control unit devices were used to control the water flow rate between feeding tank and R-1. Nevertheless, the outlet and inlet pipe level was adjusted to create gravity flow along the system. Water samples for analysis were collected from the outlet, fixed at the bottom of each reactor. Operation of the laboratory scale system The reactor set was operated in the room where the temperature was 20e22 C. During the startup period, the feed water with a chloramine concentration of about 1.0 mg-Cl2 L1 (TCl:TAN ratio, 4:1) was prepared. About 5 L water was fed continuously per day and water volume was maintained constant within each reactor to gain a retention time of 24 h. The various nitrification conditions (from no nitrification to severe nitrification) which generally occur in a full scale distribution system were created along the reactors. Chloraminated water samples were collected from the Sydney water distribution system with a high nitrite concentration (nitrite> 0.1 mg-N L1); to facilitate nitrification and to acquire specific inoculum usually present in distribution system. The water samples were placed as seed microorganisms in each reactor, except for R-1. The lab scale reactor system was continuously operated for 72 days of experimental period. Within a week, chloramine concentration in the feed water tank was increased up to 1.2e1.5 mg-Cl2 L1 and was maintained the same in the first 20 days. Between 20 and 30 days, it was adjusted to 0.7 mg-Cl2 L1 and then from 30 days onward 1.2 mg-Cl2 L1. Parameters such as TCl residuals, TAN, nitrite and nitrate were continuously monitored along the reactors (R-1 to R-4) continuously. Results and discussion
Laboratory scale system set-up The laboratory scale system was set up at Environmental Engineering Laboratory in University of Western Sydney. The system (Fig. 1) consisted of four reactors, namely R-1, R-2, R-3 and R-4, connected in series through HDPE pipes. Each reactor was 5 L in
Nitrogenous species and chloramine decay during experimental period More than 0.01 mg-N L1 nitrite level and simultaneous reduction of TAN level are hints of the occurrence of onset of
Fig. 1. Schematic diagram of laboratory scale system setup.
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nitrification (Sathasivan et al., 2008). In the first 20 days of the operational period chloramine decayed with constant and low nitrite level (less than 0.004 mg-N L1) stable TAN and nitrate levels along the reactors (Fig. 2AeD). The results confirmed there was no severe nitrification, but mild nitrification. Lowest chloramine level observed was 0.6 mg-Cl2 L1 in the fourth reactor which may not have encouraged the growth of nitrifying bacteria. Therefore, the chloramine residual in the feed was reduced to 0.7 mg-Cl2 L1 (Cl:TAN ¼ 4:1) for the next 10 days. However, severe nitrification could not be observed even in R-4 with the low chloramine level (0.15 mg/L). Even when the feed chloramine residual was increased to between 1.2 and 1.0 mg-Cl2 L1 (from 30 days onward), nitrite and nitrate levels in all reactors remained steady and less than 0.012 mg-N L1 and 0.35 mg-N L1 respectively, which confirmed all reactors were in mildly nitrifying stage. After the introduction of the new feed water with low DOC level on day 52, a simultaneous reduction of TAN level (Fig. 2B), a significant increase of nitrite level (Fig. 2C) and acceleration of nitrate level (Fig. 2D) indicated the system experienced severe nitrification. Presence of chloramine decaying organisms (CDOs) - confirmation by decay rate coefficients Fig. 3 illustrates the chloramine decay coefficient for all reactors (R-1 to R-4). The decay coefficients of reactors generally increased along the reactors (kR4 > kR3 > kR2 > kR1). This has occurred during both when severe nitrification took place and not taking place.
In the first 23 days of high DOC feed, observed kRT for all reactors were stable (Fig. 3). After the adjustment of chloramine residual concentration of feed water (to 0.7 mg-Cl2 L1), maximum kRT in all reactors increased from 0.014 h1 to 0.029 h1 between 24th and 30th days with neither significant increase of nitrite and nitrate nor decrease of TAN (Fig. 2C, B and D). During the period when chloramine residual was maintained at 1.2 mg-Cl2 L1, kRT considerably increased from 0.029 h1 to 0.06 h1. This was associated with low nitrite (0.25 mg-N/L). Bal Krishna et al. (2013) observed similar increase in decay coefficients following severe nitrification, but mildly nitrifying conditions did not result in such high decay coefficients. The results, therefore, for the first time demonstrate that microbes other than nitrifiers could accelerate chloramine decay to the same degree as nitrifying reactors. Did high DOC reactor have suitable condition for nitrification? It could be questioned if chloramine and ammonia levels were sufficient for the onset of nitrification in reactors R-3 and R-4 in case of high DOC fed reactors and too much free ammonia and less chloramine in the low DOC reactors. To show that there were suitable conditions for AOB to thrive, the data was checked against the BRC curves proposed by Sathasivan et al. (2008). A nitrite level stayed lower than or equal to 0.012 ± 0.002 mg-N L1 during high DOC fed period and this level was considered non-nitrifying. (Severe) Nitrification could not be observed when reactors fed with 10e12 mg-C L1 DOC contained water. Therefore, BRC curve did not produce two zones such as nitrifying and non-nitrifying; open circles representing the no nitrification were evenly distributed on both sides of the BRC curve (Fig. 4A). After the introduction of the low DOC contained water, clearly distinguished nitrifying and non-nitrifying zones separated by the BRC curve was observed (Fig. 4B). Therefore, there existed suitable conditions inside the reactor to cause nitrification in both high and low DOC water fed periods showing that it is the suppression of AOB or encouragement of other microbes by increased presence of DOC that has caused the phenomenon. Implications for further research and practice Traditionally chloramine stability is controlled by controlling nitrification and the research is directed towards identifying nitrifying organisms that are present in the water. Earlier findings (Sathasivan et al., 2005, 2008; Bal Krishna et al., 2013) showed that the behavior (mild nitrification mediated by CDOs) under high chloramine residual is noticed until the onset of nitrification.
However, the finding of this research, although at a high DOC level, showed that microbes other than nitrifiers can be present and can equally decay chloramine at all levels of chloramine. If these microbes (CDOs) co-exist with nitrifiers under low chloramine residuals, they can equally decay the chloramine under severely nitrifying condition. Chloraminated water supply systems experience nitrification mostly during summer (usually three months) but other times of the year, mild nitrification controlled by CDOs prevail. When mildly nitrifying conditions decrease the residual below the BRC level (Equation (1)), the onset of (severe) nitrification starts. Therefore, the key to controlling nitrification is controlling CDOs which prepare the conditions conducive to nitrification. The stability of chloramine cannot be determined by traditional nitrification related indicators and hence microbial decay coefficients need to be measured regularly using either microbial decay factor (Fm) method (Sathasivan et al., 2005) or reservoir acceleration factor (Fra) method (Sathasivan et al., 2010). As conditions conducive to the proliferation and activity of these bacteria (CDOs) are unknown, further research is needed. The factors could be high amount of assimilable organic carbon (not measured in this research), temperature and chloramine residual. Hence, the findings of the research underscore the importance of the organisms present under mildly nitrifying conditions for the stability of chloramine and the importance of research in this area. Conclusion It is traditionally believed the chloramine stability is microbiologically challenged by nitrifying organisms and much research is focused on understanding the behaviour of the organisms. However, the findings depicted that microbes other than nitrifiers could equally decay chloramine. A lab-scale reactor was fed after chloraminating the water containing high DOC (10e12 mg-C/L) followed by the one with low DOC (4 mg-C/L) from the same source. High DOC greatly suppressed the activity of nitrifying organisms but still maximum chloramine decay rate was registered to be 0.06 h1. When supplying low DOC containing water, onset of nitrification was noted as predicted by the biostability concept proposed earlier and a maximum decay rate of 0.085 h1 was noted.
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A
B
Fig. 4. A and B: Testing conformance with the BRC concept using the literature proposed coefficients of Sathasivan et al., 2008 (mm/kd ¼ 2 mg-Cl L1 and Ks ¼ 0.18 mg-N L1) for the performance of reactor set operated with high and low DOC levels.
Parameters affecting the chloramine decaying organisms (CDOs), microbes accelerating chloramine decay without causing severe nitrification, are not known and needs further research. This research also underscores that it is essential to understand the stability of chloramine not only through nitrification indicators but also through microbial decay factor or reservoir acceleration factor methods, as activity of these microbes cannot be determined otherwise. References APHA, AWWA, WEF, 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association, Washington, DC. Bal Krishna, K.C., Sathasivan, A., 2010. Does an unknown mechanism accelerate chemical chloramine decay in nitrifying waters? J. Am. Water Works Assoc. 102, 82e90. Bal Krishna, K.C., Sathasivan, A., Ginige, M.P., 2013. Microbial community changes with decaying chloramine residuals in a lab scale system. Water Res. 47, 4666e4679. Bal Krishna, K.C., Sathasivan, A., Sarker, D.C., 2012. Evidence of soluble microbial products accelerating chloramine decay in nitrifying bulk water samples. Water Res. 46, 3977e3988. Bond, T., Huang, J., Templeton, M.R., Graham, N., 2011. Occurrence and control of nitrogenous disinfection by-products in drinking water. Water Res. 45, 4341e4354. Cunliffe, D.A., 1991. Bacterial nitrification in chloraminated water supply. J. Appl. Environ. Microbiol. 57, 3399e3402. Daum, M., Zimmer, W., Papen, H., Kloos, K., Nawrath, K., Bothe, H., 1998. Physiological and molecular biological characterization of ammonia oxidation of the heterotrophic nitrifier Pseudomonas putida. Curr. Microbiol. 37, 281e288. Fleming, K.K., Harrington, G.W., Noguera, D.R., 2005. Nitrification potential curves: a new strategy for nitrification prevention. J. Am. Water Works Assoc. 97, 90e99. Hoefel, D., Phillips, R., O’ Reilly, L., Kovac, S., Lucas, J., Monis, P.T., 2011. Flow cytometry and pcr approaches to investigate chloramine instability. Aust. Water Assoc. (AWA) 38, 75e78. Matilainen, A., Vepsalainen, M., Sillapaa, M., 2010. Natural organic matter removal by coagulation during drinking water treatment: a review. Adv. Colloid Interface Sci. 159, 189e197. Odell, L.H., Kirmeyer, G.J., Wilczak, A., Jacangelo, J.G., Marcinko, J.P., Wolfe, R.L., 1996. Controlling nitrification in chloraminated systems. J. Am. Water Works Assoc. 88, 86e98.
Ohashi, A., De Silva, D.G.V., Mobarry, B., Manem, J.A., Stahl, D.A., Rittmann, B.E., 1995. Influence of substrate C/N ratio on the structure of multi-species biofilms consisting of nitrifiers and heterotrophs. Water Sci. Technol. 2, 75e84. Regan, J.M., Harrington, G.W., Nouera, D.R., 2002. Ammonia and nitrite oxidizing bacterial communities in a pitot scale chloraminated drinking water distribution system. Appl. Environ. Microbiol. 68, 73e81. Sarker, D.C., Sathasivan, A., 2011. Effect of temperature on onset of nitrification on chloraminated distribution system. Destin. Water Treat. 32, 95e99. Sarker, D.C., Sathasivan, A., Joll, C.A., Heitz, A., 2013. Modelling temperature effects on ammonia-oxidizing bacterial biostability in chloraminated systems. Sci. Total Environ. 454e455, 88e98. Sathasivan, A., Bal Krishna, K.C., Fisher, I., 2010. Developmentand application of a method for quantifying factors affectingchloramine decay in service reservoirs. Water Research 44, 4463e4472. Sathasivan, A., Fisher, I., Kastle, G., 2005. Simple method for quantifying microbiologically assisted chloramine decay in drinking water. Environ. Sci. Technol. 39, 5407e5413. Sathasivan, A., Fisher, I., Tam, T., 2008. Onset of severe nitrification in mildly nitrifying chloraminated bulk waters and its relation to biostability. Water Res. 42, 3623e3632. Skadsen, J., 1993. Nitrification in a distribution system. J. Am. Water Works Assoc. 85, 95e103. Valentine, R.L., Wilber, G.G. (Eds.), 1987. Water Chlorination Chemistry. Lewis Publishers, Chelsea. Verhagen, F.J.M., Laanbroek, H.J., 1991. Competition for ammonium between nitrifying and heterotrophic bacteria in dual energy limited chemostats. Appl. Environ. Microbiol. 57, 3255e3263. Vikesland, J.P., Ozekin, K., Valentine, L.R., 2001. Monochloramine decay in model and distribution system waters. Water Res. 35, 1766e1776. Wilczak, A., Jacangelo, J.G., Marcinko, J.P., Odell, L.H., Kirmeyer, G.J., 1996. Occurrence of nitrification in chloraminated system. J. Am. Water Works Assoc. 88, 74e85. Williams, M.M., Domingo, J.W.S., Meckes, M.C., Kelty, C.A., Rochon, H.S., 2004. Phylogenetic diversity of drinking water bacteria in a distribution system simulator. J. Appl. Microbiol. 96, 954e964. Wolfe, R.L., Lieu, N.I., Izaguirre, G., Means, E.G., 1990. Ammonia-oxidizing bacteria in a chloraminated distribution system: seasonal occurrence, distribution and disinfection resistance. J. Appl. Environ. Microbiol. 56, 451e462. Woolschlager, J.E., Rittmann, B., Piriou, P., Schwartz, B., 2001. Using a comprehensive model to identify the major mechanisms of chloramine decay in distribution systems. Water Sci. Technol. Water Supply 1, 103e110. Zaitsev, N., Dror, S., 2013. Water quality function deployment. Qual. Eng. 25, 356e369. Zhang, Y., Love, N., Edwards, M., 2009. Nitrification in drinking water systems. Crit. Rev. Environ. Sci. Technol. 39, 153e208.
Contents lists available at ScienceDirect
International Biodeterioration & Biodegradation journal homepage: www.elsevier.com/locate/ibiod
Can microbes significantly accelerate chloramine decay without severe nitrification? Bhagya S. Herath, Arumugam Sathasivan*, Hoi Ian Lam School of Computing Engineering and Mathematics, University of Western Sydney, Kingswood, NSW, 2747, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 January 2015 Received in revised form 13 March 2015 Accepted 13 March 2015 Available online 5 May 2015
The ability of microbes to accelerate chloramine decay to the same degree as under severe nitrification, but without the signs of severe nitrification is reported. Traditionally, only nitrification is believed to microbiologically challenge the stability of chloramine. Chloraminated water containing high amount of natural organic matter (10e12 mg L1 of dissolved organic carbon (DOC)) was fed to four lab scale reactors connected in series. Each reactor had one day retention time with a total of four days in total. The decay coefficient was observed to be a maximum of 0.06 h1 without substantial changes in ammonia, nitrite or nitrate levels. Despite very low chloramine residuals, nitrite only increased to less than 0.012 mg-N L1, indicating a mildly nitrifying condition. Previously reported decay coefficient (0.001e0.006 h1) for the condition was an order less. Changing of the feed to a new water from the same source, but with a low DOC (of 4 mg L1) led to the onset of nitrification complying biostability. The maximum observed chloramine decay coefficient with severe nitrification was 0.085 h1. Therefore, microbes present under mildly nitrifying condition can be as destructive as that in severely nitrifying condition. For better control of chloramine, attention on microbes present under mild nitrification is needed. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Chloramine Chloramine decay Nitrification Heterotrophic bacteria Biostability Dissolved organic carbon DOC
Introduction Chloramine especially monochloramine, behind chlorine, is the second most popular disinfectant used in water supply systems. The typical purpose of chloramine is to provide a longer-lasting residual as the water moves through pipes to consumers. Chloramine is formed when ammonia and chlorine is combined. Subsequently chloramine is not as reactive as chlorine, it forms fewer regulated disinfection by-products such as the Trihalomethanes (THMs) and Halo acetic acids (HAAs) (Bond et al., 2011). The longer lasting nature provides enhanced protection against bacterial regrowth in systems with large storage tanks. Apart from that, ability to minimise taste and odour is an additional advantage (Zaitsev and Dror, 2013). Maintaining an effective chloramine residual throughout a distribution system can be a challenge at times due to accelerated chloramine decay occurring through chemical and microbial reactions (Sathasivan et al., 2005). Auto-decomposition and direct
* Corresponding author. E-mail addresses: [email protected] (B.S. Herath), A.Sathasivan@ uws.edu.au (A. Sathasivan), [email protected] (H.I. Lam). http://dx.doi.org/10.1016/j.ibiod.2015.03.018 0964-8305/© 2015 Elsevier Ltd. All rights reserved.
reaction with chloramine demanding compounds including natural organic matter (NOM) present in water distribution system contribute towards chemical decay of monochloramine (Vikesland et al., 2001). Usually, nitrification is associated with chloramine loss or difficulty in maintaining an adequate disinfectant residual (Wolfe et al., 1990; Cunliffe, 1991; Skadsen, 1993; Odell et al., 1996; Wilczak et al., 1996) and hence nitrification related parameters and disinfectant residual are measured to judge the stability of a disinfectant. Nitrification is the biological oxidation of ammonia with oxygen into nitrite followed by the oxidation of nitrite into nitrate. The first step is performed by ammonia-oxidizing bacteria (AOB) and the second by nitrite oxidizing bacteria (NOB). However, recent findings have shown that other species e e.g., Nitrosomonas oligotropha (Regan et al. 2002), nitrifying Archaea (Hoefel et al., 2011), and heterotrophic nitrifiers (Daum et al., 1998) can be present and contribute to nitrification. In a chloraminated system, the highest residual at which onset of nitrification occurs for a given free ammonia and temperature is reported to follow biostability concept (Sathasivan et al., 2008; Sarker and Sathasivan, 2011; Sarker et al., 2013). According to the biostability concept (Woolschlager et al., 2001) the regrowth of
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AOB can be prevented if the inactivation rate equals or exceeds the bacterial growth rate at each location within the distribution system (Sarker et al., 2013). A biostable residual concentration (BRC) is the chloramine residual below which potential for nitrification occurrence exists. The equation for BRC was implemented by Fleming et al. (2005) and further modified by Sathasivan et al. (2008) as follows:
BRC ¼
mm free ammonia N $ kd Ks þ free ammonia N
(1)
where mm is the maximum specific growth rate of AOB (d1); free ammonia_N ¼ ammonia (NH3eN) plus ammonium (NHþ 4 -N) concentrations (mg-N L1); Ks ¼ half saturation constant for AOB (mg-N L1); kd ¼ the rate constant for inactivation of AOB by disinfectant (L d1 mg-Cl1 2 ); BRC is measured as total chlorine concentration (mg Cl2 L1). Sathasivan et al. (2008) proposed the values for mm/kd ¼ 2 mg-Cl2 L1and Ks ¼ 0.18 mg-N L1 at 20 C. In addition to nitrification, Sathasivan et al. (2005, 2008) and later Bal Krishna et al. (2012) observed microbiologically assisted chloramine decay with a lower but a steady production of nitrite until the onset of nitrification. The former authors termed this behaviour as mild nitrification. Bal Krishna et al. (2013) identified the microbes present under mildly nitrifying conditions; Microbes include heterotrophic bacteria such as Solibacteres, Sphingomonas, Actinobacteria, Psuedomonas and Methylobacterium. These results confirmed the findings of Williams et al. (2004) who analyzed the heterotrophic bacterial community in a chloraminated system and reported the dominance of Alphaproteobacteria. Sequences aligned with Alphaproteobacteria including Afipia, Sphingomonas, Brevundimonas, Blastomonas, Hyphomicrobium, Methylocystis and Bradyrhizobium were reported in the systems. Nitrification normally occurs in reservoirs which are situated far from the water treatment plant during warmer months. On other times, i.e., majority of the time, water in reservoirs experience mild nitrification and it is mild nitrification that accelerates chloramine decay to bring chloramine residual below BRC before the onset of nitrification occurs (Sathasivan et al., 2008). Therefore, investigation of accelerated chloramine decay in mild nitrification stage is crucial. Factors affecting the mildly nitifying microbes are not known but logically it could include organic carbon, chloramine residual and temperature. Organic carbon is known to inhibit nitrification in wastewater treatment systems, but in chloraminated systems the role remains indecisive (Zhang et al., 2009). Others state that nitrifiers are strongly dominated by heterotrophs at a high organic carbon-to-ammonia ratio (Verhagen and Laanbroek, 1991; Ohashi et al., 1995). As a source of organic carbon, natural organic matter (NOM) may also encourage the growth of microbes present under mildly nitrifying conditions and suppress severe nitrification. The NOM, an integral part of water sources used for drinking water purposes, is derived mainly from decaying vegetation and consists of a complex mixture of organic compounds. Therefore, the amount, character and properties of NOM could differ significantly according to origin and biogeochemical cycles of the surrounding environments. The NOM, which is measured as dissolved organic carbon (DOC) also vary on the same location seasonally (Matilainen et al., 2010). Studying such variation may help understanding the dynamics. In this paper, an unusually high DOC (10e12 mg-C L1) observed in natural water and a low DOC (4 mg-C L1) experienced soon after a few months in the same source are tested for its ability to cause/ inhibit nitrification or to encourage microbes causing mild nitrification. The conformance with the bio-stability concept was also tested.
Materials and methods Analytical procedures Total chlorine (TCl), total ammoniacal nitrogen (TAN), nitrite and nitrate were measured immediately after collecting the samples. TCl is the total chlorine and TAN is the summation of NH3eN, NHþ 4 -N and nitrogen associated with chloramine. The Gallery (Thermo Scientific), a high precision chemistry automated analyzer, was adopted for measuring TAN, nitrite and NOx concentrations. It performs discrete, spectrophotometric analysis on optical multi-cell cuvette. Available ammonia reacts with hypochlorite ions generated by the alkaline hydrolysis of sodium dichloroisocyanurate to form monochloramine which reacts with salicylate ions in the presence of sodium nitroprusside at around pH 12.6 to form a blue compound. The compound is measured spectrophotometrically at 660 nm. Nitrite is measured by reaction with sulphanilamide and N-(1-naphthyl)-ethylenediamine dihydorchloride to form a highly coloured azo-dye, thus, the absorbance is measured spectrophotometrically at 540 nm or 520 nm (APHA et al., 1998). The determination of nitrate is by catalytically reducing the nitrate ions into nitrite ions (by nitrate reductive enzyme in the presence of reduced nicotinaminde dinucleotide), the total nitrite ions are then measured by sulphanilamide method as the NOx, and nitrate is obtained by deduction of nitrite from the NOx. The analyzer has the detection limit for TAN, nitratre and nitrite of 0.002 mg-N L1 and NOx has an error of 0.005 mg-N/L. Standard curves for TAN, nitrite and NOx were calibrated for the range 0.0e1.0 mg-N L1 using stock solutions of ammonium chloride, sodium nitrite and sodium nitrate, respectively. The experimental errors were 1.5% for TAN, nitrite and nitrate measurement. TCl residuals were measured by DPD colorimetric method using a HACH pocket colorimeter. It was assumed that more than 99% of the chloramine was present in the form of monochloramine given that pH was above 8.0 and the Cl to TAN mass ratio was approximately 4.0 or less (Valentine and Wilber, 1987). TCl measurement had an experimental error of ±0.03 mg-Cl2 L1. Supplementary details of the analytical methods can be found in Bal Krishna and Sathasivan (2010). Total organic carbon (TOC) was measured using Shimadzu Total Organic Carbon Analyser with an experimental error for TOC of ±5%. DOC was measured using the same analyser, however the sample was filtered through pre-washed 0.45 mm polycarbonate membrane filter papers. After placing the filter paper on the filtration apparatus, around 100 ml of Milli-Q water was passed through the filter device for minimising the contamination of DOC from filter paper into the sample. Free ammoniacal nitrogen was determined (Equation (2)) by assuming all total Cl2 was present as chloramine.
Free Ammonia N ¼ TAN
Total Cl2 5
(2)
where, free ammonia nitrogen concentration is a function of the TAN concentration and the chloramine concentration is measured as total Cl2. Total chloramine decay coefficient was calculated using Equation (3) as defined by Sathasivan et al. (2010).
krt ¼
1 Clin 1 q Clout
(3)
where, Clout and Clin are outlet and inlet chloramine (as TCl) residuals respectively, kRt is the total chloramine decay coefficient and Ɵ is the water retention time in the reactors.
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Portable pH meter (HACH 40d) was used to measure pH values and the measurement error was ±0.1. Stock chemical solutions preparation Stock solutions for all chemicals were prepared using analytical grade chemicals in Milli-Q water. Monochloramine solution was prepared using stock solutions of ammonium chloride (1 g-N L1) and sodium hypochlorite (1 g-Cl2 L1). 1 M sulfuric acid (H2SO4) and 1 M sodium hydroxide (NaOH) solutions were prepared for pH adjustment. Stock solutions of sodium nitrite (NaNO2) and sodium nitrate (NaNO3) for the later experiments were also prepared with the concentration of 1 g-N.L1. Feed water sample collection and feed preparation The water from Warragamba dam, 1.0 m3 in volume, was collected at the point before the disinfectant was added and was transported to University of Western Sydney and stored in High Density Poly-ethylene (HDPE) tank at room temperature. To eliminate contamination by indigenous microbes present in feed water, chlorine (from a stock solution of 1 g-Cl2 L1 sodium hypochlorite) was added to achieve a higher chlorine residual than required at least 3 h before ammonia (from a stock solution of 1 g-N L1 ammonium chloride) was dosed. If chlorine dropped below required level at the time of ammonia dosing, additional (usually a maximum of 0.3 mg-Cl2 L1) amount of chlorine was added. This was kept in 20 L tank (HDPE), as a feed water tank. Feed was prepared every 4 days. The DOC of feed water was 10e12 mg-C L1 and 4 ± 0.5 mg-C L1 during the first 52 days and the next 20 days of the experimental period, respectively. The pH in the feeding tank was adjusted to 8.0 ± 0.1 before feeding the reactors.
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volume and was made of HDPE closed with a lid (HDPE) and the feed water tank was also made of HDPE with a lid (HDPE). The system was facilitated with automatic water flow; control unit devices were used to control the water flow rate between feeding tank and R-1. Nevertheless, the outlet and inlet pipe level was adjusted to create gravity flow along the system. Water samples for analysis were collected from the outlet, fixed at the bottom of each reactor. Operation of the laboratory scale system The reactor set was operated in the room where the temperature was 20e22 C. During the startup period, the feed water with a chloramine concentration of about 1.0 mg-Cl2 L1 (TCl:TAN ratio, 4:1) was prepared. About 5 L water was fed continuously per day and water volume was maintained constant within each reactor to gain a retention time of 24 h. The various nitrification conditions (from no nitrification to severe nitrification) which generally occur in a full scale distribution system were created along the reactors. Chloraminated water samples were collected from the Sydney water distribution system with a high nitrite concentration (nitrite> 0.1 mg-N L1); to facilitate nitrification and to acquire specific inoculum usually present in distribution system. The water samples were placed as seed microorganisms in each reactor, except for R-1. The lab scale reactor system was continuously operated for 72 days of experimental period. Within a week, chloramine concentration in the feed water tank was increased up to 1.2e1.5 mg-Cl2 L1 and was maintained the same in the first 20 days. Between 20 and 30 days, it was adjusted to 0.7 mg-Cl2 L1 and then from 30 days onward 1.2 mg-Cl2 L1. Parameters such as TCl residuals, TAN, nitrite and nitrate were continuously monitored along the reactors (R-1 to R-4) continuously. Results and discussion
Laboratory scale system set-up The laboratory scale system was set up at Environmental Engineering Laboratory in University of Western Sydney. The system (Fig. 1) consisted of four reactors, namely R-1, R-2, R-3 and R-4, connected in series through HDPE pipes. Each reactor was 5 L in
Nitrogenous species and chloramine decay during experimental period More than 0.01 mg-N L1 nitrite level and simultaneous reduction of TAN level are hints of the occurrence of onset of
Fig. 1. Schematic diagram of laboratory scale system setup.
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nitrification (Sathasivan et al., 2008). In the first 20 days of the operational period chloramine decayed with constant and low nitrite level (less than 0.004 mg-N L1) stable TAN and nitrate levels along the reactors (Fig. 2AeD). The results confirmed there was no severe nitrification, but mild nitrification. Lowest chloramine level observed was 0.6 mg-Cl2 L1 in the fourth reactor which may not have encouraged the growth of nitrifying bacteria. Therefore, the chloramine residual in the feed was reduced to 0.7 mg-Cl2 L1 (Cl:TAN ¼ 4:1) for the next 10 days. However, severe nitrification could not be observed even in R-4 with the low chloramine level (0.15 mg/L). Even when the feed chloramine residual was increased to between 1.2 and 1.0 mg-Cl2 L1 (from 30 days onward), nitrite and nitrate levels in all reactors remained steady and less than 0.012 mg-N L1 and 0.35 mg-N L1 respectively, which confirmed all reactors were in mildly nitrifying stage. After the introduction of the new feed water with low DOC level on day 52, a simultaneous reduction of TAN level (Fig. 2B), a significant increase of nitrite level (Fig. 2C) and acceleration of nitrate level (Fig. 2D) indicated the system experienced severe nitrification. Presence of chloramine decaying organisms (CDOs) - confirmation by decay rate coefficients Fig. 3 illustrates the chloramine decay coefficient for all reactors (R-1 to R-4). The decay coefficients of reactors generally increased along the reactors (kR4 > kR3 > kR2 > kR1). This has occurred during both when severe nitrification took place and not taking place.
In the first 23 days of high DOC feed, observed kRT for all reactors were stable (Fig. 3). After the adjustment of chloramine residual concentration of feed water (to 0.7 mg-Cl2 L1), maximum kRT in all reactors increased from 0.014 h1 to 0.029 h1 between 24th and 30th days with neither significant increase of nitrite and nitrate nor decrease of TAN (Fig. 2C, B and D). During the period when chloramine residual was maintained at 1.2 mg-Cl2 L1, kRT considerably increased from 0.029 h1 to 0.06 h1. This was associated with low nitrite (0.25 mg-N/L). Bal Krishna et al. (2013) observed similar increase in decay coefficients following severe nitrification, but mildly nitrifying conditions did not result in such high decay coefficients. The results, therefore, for the first time demonstrate that microbes other than nitrifiers could accelerate chloramine decay to the same degree as nitrifying reactors. Did high DOC reactor have suitable condition for nitrification? It could be questioned if chloramine and ammonia levels were sufficient for the onset of nitrification in reactors R-3 and R-4 in case of high DOC fed reactors and too much free ammonia and less chloramine in the low DOC reactors. To show that there were suitable conditions for AOB to thrive, the data was checked against the BRC curves proposed by Sathasivan et al. (2008). A nitrite level stayed lower than or equal to 0.012 ± 0.002 mg-N L1 during high DOC fed period and this level was considered non-nitrifying. (Severe) Nitrification could not be observed when reactors fed with 10e12 mg-C L1 DOC contained water. Therefore, BRC curve did not produce two zones such as nitrifying and non-nitrifying; open circles representing the no nitrification were evenly distributed on both sides of the BRC curve (Fig. 4A). After the introduction of the low DOC contained water, clearly distinguished nitrifying and non-nitrifying zones separated by the BRC curve was observed (Fig. 4B). Therefore, there existed suitable conditions inside the reactor to cause nitrification in both high and low DOC water fed periods showing that it is the suppression of AOB or encouragement of other microbes by increased presence of DOC that has caused the phenomenon. Implications for further research and practice Traditionally chloramine stability is controlled by controlling nitrification and the research is directed towards identifying nitrifying organisms that are present in the water. Earlier findings (Sathasivan et al., 2005, 2008; Bal Krishna et al., 2013) showed that the behavior (mild nitrification mediated by CDOs) under high chloramine residual is noticed until the onset of nitrification.
However, the finding of this research, although at a high DOC level, showed that microbes other than nitrifiers can be present and can equally decay chloramine at all levels of chloramine. If these microbes (CDOs) co-exist with nitrifiers under low chloramine residuals, they can equally decay the chloramine under severely nitrifying condition. Chloraminated water supply systems experience nitrification mostly during summer (usually three months) but other times of the year, mild nitrification controlled by CDOs prevail. When mildly nitrifying conditions decrease the residual below the BRC level (Equation (1)), the onset of (severe) nitrification starts. Therefore, the key to controlling nitrification is controlling CDOs which prepare the conditions conducive to nitrification. The stability of chloramine cannot be determined by traditional nitrification related indicators and hence microbial decay coefficients need to be measured regularly using either microbial decay factor (Fm) method (Sathasivan et al., 2005) or reservoir acceleration factor (Fra) method (Sathasivan et al., 2010). As conditions conducive to the proliferation and activity of these bacteria (CDOs) are unknown, further research is needed. The factors could be high amount of assimilable organic carbon (not measured in this research), temperature and chloramine residual. Hence, the findings of the research underscore the importance of the organisms present under mildly nitrifying conditions for the stability of chloramine and the importance of research in this area. Conclusion It is traditionally believed the chloramine stability is microbiologically challenged by nitrifying organisms and much research is focused on understanding the behaviour of the organisms. However, the findings depicted that microbes other than nitrifiers could equally decay chloramine. A lab-scale reactor was fed after chloraminating the water containing high DOC (10e12 mg-C/L) followed by the one with low DOC (4 mg-C/L) from the same source. High DOC greatly suppressed the activity of nitrifying organisms but still maximum chloramine decay rate was registered to be 0.06 h1. When supplying low DOC containing water, onset of nitrification was noted as predicted by the biostability concept proposed earlier and a maximum decay rate of 0.085 h1 was noted.
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A
B
Fig. 4. A and B: Testing conformance with the BRC concept using the literature proposed coefficients of Sathasivan et al., 2008 (mm/kd ¼ 2 mg-Cl L1 and Ks ¼ 0.18 mg-N L1) for the performance of reactor set operated with high and low DOC levels.
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