- Email: [email protected]
Evaluation of zinc and copper for co-inhibition of nitrification in mild nitrified drinking water
Journal of Environmental Chemical Engineering 6 (2018) 2939–2943
Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Evaluation of zinc and copper for co-inhibition of nitrification in mild nitrified drinking water
T
⁎
Dipok Chandra Sarker , Chandni Manji Patel, Anna Heitz, A.H.M. Faisal Anwar Department of Civil and Construction Engineering, Curtin University, GPO Box U1987, Perth WA 6845, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Chloramine decay Biostability analysis Disinfectant and metal inhibition
Maintaining adequate chloramine and overcoming nitrification are major challenges faced by water utilities where chloramine is used as a disinfectant. Laboratory batch experiments were carried out to evaluate the effectiveness of using zinc and copper as metal inhibitors in drinking water systems under nitrified conditions. The experiments were conducted for bulk water samples that were collected from a real drinking water distribution system. From the batch experimental results, it was found that 0.25 mg/L zinc inhibited nitrification and reduced the chloramine decay rate but that 0.25 mg/L zinc together with 0.2 mg/L copper significantly controlled chloramine decay and inhibited nitrification completely. Biostability analysis showed that the biostable residual concentration (BRC) was lowered with the addition of zinc, and zinc with higher copper concentrations i.e. the lowest BRC was calculated for 0.25 mg/L zinc with 0.2 mg/L copper. Therefore, it can be concluded that zinc alone can be used as an inhibitor of nitrification and chloramine decay but co-inhibition using zinc and copper provides better control of chloramine decay.
1. Introduction Many water utilities are facing challenges with disinfectant control in drinking water systems due to the effects of rising temperatures and climate change. Temperature increases lead to high levels of disinfectant decay which increase the risks associated with waterborne pathogens. To provide water that is free from microbial contamination, water utilities use free chlorine as a disinfectant but chlorine produces higher concentrations of disinfection by-products (DBPs) as well as taste and odour issues. Therefore many water utilities now utilise monochloramine as a final disinfectant [1]. Despite stringent controls on DBP concentrations, it is crucial to maintain water quality to avoid any possible waterborne diseases. Although chloramine produces less DBPs, and taste and odour compounds in comparison to free chlorine, it enhances nitrification activity in the system which causes chloramine decay [2]. Normally, chloramination is carried out by adding chlorine followed by ammonia or by adding chlorine and ammonia simultaneously. Chloramine decays in two ways: chemical decay and microbiologically assisted decay. The chemical decay of monochloramine (NH2Cl) in drinking water can be due to auto-decomposition and reactions with organic or inorganic constituents [3,4]. According to Valentine and Jafvert [5] auto-decomposition of NH2Cl occurs according to the reaction presented in Eq. (1). During the auto-decomposition reaction,
⁎
ammonia from chloramine is oxidised to nitrogen gas with production of smaller quantities of nitrate (NO3−) [Eq. (2)] [6]. The direct reaction between NH2Cl and NO2− is described by Eq. (3). 3NH2Cl → N2 + NH3 + 3Cl− + 3H+
(1)
4NH2Cl + 3H2O → 3NH3 + NO3− + 4Cl− + 5H+ NH2Cl
+ NO2− +
H2O
→ NO3− +
+
−
NH4 + Cl
(2) (3)
It is thought that the main cause of rapid chloramine loss is nitrification. Nitrification is a two-stage oxidation process where ammonia is oxidised to form nitrite due to the presence of ammonia oxidising bacteria (AOB). The nitrite is then oxidised to form nitrate with the aid of nitrite oxidising bacteria (NOB). These nitrifying bacteria obtain their energy for oxidation from chloramine which leads to an enhancement in chloramine decay. Nitrification also leads to a depletion of dissolved oxygen and reduction of pH and alkalinity which can cause corrosion in pipe systems [2,7]. The nitrification process is influenced by the initial chloramine concentration, free ammonia, temperature, pH and pipe material [8]. Nitrification has been observed more frequently at low chloramine residuals [9–15]. Chloramine levels of 1.0 to 2.0 mg/L used for potable water should be sufficient to overcome nitrifiers [16] but once nitrification has commenced, it is very hard to control even by increasing chloramine residuals up to 8.0 mg/L [10].
Corresponding author. E-mail address: [email protected] (D.C. Sarker).
https://doi.org/10.1016/j.jece.2018.04.028 Received 4 December 2017; Received in revised form 28 March 2018; Accepted 15 April 2018 Available online 18 April 2018 2213-3437/ © 2018 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 6 (2018) 2939–2943
D.C. Sarker et al.
2.2. Preparation of sample bottles and glassware
Maintenance of adequate chloramine residuals is a challenge during hot weather and for long distribution systems due to increasing nitrifying activities. Previous studies [17–19,8] concluded that chloramine residual combined with the availability of free ammonia can play a significant role in the onset of nitrification in distribution systems. In order to determine the onset of nitrification, [17] proposed the biostability concept, which was adopted by Harrington et al. [20] and used by Fleming et al. [18] to determine the residual below which potential for nitrification occurs in a laboratory scale system. This is based on the balance between the growth rate of microbes and the disinfection rate, using ammonia as the substrate for controlling growth of AOB and dichloramine as the disinfectant. Sathasivan et al. [8] considered total chlorine as a disinfectant and proposed the following Equation:
μm (free ammonia as N ) (K S + free ammonia as N )
Sample bottles (material: polyethylene terephthalate (PET), volume = 600 ml) and glassware were soaked for 24 h in 2–3% sodium hypochlorite prior to rinsing six times with reverse osmosis (RO) treated laboratory water to ensure they were free of chlorine. 2.3. Preparation of stock chemical solutions Stock solutions of reagents and standards were prepared by mixing analytical-grade chemicals with Milli-Q ultra-pure water (18 MΩ/ cm, < 100 ppb-C L−1). Stock solutions of total chlorine (TCl; 1000 mg/ L), ammonia-nitrogen (500 mg/L), nitrite-nitrogen (NO2-N; 500 mg/L) and nitrate-nitrogen (NOx-N; 500 mg/L) were prepared using sodium hypochlorite (12.5% w/v, Analytical reagent, Rowe Scientific Pty Ltd.), ammonium chloride (Analytical reagent, Biolab Australia Ltd.), sodium nitrite (Analytical reagent, Thermo Fisher Scientific Australia Pty. Ltd) and sodium nitrate (Analytical grade, Thermo Fisher Scientific Australia Pty. Ltd), respectively. Monochloramine (NH2Cl) solution was prepared using stock solutions of ammonium chloride (500 mg/L as N) and sodium hypochlorite (1000 mg/L as Cl2) by maintaining the proportions of chlorine and nitrogen as TCl:TAN = 4.1:1. Total ammoniacal nitrogen (TAN) is the sum of NH3-N, ammonium (NH4+-N) and ammonia nitrogen associated with chloramine. For determination of calibration standard curves, working solutions (1 mg/L) were prepared using ammonium chloride, sodium nitrite and sodium nitrate for ammonia-nitrogen, NO2-N and NOx-N respectively. Standard copper sulphate solution (1000 mg/L-Cu) was prepared by mixing copper sulphate (CuSO4·5H2O; Analytical reagent, Chem-Supply Pty Ltd) into Milli-Q ultra-pure water. A zinc solution was prepared (1000 mg/L-Zn) by mixing zinc chloride (ZnCl2; Analytical reagent, Rowe Scientific Pty Ltd.) with Milli-Q ultra-pure water. The pH of the standard solution of copper sulphate was maintained below 6.0 to preserve copper as cupric ions. The pH was adjusted using 1 M hydrochloric acid (General laboratory reagent, Rowe Scientific Pty Ltd.) and 1 M sodium hydroxide (Analytical reagent, Rowe Scientific Pty Ltd.).
= kd × BRC (4)
where, μm is the maximum specific growth rate of AOB (day−1); Ks is the half saturation constant for AOB (mg/L nitrogen); kd is the rate constant for inactivation of AOB by disinfectant (L day−1 mg/LChlorine); BRC is the total chlorine concentration (mg/L) referred to as biostable residual concentration. Several approaches have been taken previously to overcome nitrification. These include decreasing detention times, increasing chloramine concentration, flushing affected areas, breakpoint chlorination, increasing chlorine to ammonia ratio and metal inhibition [20–22]. These strategies have not been very effective and thus, more research needs to be conducted on possible inhibitors. In metal inhibition, metals are used to inactivate and kill nitrifiers by blocking their enzymatic functions. Previously, research has been carried out by using copper, silver and phosphorus as inhibitors in drinking water systems [23–25]. Silver is used by the water utilities in Europe as it is an effective inhibitor in lower temperatures, however, more research needs to be carried out to find the optimum concentration of silver for use in warmer climates [26]. Research was also done on copper inhibition for a lab scale system, which was effective but research is required to determine the effectiveness of using copper in a full-scale system [27]. Studies on phosphorus inhibition were only carried out for copper pipes and thus, may not be applicable for other pipe material [24,28]. Studies of inhibition by zinc and nickel were carried out in wastewater treatment plants (WWTP) and further research is needed to determine their applicability to drinking water distribution systems [29]. As zinc was an effective inhibitor in WWTP and copper has been shown to have some effect, a combination of both zinc and copper could be used as inhibitors to overcome chloramine decay. The aim of this study is to examine the effectiveness of zinc and copper co-inhibition for controlling chloramine decay in a mildly nitrified drinking water under conditions typical to potable water distribution systems.
2.4. Analytical procedures TAN, NO2-N, and NOx-N concentrations were measured using an Aquakem-200, a high precision wet chemistry automated analyser. TAN was measured using the phenolate method [30]. NO2-N was measured using the sulphanilamide method (4500-NO2 B; [31] and NOx-N was measured by enzymatic reduction [32,33]. Aquakem-200 has a high efficiency with a detection limit of 0.002 mg/L for the measurement of TAN, NO2-N and NOx-N. The experimental error was found to be ± 1.5% for TAN and NO2-N, and ± 2% for NOx-N measurement. TCl residuals were measured using the DPD colorimetric method with a HACH pocket colorimeter. TCl measurement had an experimental error of ± 0.03 mg/L [34] reported that more than 99% of chloramine is present in NH2Cl form when pH is above 7.5. Hence, TCl represents mainly NH2Cl residual at pH 8. A portable pH meter (HACH 40d) with temperature compensation was used to measure pH values and the measurement error was ± 0.10.
2. Materials and methods 2.1. Sample collection To examine the effectiveness of using copper and zinc inhibitors to reduce nitrification and maintain chloramine residual, bulk water samples were collected from the public taps of a water supply system pipeline. The water was collected in 25L high-density polyethylene tanks (pre-cleaned with sodium hypochlorite to remove impurities), transported and stored at room temperature in the laboratory. In order to determine the nitrification status of the samples, total chlorine was measured onsite. In addition total chlorine, ammonia nitrite, and NOxN levels were monitored daily in the laboratory. The samples were stored in the laboratory and the temperature inside the container was kept at 25 °C by using glass heaters to enhance the growth of nitrifying bacteria.
2.5. Experimental design Two sets of batch test were carried out to understand the effect of zinc and copper inhibition and the outline of the different tests are shown in Fig. 1. There was a control sample for each set (no metal addition). Duplicate samples were prepared for each set and average results of each set are presented. The initial weight ratio of TCl:TAN was maintained at 4.1:1 by adding appropriate amounts of ammonium followed by chlorine (sodium hypochlorite). The pH of each sample was maintained at 8.0 ± 0.1 by adding HCl or NaOH. The first set of experiments was designed to check the inhibitory 2940
Journal of Environmental Chemical Engineering 6 (2018) 2939–2943
D.C. Sarker et al.
Table 1 General water quality characteristics of sample at the time of collection and initial conditions for batch test.
effects of zinc on AOB activity. The nitrified bulk waters were separated into three sub-sets of sample. The first sub-set remained unchanged; the second and third sub-sets were dosed with 0.25 mg/L and 0.50 mg/L zinc respectively. The three sub-sets were referred as ‘Zn-0′, ‘Zn-0.25′ and ‘Zn-0.50′ respectively. A second set of experiments was conducted to determine how zinc combined with copper impact on nitrification inhibition for mildly nitrified bulk waters. The nitrified bulk waters were separated into three subsets of samples. The first sub-set remained unchanged, as a control; the second and third sub-sets were dosed with 0.10 mg/L and 0.20 mg/ L copper respectively after subsequent addition of 0.25 mg/L zinc. The three sub-sets were referred as ‘Zn-0 + Cu-0′, ‘Zn-0.25 + Cu-0.1′ and ‘Zn-0.25 + Cu-0.2′ respectively. Samples were incubated in a constant temperature water bath at 25 °C. Levels of TCl, TAN, NO2-N and NOx-N were monitored regularly in all samples.
TCl (mg/L) TAN (mg/L) NO2-N (mg/L) NOx-N (mg/L) pH Temperature (°C)
1.31 0.596 0.181 0.546 7.42 NA
2.00 0.600 0.185 0.546 8.0 ± 0.3 20 ± 1
Zinc was added to samples at two concentrations (0.25 and 0.50 mg/L; experimental set 1, Fig. 1) in order to test the effect of zinc on inhibition of nitrification to maintain chloramine residual. In the TCl profiles of zinc inhibited bulk water samples (0.25 and 0.50 mg/L Zn added) an initial chloramine loss, possibly partly due to auto-decomposition, occurred in both the samples. However, significantly higher concentrations of TCl were maintained throughout in the sample with the higher dose of Zn (0.5 mg/L; Fig. 3A). The kt values for 0.25 and 0.50 mg/L zinc inhibited samples were determined to be 0.0013 and 0.0004 h−1 respectively. Therefore, the higher concentration of zinc (0.5 mg/L) appeared to be effective in inhibiting nitrification which shows that the higher concentration of metal should have an increased effect on inhibition [35]. Thus, only results of 0.5 mg/L zinc dosed sample are presented in the following discussion. Trends for TCl and TAN profiles decreased, together with an upward trend of the NOx-N profile for the unprocessed sample after 50 h (Fig. 3B). On the other hand, TCl, TAN and NOx-N profiles remained constant throughout the incubation period for zinc inhibited samples. As the samples were mildly nitrified, there was a slower chloramine decay. Although the kt values of unprocessed and zinc inhibited samples were low, the addition of zinc reduced the decay rate constant from 0.0042 to 0.0004 h−1, a 10-fold reduction. The stable pattern of NOx-N in the zinc inhibited sample indicated that zinc was effective in
First-order decay kinetics were used to determine the total chloramine decay rate according to the following equation (5)
where TClt is the total chloramine residual (mg/L) at time t (hours), TCl0 is the initial total chloramine concentration (mg/L), t is incubation time in hours and kt is the total chloramine decay coefficient per hour. The free ammonia present in a sample is calculated by the following equation
TClt 5
Initial concentration of water quality parameters in batch test
3.2. Impact of zinc on mild nitrified bulk water
2.6. Determination of total chloramine decay rate and calculation of free ammonia
Free ammonia = TAN −
Typical conditions onsite water quality parameters during sampling
the samples. Prior to the commencement of the test, the chloramine concentration of the sample was adjusted to TCl = 2.0 mg/L and TAN = 0.6 mg/L and pH was adjusted to 8.0 (Table 1). Surrogate parameters were monitored over the course of a week while maintaining the sample temperature at 25 °C. TCl and TAN gradually decreased with a gradual increase in NOx-N, which confirmed the presence of nitrifying activity in the sample. TCl and TAN profiles started to decrease while the NOx-N profile increased after 50 h. After 75 h incubation time, it was observed that the NO2-N concentration was 0.185 mg/L with a calculated kt of 0.0072 h−1. These conditions were classified as mild nitrification as defined by Sathasivan et al. [8] (Fig. 2).
Fig. 1. Schematic diagram of experimental method. Units of TCl, TAN, copper and zinc are mg/L.
TClt = TCl 0⋅e(-kt)
Parameters
(6)
where, the factor 5 equals 70 mg-Cl2 per 14 mg-N. 3. Results and discussion 3.1. General characteristics of the collected samples Nitrification was shown to be established in samples after the first 75 h of incubation. Table 1 presents the water quality parameters of the bulk water sample collected from publicly accessible taps in two regional towns in Western Australia that are supplied with chloraminated water. Three sets of sample were collected and analysed in the laboratory. As all the samples showed similar water quality characteristics, the typical values are presented in Table 1. The recorded TCl and NO2-N concentrations of the sample during the sampling period were 1.31 mg/L and 0.181 mg/L respectively. A batch test was conducted to assess the nitrification conditions of
Fig. 2. Profiles of TCl and Nitrogenous compounds over 75 h of incubation. 2941
Journal of Environmental Chemical Engineering 6 (2018) 2939–2943
D.C. Sarker et al.
Fig. 4. Profiles of TCl for zinc and copper inhibited samples. Table 2 Effect of Zn and Cu, individually and in combination, on chloramine decay rate constant.
Fig. 3. A Variation of TCl profiles for 0.25 and 0.50 mg/L zinc inhibited bulk waters. B Profiles of TCl and nitrogenous compounds for mild nitrifying samples.
Type and concentration of metal added
Chloramine decay rate constant (hr−1)
Zn-0 + Cu-0 Zn-0.25 Zn-0.50 Zn-0.25 + Cu-0.10 Zn-0.25 + Cu-0.20
0.0072 0.0019 0.0009 0.0016 0.0008
± ± ± ± ±
0.0015 0.0004 0.0002 0.0003 0.0001
between Zn-0.25 and Zn-0.25 + Cu-0.10 when considering the experimental error but the lowest kt value was observed in Zn-0.25 + Cu-0.20 sample. This confirms that ‘zinc only’ inhibition is not as effective as when Zn is used in combination with copper as stated in previous research [21]. Additionally, metals cause greater inhibition when applied in higher concentrations. The results clearly revealed that adding zinc and copper in combination have a significant controlling effect on total chloramine decay in the bulk water.
inhibition of nitrification. Higher TCl and TAN profiles in the zinc modified samples indicate that zinc provides better stability of chloramine residual. TAN loss was slightly higher (0.10 mg/L) than Total inorganic nitrogen (TIN) loss in unprocessed samples which might be due to the presence of natural organic matter (NOM) and soluble microbial products (SMPs). NOM oxidation is a slow process and could be a dominant mechanism due to long incubation of the sample [36,37]. However, TAN loss was the same as TIN loss in zinc inhibited samples which suggests that zinc is beneficial in preventing the chloramine decay caused by NOM and SMPs.
3.4. Applicability of the biostability concept in mild nitrified bulk waters As nitrification is an important phenomenon in chloramine disinfection, a BRC curve assists water utilities to prevent the growth of nitrifying bacteria in water distribution systems [38]. Since BRC is defined as the residual below which potential for nitrification occurs, water dosed with a combined Zn/Cu mixture should be free of nitrification at much lower chloramine residual than water containing no metals or lower doses of metals. Therefore, the BRC values with respect to different TAN concentrations for the different samples were calculated according to Eq. (4) (Fig. 5). Ks = 0.18 [8], μm/kd = 2.046 for 25 °C [39] and the free ammonia is calculated according to Eq. (6) are used to calculate the BRC. The required residual concentration e.g. BRC value of Zn-0 + Cu0 sample is higher than the Zn-0.25, Zn-0.25 + Cu-0.1 and Zn0.25 + Cu-0.2 samples for the same concentration of TAN (Fig. 5). This indicates that zinc dosing alone or with copper would be sufficient in mildly nitrified chloraminated bulk waters to stop nitrification. A zinc concentration of 0.25 mg/L is sufficient to control nitrification significantly but zinc with copper (Zn-0.25 + Cu-0.20) could stop the excessive chloramine decay rates observed during the nitrifying phase probably due to NOM and SMPs that are likely to present in these samples. To take advantage of using metal co-inhibition, utilities could dose metals before the onset of nitrification especially at times when chloramine residuals cannot be maintained above the BRC. Such temporary metal co-inhibition would be helpful as an early intervention to stop the onset of nitrification and thus to protect chloramine residual.
3.3. Impact of zinc and copper co-inhibition on mildly nitrified bulk water Since zinc at 0.25 mg/L was found to have some effect in the control of nitrification and copper at 0.25 mg/L was sufficient to inhibit/inactivate nitrifying bacteria in nitrified bulk water samples [27], experiments were conducted to determine the effect of these two elements in combination. The impact of copper (0.10 or 0.20 mg/L) in combination with of zinc (0.25 mg/L) on chloramine in mildly nitrified bulk water samples was tested. Both copper and zinc inhibited nitrification with an increase in the effect with increasing total metal concentrations (Fig. 4). Chloramine decay was rapid for the unprocessed sample during the first 6 h of the experiment from 1.92 to 1.77 mg/L and it continued to decay over the full period of the test. However, the samples treated with both copper and zinc had a much more stable chloramine residual. For the zinc and copper inhibited samples, all three showed similar trends in chloramine decay profiles. The total chloramine decay coefficients demonstrated the effectiveness of using Zn alone and Zn combined with Cu (Table 2). It was observed that the Zn-0 + Cu-0 sample had the highest kt value whereas the Zn-0.25 + Cu-0.20 sample had the lowest value. Addition of high dose of zinc (Zn-0.50) and co-inhibition of Zn and Cu can reduce the chloramine decay and thus provide better residual stability in the bulk waters. There was no significant difference in kt values 2942
Journal of Environmental Chemical Engineering 6 (2018) 2939–2943
D.C. Sarker et al.
1987, pp. 819–832. [7] K.C. Bal Krishna, A. Sathasivan, D.C. Sarker, Evidence of soluble microbial products accelerating chloramine decay in nitrifying bulk water samples, Water Res. 46 (2012) 3977–3988. [8] A. Sathasivan, I. Fisher, T. Tam, Onset of severe nitrification in mildly nitrifying chloraminated bulk waters and its relation to biostability, Water Res. 42 (2008) 3623–3632. [9] N.I. Lieu, R.L. Wolfe, I.E.G. Means, Optimizing chloramine disinfection for the control of nitrification, J. Am. Water Works Assoc. 85 (1993) 84–90. [10] J. Skadsen, Nitrification in a distribution system, J. Am. Water Works Assoc. 85 (7) (1993) 95–103. [11] L.H. Odell, G.J. Kirmeyer, A. Wilczak, G.J. Joseph, P.M. Joseph, R.L. Wolfe, Controlling nitrification in chloraminated systems, J. Am. Water Works Assoc. 86 (1996) 86–98. [12] A. Wilczak, J.G. Jacangelo, J.P. Marcinko, L.H. Odell, G.J. Kirmeyer, Occurrence of nitrification in chloraminated systems, J. Am. Water Works Assoc. 88 (1996) 74–85. [13] S.B. Liu, J.S. Taylor, A.A. Randall, J.D. Dietz, Nitrification modelling in chloraminated distribution system, J. Am. Water Works Assoc. 97 (10) (2005) 98–108. [14] Y. Zhang, M. Edwards, Accelerated chloramine decay and microbial growth by nitrification in premise plumbing, J. Am. Water Works Assoc. 101 (11) (2009) 51–61. [15] C. Nguyen, C. Elfland, M. Edwards, Impact of advanced water conservation features and new copper pipe on rapid chloramine decay and microbial regrowth, Water Res. 36 (3) (2012) 611–621. [16] R.L. Wolfe, N.I. Lieu, G.E. Means, Ammonia-oxidizing bacteria in a chloraminated distribution system: seasonal occurrence, distribution, and disinfection resistance, Appl. Environ. Microbiol. 56 (1990) 451–462. [17] J.E. Woolschlager, B. Rittmann, P. Piriou, B. Schwartz, Using a comprehensive model to identify the major mechanisms of chloramine decay in distribution systems, Water Sci. Technol.: Water Supply 1 (2001) 103–110. [18] K.K. Fleming, G.W. Harrington, D.R. Noguera, Nitrification potential curves: a new strategy for nitrification prevention, J. Am. Water Works Assoc. 97 (2005) 90–99. [19] K.D. Pintar, W.B. Anderson, R.M. Slawson, E.F. Smith, Assessment of a distribution system nitrification critical threshold concept, J. Am. Water Works Assoc. 97 (2005) 116–129. [20] G.W. Harrington, D.R. Noguera, A.I. Kandou, D.J. VanHoven, Pilot scale evaluation of nitrification control strategies, J. Am. Water Works Assoc. 94 (2002) 78–89. [21] Z. Hu, K. Chandran, D. Grasso, B.F. Smets, Impact of metal sorption and internalization on nitrification inhibition, Environ. Sci. Technol. 37 (4) (2003) 728–734. [22] D.C. Sarker, Modelling Inhibition of Microbes Responsible for Acceleration of Chloramine Decay in Water Supply System. Doctoral Thesis, Curtin University, Bentley, Western Australia, 2012. [23] W. Zhan, Effect of Iron Corrosion on the Fate of Dosed Copper to Inhibit Nitrification in Chloraminated Water Distribution System. Master Thesis, Curtin University, Bentley, Western Australia, 2011. [24] X. Li, V. Kapoor, C. Impelliteri, K. Chandran, J.W.S. Domingo, Measuring nitrification inhibition by metals in wastewater treatment systems: current state of science and fundamental research needs, Crit. Rev. Environ. Sci. Technol. 46 (3) (2016) 249–289. [25] H. Gruettner, M. Winther-Nielsen, L. Jorgensen, P. Bogebjerg, O. Sinkjaer, Inhibition of the nitrification process in municipal wastewater treatment plants by industrial discharges, Water Sci. Technol. 29 (9) (1994) 69–77. [26] K.C. Bal Krishna, A. Sathasivan, Effect of silver in severely nitrified chloraminated bulk waters, Water Sci. Technol. Water Supply 12 (4) (2012) 415–421. [27] D.C. Sarker, A. Sathasivan, B.E. Rittmann, Modelling combined effect of chloramine and copper on ammonia oxidizing microbial activity using a biostability approach, Water Res. 84 (2015) 190–197. [28] M. Edwards, H. Loay, G. Dawn, Phosphate inhibition of soluble copper corrosion byproduct release, Corros. Sci. 44 (5) (2002) 1057–1071. [29] Z. Hu, K. Chandran, D. Grasso, B.F. Smets, Comparison of nitrification inhibition by metals in batch and continuous flow reactors, Water Res. 38 (2004) 3949–3959. [30] Methods for the Examination of Waters and Associated Materials, Ammonia in Waters, (1981). [31] APHA, WEF, AWWA, Standard Methods for the Examination of Water and Wastewater, 20th ed., (1998) (Washington, DC). [32] W.H. Campbell, P. Song, G.G. Barbier, Nitrate reductase for nitrate analysis in water, Environ. Chem. Lett. 4 (2006) 69–73. [33] C.J. Patton, A.E. Fischer, W.H. Campbell, E.R. Campbell, Corn leaf nitrate reductase: a nontoxic alternative to cadmium for photometric nitrate determinations in water samples by air-segmented continuous-flow analysis, Environ. Sci. Technol. 36 (2002) 729–735. [34] R.J. Valentine, Opinion as to Probable Chloramine Speciation in San Francisco Public Utilities Drinking Water Distribution System, (2007) sfwater.org/Files/ Other/Chloramine%20Speciation%20Opinion.pdf. (Accessed 18.06.08). [35] S.R. Juliastuti, J. Baeyens, C. Creemers, Inhibition of nitrification by heavy metals and organic compounds: the ISO 9509 Test, Environ. Eng. Sci. 20 (2) (2003) 79–90. [36] A. Sathasivan, K.C.B. Krishna, Major Mechanism (s) of chloramine decay in rechloraminated laboratory scale system waters, Desalin. Water Treat. 47 (1-3) (2012) 112–119. [37] J.P. Vikesland, K. Ozekin, L.R. Valentine, Monochloramine decay in model and distribution system waters, Water Res. 35 (2001) 1766–1776. [38] S. Srinivasan, G.W. Harrington, Biostability analysis for drinking water distribution systems, Water Res. 41 (10) (2007) 2127–2138. [39] D.C. Sarker, A. Sathasivan, A.J. Cynthia, A. Heitz, Modelling temperature effects on ammonia-oxidising bacterial biostability in chloraminated systems, Sci. Total Environ. 454–455 (2013) 88–98.
Fig. 5. Inhibitory effects of metal on nitrification in terms of BRC.
3.5. Implication for nitrification control and residual management Nitrification plays an important role in causing chloramine decay in distribution systems where chloramine is used as a disinfectant and water utilities usually increase operational monitoring to determine causes and methods of control of nitrification. The traditional approaches to overcome nitrification and chloramine decay by break point chlorination, adjusting chlorine to ammonia ratio or re-chloramination have been relatively ineffective. In chloraminated samples, nitrification inhibition by metal addition has been shown to have some positive effects for maintaining chloramine residual although this depends on various factors such as pH, availability of natural organic matter and their complexing ability with metal, and substrate concentration (free ammonia). Although, a preventive approach to avoid triggers of nitrification is needed for long term control, addition of metal before or just upon the onset of nitrification could assist in better control of chloramine residual. While the experimental results in this study showed that co-inhibition of copper and zinc reduce chloramine decay, further research is needed to demonstrate its impact on other mechanisms in the nitrification process. 4. Conclusion For better understanding of how addition of the metals zinc and copper to chloraminated water would act as an inhibitor, experiments were performed on mild nitrifying chloraminated bulk water samples. Though zinc alone is able to inhibit nitrification and control chloramine decay, the co-inhibition effects of zinc and copper reduced the chloramine decay rate from 0.0072 h−1 to 0.0004 h−1. Therefore, zinc alone inhibited nitrification and thus reduced the chloramine decay rate but the combination of zinc with copper strongly reduced chloramine decay in chloraminated nitrifying bulk waters. Water utilities can take advantage of these findings if they opt to dose zinc and copper during warmer months when there is higher likelihood of chloramine concentrations decreasing below the BRC values. References [1] K.C. Bal Krishna, A. Sathasivan, Does an unknown mechanism accelerate chemical chloramine decay in nitrifying waters? J. Am. Water Works Assoc. 102 (10) (2010) 82–90. [2] Nitrification. Office of Ground Water and Drinking Water: Distribution System Issue Paper, (2002). [3] C.T. Jafvert, R.L. Valentine, Dichloramine decomposition in the presence of excess ammonia, Water Res. 21 (1987) 967–973. [4] Q. Zhang, E.G.R. Davies, J. Bolton, Y. Liu, Monochloramine loss mechanisms in tap water, Water Environ. Res. (2017) 1999–2005. [5] R.L. Valentine, C.T. Jafvert, General acid catalysis of monochloramine disproportionation, Environ. Sci. Technol. 22 (1988) 691–696. [6] R.L. Valentine, G.G. Wilber, R.L. Jolley, L.W. Condie, J.D. Johnson, S. Katz, R.A. Minear, J.S. Mattice, V.A. Jacobs (Eds.), Water Chlorination Chemistry: Environmental Impact and Health Effects, vol. 6, Lewis Publishers, Chelsea, MI,
2943
Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Evaluation of zinc and copper for co-inhibition of nitrification in mild nitrified drinking water
T
⁎
Dipok Chandra Sarker , Chandni Manji Patel, Anna Heitz, A.H.M. Faisal Anwar Department of Civil and Construction Engineering, Curtin University, GPO Box U1987, Perth WA 6845, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Chloramine decay Biostability analysis Disinfectant and metal inhibition
Maintaining adequate chloramine and overcoming nitrification are major challenges faced by water utilities where chloramine is used as a disinfectant. Laboratory batch experiments were carried out to evaluate the effectiveness of using zinc and copper as metal inhibitors in drinking water systems under nitrified conditions. The experiments were conducted for bulk water samples that were collected from a real drinking water distribution system. From the batch experimental results, it was found that 0.25 mg/L zinc inhibited nitrification and reduced the chloramine decay rate but that 0.25 mg/L zinc together with 0.2 mg/L copper significantly controlled chloramine decay and inhibited nitrification completely. Biostability analysis showed that the biostable residual concentration (BRC) was lowered with the addition of zinc, and zinc with higher copper concentrations i.e. the lowest BRC was calculated for 0.25 mg/L zinc with 0.2 mg/L copper. Therefore, it can be concluded that zinc alone can be used as an inhibitor of nitrification and chloramine decay but co-inhibition using zinc and copper provides better control of chloramine decay.
1. Introduction Many water utilities are facing challenges with disinfectant control in drinking water systems due to the effects of rising temperatures and climate change. Temperature increases lead to high levels of disinfectant decay which increase the risks associated with waterborne pathogens. To provide water that is free from microbial contamination, water utilities use free chlorine as a disinfectant but chlorine produces higher concentrations of disinfection by-products (DBPs) as well as taste and odour issues. Therefore many water utilities now utilise monochloramine as a final disinfectant [1]. Despite stringent controls on DBP concentrations, it is crucial to maintain water quality to avoid any possible waterborne diseases. Although chloramine produces less DBPs, and taste and odour compounds in comparison to free chlorine, it enhances nitrification activity in the system which causes chloramine decay [2]. Normally, chloramination is carried out by adding chlorine followed by ammonia or by adding chlorine and ammonia simultaneously. Chloramine decays in two ways: chemical decay and microbiologically assisted decay. The chemical decay of monochloramine (NH2Cl) in drinking water can be due to auto-decomposition and reactions with organic or inorganic constituents [3,4]. According to Valentine and Jafvert [5] auto-decomposition of NH2Cl occurs according to the reaction presented in Eq. (1). During the auto-decomposition reaction,
⁎
ammonia from chloramine is oxidised to nitrogen gas with production of smaller quantities of nitrate (NO3−) [Eq. (2)] [6]. The direct reaction between NH2Cl and NO2− is described by Eq. (3). 3NH2Cl → N2 + NH3 + 3Cl− + 3H+
(1)
4NH2Cl + 3H2O → 3NH3 + NO3− + 4Cl− + 5H+ NH2Cl
+ NO2− +
H2O
→ NO3− +
+
−
NH4 + Cl
(2) (3)
It is thought that the main cause of rapid chloramine loss is nitrification. Nitrification is a two-stage oxidation process where ammonia is oxidised to form nitrite due to the presence of ammonia oxidising bacteria (AOB). The nitrite is then oxidised to form nitrate with the aid of nitrite oxidising bacteria (NOB). These nitrifying bacteria obtain their energy for oxidation from chloramine which leads to an enhancement in chloramine decay. Nitrification also leads to a depletion of dissolved oxygen and reduction of pH and alkalinity which can cause corrosion in pipe systems [2,7]. The nitrification process is influenced by the initial chloramine concentration, free ammonia, temperature, pH and pipe material [8]. Nitrification has been observed more frequently at low chloramine residuals [9–15]. Chloramine levels of 1.0 to 2.0 mg/L used for potable water should be sufficient to overcome nitrifiers [16] but once nitrification has commenced, it is very hard to control even by increasing chloramine residuals up to 8.0 mg/L [10].
Corresponding author. E-mail address: [email protected] (D.C. Sarker).
https://doi.org/10.1016/j.jece.2018.04.028 Received 4 December 2017; Received in revised form 28 March 2018; Accepted 15 April 2018 Available online 18 April 2018 2213-3437/ © 2018 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 6 (2018) 2939–2943
D.C. Sarker et al.
2.2. Preparation of sample bottles and glassware
Maintenance of adequate chloramine residuals is a challenge during hot weather and for long distribution systems due to increasing nitrifying activities. Previous studies [17–19,8] concluded that chloramine residual combined with the availability of free ammonia can play a significant role in the onset of nitrification in distribution systems. In order to determine the onset of nitrification, [17] proposed the biostability concept, which was adopted by Harrington et al. [20] and used by Fleming et al. [18] to determine the residual below which potential for nitrification occurs in a laboratory scale system. This is based on the balance between the growth rate of microbes and the disinfection rate, using ammonia as the substrate for controlling growth of AOB and dichloramine as the disinfectant. Sathasivan et al. [8] considered total chlorine as a disinfectant and proposed the following Equation:
μm (free ammonia as N ) (K S + free ammonia as N )
Sample bottles (material: polyethylene terephthalate (PET), volume = 600 ml) and glassware were soaked for 24 h in 2–3% sodium hypochlorite prior to rinsing six times with reverse osmosis (RO) treated laboratory water to ensure they were free of chlorine. 2.3. Preparation of stock chemical solutions Stock solutions of reagents and standards were prepared by mixing analytical-grade chemicals with Milli-Q ultra-pure water (18 MΩ/ cm, < 100 ppb-C L−1). Stock solutions of total chlorine (TCl; 1000 mg/ L), ammonia-nitrogen (500 mg/L), nitrite-nitrogen (NO2-N; 500 mg/L) and nitrate-nitrogen (NOx-N; 500 mg/L) were prepared using sodium hypochlorite (12.5% w/v, Analytical reagent, Rowe Scientific Pty Ltd.), ammonium chloride (Analytical reagent, Biolab Australia Ltd.), sodium nitrite (Analytical reagent, Thermo Fisher Scientific Australia Pty. Ltd) and sodium nitrate (Analytical grade, Thermo Fisher Scientific Australia Pty. Ltd), respectively. Monochloramine (NH2Cl) solution was prepared using stock solutions of ammonium chloride (500 mg/L as N) and sodium hypochlorite (1000 mg/L as Cl2) by maintaining the proportions of chlorine and nitrogen as TCl:TAN = 4.1:1. Total ammoniacal nitrogen (TAN) is the sum of NH3-N, ammonium (NH4+-N) and ammonia nitrogen associated with chloramine. For determination of calibration standard curves, working solutions (1 mg/L) were prepared using ammonium chloride, sodium nitrite and sodium nitrate for ammonia-nitrogen, NO2-N and NOx-N respectively. Standard copper sulphate solution (1000 mg/L-Cu) was prepared by mixing copper sulphate (CuSO4·5H2O; Analytical reagent, Chem-Supply Pty Ltd) into Milli-Q ultra-pure water. A zinc solution was prepared (1000 mg/L-Zn) by mixing zinc chloride (ZnCl2; Analytical reagent, Rowe Scientific Pty Ltd.) with Milli-Q ultra-pure water. The pH of the standard solution of copper sulphate was maintained below 6.0 to preserve copper as cupric ions. The pH was adjusted using 1 M hydrochloric acid (General laboratory reagent, Rowe Scientific Pty Ltd.) and 1 M sodium hydroxide (Analytical reagent, Rowe Scientific Pty Ltd.).
= kd × BRC (4)
where, μm is the maximum specific growth rate of AOB (day−1); Ks is the half saturation constant for AOB (mg/L nitrogen); kd is the rate constant for inactivation of AOB by disinfectant (L day−1 mg/LChlorine); BRC is the total chlorine concentration (mg/L) referred to as biostable residual concentration. Several approaches have been taken previously to overcome nitrification. These include decreasing detention times, increasing chloramine concentration, flushing affected areas, breakpoint chlorination, increasing chlorine to ammonia ratio and metal inhibition [20–22]. These strategies have not been very effective and thus, more research needs to be conducted on possible inhibitors. In metal inhibition, metals are used to inactivate and kill nitrifiers by blocking their enzymatic functions. Previously, research has been carried out by using copper, silver and phosphorus as inhibitors in drinking water systems [23–25]. Silver is used by the water utilities in Europe as it is an effective inhibitor in lower temperatures, however, more research needs to be carried out to find the optimum concentration of silver for use in warmer climates [26]. Research was also done on copper inhibition for a lab scale system, which was effective but research is required to determine the effectiveness of using copper in a full-scale system [27]. Studies on phosphorus inhibition were only carried out for copper pipes and thus, may not be applicable for other pipe material [24,28]. Studies of inhibition by zinc and nickel were carried out in wastewater treatment plants (WWTP) and further research is needed to determine their applicability to drinking water distribution systems [29]. As zinc was an effective inhibitor in WWTP and copper has been shown to have some effect, a combination of both zinc and copper could be used as inhibitors to overcome chloramine decay. The aim of this study is to examine the effectiveness of zinc and copper co-inhibition for controlling chloramine decay in a mildly nitrified drinking water under conditions typical to potable water distribution systems.
2.4. Analytical procedures TAN, NO2-N, and NOx-N concentrations were measured using an Aquakem-200, a high precision wet chemistry automated analyser. TAN was measured using the phenolate method [30]. NO2-N was measured using the sulphanilamide method (4500-NO2 B; [31] and NOx-N was measured by enzymatic reduction [32,33]. Aquakem-200 has a high efficiency with a detection limit of 0.002 mg/L for the measurement of TAN, NO2-N and NOx-N. The experimental error was found to be ± 1.5% for TAN and NO2-N, and ± 2% for NOx-N measurement. TCl residuals were measured using the DPD colorimetric method with a HACH pocket colorimeter. TCl measurement had an experimental error of ± 0.03 mg/L [34] reported that more than 99% of chloramine is present in NH2Cl form when pH is above 7.5. Hence, TCl represents mainly NH2Cl residual at pH 8. A portable pH meter (HACH 40d) with temperature compensation was used to measure pH values and the measurement error was ± 0.10.
2. Materials and methods 2.1. Sample collection To examine the effectiveness of using copper and zinc inhibitors to reduce nitrification and maintain chloramine residual, bulk water samples were collected from the public taps of a water supply system pipeline. The water was collected in 25L high-density polyethylene tanks (pre-cleaned with sodium hypochlorite to remove impurities), transported and stored at room temperature in the laboratory. In order to determine the nitrification status of the samples, total chlorine was measured onsite. In addition total chlorine, ammonia nitrite, and NOxN levels were monitored daily in the laboratory. The samples were stored in the laboratory and the temperature inside the container was kept at 25 °C by using glass heaters to enhance the growth of nitrifying bacteria.
2.5. Experimental design Two sets of batch test were carried out to understand the effect of zinc and copper inhibition and the outline of the different tests are shown in Fig. 1. There was a control sample for each set (no metal addition). Duplicate samples were prepared for each set and average results of each set are presented. The initial weight ratio of TCl:TAN was maintained at 4.1:1 by adding appropriate amounts of ammonium followed by chlorine (sodium hypochlorite). The pH of each sample was maintained at 8.0 ± 0.1 by adding HCl or NaOH. The first set of experiments was designed to check the inhibitory 2940
Journal of Environmental Chemical Engineering 6 (2018) 2939–2943
D.C. Sarker et al.
Table 1 General water quality characteristics of sample at the time of collection and initial conditions for batch test.
effects of zinc on AOB activity. The nitrified bulk waters were separated into three sub-sets of sample. The first sub-set remained unchanged; the second and third sub-sets were dosed with 0.25 mg/L and 0.50 mg/L zinc respectively. The three sub-sets were referred as ‘Zn-0′, ‘Zn-0.25′ and ‘Zn-0.50′ respectively. A second set of experiments was conducted to determine how zinc combined with copper impact on nitrification inhibition for mildly nitrified bulk waters. The nitrified bulk waters were separated into three subsets of samples. The first sub-set remained unchanged, as a control; the second and third sub-sets were dosed with 0.10 mg/L and 0.20 mg/ L copper respectively after subsequent addition of 0.25 mg/L zinc. The three sub-sets were referred as ‘Zn-0 + Cu-0′, ‘Zn-0.25 + Cu-0.1′ and ‘Zn-0.25 + Cu-0.2′ respectively. Samples were incubated in a constant temperature water bath at 25 °C. Levels of TCl, TAN, NO2-N and NOx-N were monitored regularly in all samples.
TCl (mg/L) TAN (mg/L) NO2-N (mg/L) NOx-N (mg/L) pH Temperature (°C)
1.31 0.596 0.181 0.546 7.42 NA
2.00 0.600 0.185 0.546 8.0 ± 0.3 20 ± 1
Zinc was added to samples at two concentrations (0.25 and 0.50 mg/L; experimental set 1, Fig. 1) in order to test the effect of zinc on inhibition of nitrification to maintain chloramine residual. In the TCl profiles of zinc inhibited bulk water samples (0.25 and 0.50 mg/L Zn added) an initial chloramine loss, possibly partly due to auto-decomposition, occurred in both the samples. However, significantly higher concentrations of TCl were maintained throughout in the sample with the higher dose of Zn (0.5 mg/L; Fig. 3A). The kt values for 0.25 and 0.50 mg/L zinc inhibited samples were determined to be 0.0013 and 0.0004 h−1 respectively. Therefore, the higher concentration of zinc (0.5 mg/L) appeared to be effective in inhibiting nitrification which shows that the higher concentration of metal should have an increased effect on inhibition [35]. Thus, only results of 0.5 mg/L zinc dosed sample are presented in the following discussion. Trends for TCl and TAN profiles decreased, together with an upward trend of the NOx-N profile for the unprocessed sample after 50 h (Fig. 3B). On the other hand, TCl, TAN and NOx-N profiles remained constant throughout the incubation period for zinc inhibited samples. As the samples were mildly nitrified, there was a slower chloramine decay. Although the kt values of unprocessed and zinc inhibited samples were low, the addition of zinc reduced the decay rate constant from 0.0042 to 0.0004 h−1, a 10-fold reduction. The stable pattern of NOx-N in the zinc inhibited sample indicated that zinc was effective in
First-order decay kinetics were used to determine the total chloramine decay rate according to the following equation (5)
where TClt is the total chloramine residual (mg/L) at time t (hours), TCl0 is the initial total chloramine concentration (mg/L), t is incubation time in hours and kt is the total chloramine decay coefficient per hour. The free ammonia present in a sample is calculated by the following equation
TClt 5
Initial concentration of water quality parameters in batch test
3.2. Impact of zinc on mild nitrified bulk water
2.6. Determination of total chloramine decay rate and calculation of free ammonia
Free ammonia = TAN −
Typical conditions onsite water quality parameters during sampling
the samples. Prior to the commencement of the test, the chloramine concentration of the sample was adjusted to TCl = 2.0 mg/L and TAN = 0.6 mg/L and pH was adjusted to 8.0 (Table 1). Surrogate parameters were monitored over the course of a week while maintaining the sample temperature at 25 °C. TCl and TAN gradually decreased with a gradual increase in NOx-N, which confirmed the presence of nitrifying activity in the sample. TCl and TAN profiles started to decrease while the NOx-N profile increased after 50 h. After 75 h incubation time, it was observed that the NO2-N concentration was 0.185 mg/L with a calculated kt of 0.0072 h−1. These conditions were classified as mild nitrification as defined by Sathasivan et al. [8] (Fig. 2).
Fig. 1. Schematic diagram of experimental method. Units of TCl, TAN, copper and zinc are mg/L.
TClt = TCl 0⋅e(-kt)
Parameters
(6)
where, the factor 5 equals 70 mg-Cl2 per 14 mg-N. 3. Results and discussion 3.1. General characteristics of the collected samples Nitrification was shown to be established in samples after the first 75 h of incubation. Table 1 presents the water quality parameters of the bulk water sample collected from publicly accessible taps in two regional towns in Western Australia that are supplied with chloraminated water. Three sets of sample were collected and analysed in the laboratory. As all the samples showed similar water quality characteristics, the typical values are presented in Table 1. The recorded TCl and NO2-N concentrations of the sample during the sampling period were 1.31 mg/L and 0.181 mg/L respectively. A batch test was conducted to assess the nitrification conditions of
Fig. 2. Profiles of TCl and Nitrogenous compounds over 75 h of incubation. 2941
Journal of Environmental Chemical Engineering 6 (2018) 2939–2943
D.C. Sarker et al.
Fig. 4. Profiles of TCl for zinc and copper inhibited samples. Table 2 Effect of Zn and Cu, individually and in combination, on chloramine decay rate constant.
Fig. 3. A Variation of TCl profiles for 0.25 and 0.50 mg/L zinc inhibited bulk waters. B Profiles of TCl and nitrogenous compounds for mild nitrifying samples.
Type and concentration of metal added
Chloramine decay rate constant (hr−1)
Zn-0 + Cu-0 Zn-0.25 Zn-0.50 Zn-0.25 + Cu-0.10 Zn-0.25 + Cu-0.20
0.0072 0.0019 0.0009 0.0016 0.0008
± ± ± ± ±
0.0015 0.0004 0.0002 0.0003 0.0001
between Zn-0.25 and Zn-0.25 + Cu-0.10 when considering the experimental error but the lowest kt value was observed in Zn-0.25 + Cu-0.20 sample. This confirms that ‘zinc only’ inhibition is not as effective as when Zn is used in combination with copper as stated in previous research [21]. Additionally, metals cause greater inhibition when applied in higher concentrations. The results clearly revealed that adding zinc and copper in combination have a significant controlling effect on total chloramine decay in the bulk water.
inhibition of nitrification. Higher TCl and TAN profiles in the zinc modified samples indicate that zinc provides better stability of chloramine residual. TAN loss was slightly higher (0.10 mg/L) than Total inorganic nitrogen (TIN) loss in unprocessed samples which might be due to the presence of natural organic matter (NOM) and soluble microbial products (SMPs). NOM oxidation is a slow process and could be a dominant mechanism due to long incubation of the sample [36,37]. However, TAN loss was the same as TIN loss in zinc inhibited samples which suggests that zinc is beneficial in preventing the chloramine decay caused by NOM and SMPs.
3.4. Applicability of the biostability concept in mild nitrified bulk waters As nitrification is an important phenomenon in chloramine disinfection, a BRC curve assists water utilities to prevent the growth of nitrifying bacteria in water distribution systems [38]. Since BRC is defined as the residual below which potential for nitrification occurs, water dosed with a combined Zn/Cu mixture should be free of nitrification at much lower chloramine residual than water containing no metals or lower doses of metals. Therefore, the BRC values with respect to different TAN concentrations for the different samples were calculated according to Eq. (4) (Fig. 5). Ks = 0.18 [8], μm/kd = 2.046 for 25 °C [39] and the free ammonia is calculated according to Eq. (6) are used to calculate the BRC. The required residual concentration e.g. BRC value of Zn-0 + Cu0 sample is higher than the Zn-0.25, Zn-0.25 + Cu-0.1 and Zn0.25 + Cu-0.2 samples for the same concentration of TAN (Fig. 5). This indicates that zinc dosing alone or with copper would be sufficient in mildly nitrified chloraminated bulk waters to stop nitrification. A zinc concentration of 0.25 mg/L is sufficient to control nitrification significantly but zinc with copper (Zn-0.25 + Cu-0.20) could stop the excessive chloramine decay rates observed during the nitrifying phase probably due to NOM and SMPs that are likely to present in these samples. To take advantage of using metal co-inhibition, utilities could dose metals before the onset of nitrification especially at times when chloramine residuals cannot be maintained above the BRC. Such temporary metal co-inhibition would be helpful as an early intervention to stop the onset of nitrification and thus to protect chloramine residual.
3.3. Impact of zinc and copper co-inhibition on mildly nitrified bulk water Since zinc at 0.25 mg/L was found to have some effect in the control of nitrification and copper at 0.25 mg/L was sufficient to inhibit/inactivate nitrifying bacteria in nitrified bulk water samples [27], experiments were conducted to determine the effect of these two elements in combination. The impact of copper (0.10 or 0.20 mg/L) in combination with of zinc (0.25 mg/L) on chloramine in mildly nitrified bulk water samples was tested. Both copper and zinc inhibited nitrification with an increase in the effect with increasing total metal concentrations (Fig. 4). Chloramine decay was rapid for the unprocessed sample during the first 6 h of the experiment from 1.92 to 1.77 mg/L and it continued to decay over the full period of the test. However, the samples treated with both copper and zinc had a much more stable chloramine residual. For the zinc and copper inhibited samples, all three showed similar trends in chloramine decay profiles. The total chloramine decay coefficients demonstrated the effectiveness of using Zn alone and Zn combined with Cu (Table 2). It was observed that the Zn-0 + Cu-0 sample had the highest kt value whereas the Zn-0.25 + Cu-0.20 sample had the lowest value. Addition of high dose of zinc (Zn-0.50) and co-inhibition of Zn and Cu can reduce the chloramine decay and thus provide better residual stability in the bulk waters. There was no significant difference in kt values 2942
Journal of Environmental Chemical Engineering 6 (2018) 2939–2943
D.C. Sarker et al.
1987, pp. 819–832. [7] K.C. Bal Krishna, A. Sathasivan, D.C. Sarker, Evidence of soluble microbial products accelerating chloramine decay in nitrifying bulk water samples, Water Res. 46 (2012) 3977–3988. [8] A. Sathasivan, I. Fisher, T. Tam, Onset of severe nitrification in mildly nitrifying chloraminated bulk waters and its relation to biostability, Water Res. 42 (2008) 3623–3632. [9] N.I. Lieu, R.L. Wolfe, I.E.G. Means, Optimizing chloramine disinfection for the control of nitrification, J. Am. Water Works Assoc. 85 (1993) 84–90. [10] J. Skadsen, Nitrification in a distribution system, J. Am. Water Works Assoc. 85 (7) (1993) 95–103. [11] L.H. Odell, G.J. Kirmeyer, A. Wilczak, G.J. Joseph, P.M. Joseph, R.L. Wolfe, Controlling nitrification in chloraminated systems, J. Am. Water Works Assoc. 86 (1996) 86–98. [12] A. Wilczak, J.G. Jacangelo, J.P. Marcinko, L.H. Odell, G.J. Kirmeyer, Occurrence of nitrification in chloraminated systems, J. Am. Water Works Assoc. 88 (1996) 74–85. [13] S.B. Liu, J.S. Taylor, A.A. Randall, J.D. Dietz, Nitrification modelling in chloraminated distribution system, J. Am. Water Works Assoc. 97 (10) (2005) 98–108. [14] Y. Zhang, M. Edwards, Accelerated chloramine decay and microbial growth by nitrification in premise plumbing, J. Am. Water Works Assoc. 101 (11) (2009) 51–61. [15] C. Nguyen, C. Elfland, M. Edwards, Impact of advanced water conservation features and new copper pipe on rapid chloramine decay and microbial regrowth, Water Res. 36 (3) (2012) 611–621. [16] R.L. Wolfe, N.I. Lieu, G.E. Means, Ammonia-oxidizing bacteria in a chloraminated distribution system: seasonal occurrence, distribution, and disinfection resistance, Appl. Environ. Microbiol. 56 (1990) 451–462. [17] J.E. Woolschlager, B. Rittmann, P. Piriou, B. Schwartz, Using a comprehensive model to identify the major mechanisms of chloramine decay in distribution systems, Water Sci. Technol.: Water Supply 1 (2001) 103–110. [18] K.K. Fleming, G.W. Harrington, D.R. Noguera, Nitrification potential curves: a new strategy for nitrification prevention, J. Am. Water Works Assoc. 97 (2005) 90–99. [19] K.D. Pintar, W.B. Anderson, R.M. Slawson, E.F. Smith, Assessment of a distribution system nitrification critical threshold concept, J. Am. Water Works Assoc. 97 (2005) 116–129. [20] G.W. Harrington, D.R. Noguera, A.I. Kandou, D.J. VanHoven, Pilot scale evaluation of nitrification control strategies, J. Am. Water Works Assoc. 94 (2002) 78–89. [21] Z. Hu, K. Chandran, D. Grasso, B.F. Smets, Impact of metal sorption and internalization on nitrification inhibition, Environ. Sci. Technol. 37 (4) (2003) 728–734. [22] D.C. Sarker, Modelling Inhibition of Microbes Responsible for Acceleration of Chloramine Decay in Water Supply System. Doctoral Thesis, Curtin University, Bentley, Western Australia, 2012. [23] W. Zhan, Effect of Iron Corrosion on the Fate of Dosed Copper to Inhibit Nitrification in Chloraminated Water Distribution System. Master Thesis, Curtin University, Bentley, Western Australia, 2011. [24] X. Li, V. Kapoor, C. Impelliteri, K. Chandran, J.W.S. Domingo, Measuring nitrification inhibition by metals in wastewater treatment systems: current state of science and fundamental research needs, Crit. Rev. Environ. Sci. Technol. 46 (3) (2016) 249–289. [25] H. Gruettner, M. Winther-Nielsen, L. Jorgensen, P. Bogebjerg, O. Sinkjaer, Inhibition of the nitrification process in municipal wastewater treatment plants by industrial discharges, Water Sci. Technol. 29 (9) (1994) 69–77. [26] K.C. Bal Krishna, A. Sathasivan, Effect of silver in severely nitrified chloraminated bulk waters, Water Sci. Technol. Water Supply 12 (4) (2012) 415–421. [27] D.C. Sarker, A. Sathasivan, B.E. Rittmann, Modelling combined effect of chloramine and copper on ammonia oxidizing microbial activity using a biostability approach, Water Res. 84 (2015) 190–197. [28] M. Edwards, H. Loay, G. Dawn, Phosphate inhibition of soluble copper corrosion byproduct release, Corros. Sci. 44 (5) (2002) 1057–1071. [29] Z. Hu, K. Chandran, D. Grasso, B.F. Smets, Comparison of nitrification inhibition by metals in batch and continuous flow reactors, Water Res. 38 (2004) 3949–3959. [30] Methods for the Examination of Waters and Associated Materials, Ammonia in Waters, (1981). [31] APHA, WEF, AWWA, Standard Methods for the Examination of Water and Wastewater, 20th ed., (1998) (Washington, DC). [32] W.H. Campbell, P. Song, G.G. Barbier, Nitrate reductase for nitrate analysis in water, Environ. Chem. Lett. 4 (2006) 69–73. [33] C.J. Patton, A.E. Fischer, W.H. Campbell, E.R. Campbell, Corn leaf nitrate reductase: a nontoxic alternative to cadmium for photometric nitrate determinations in water samples by air-segmented continuous-flow analysis, Environ. Sci. Technol. 36 (2002) 729–735. [34] R.J. Valentine, Opinion as to Probable Chloramine Speciation in San Francisco Public Utilities Drinking Water Distribution System, (2007) sfwater.org/Files/ Other/Chloramine%20Speciation%20Opinion.pdf. (Accessed 18.06.08). [35] S.R. Juliastuti, J. Baeyens, C. Creemers, Inhibition of nitrification by heavy metals and organic compounds: the ISO 9509 Test, Environ. Eng. Sci. 20 (2) (2003) 79–90. [36] A. Sathasivan, K.C.B. Krishna, Major Mechanism (s) of chloramine decay in rechloraminated laboratory scale system waters, Desalin. Water Treat. 47 (1-3) (2012) 112–119. [37] J.P. Vikesland, K. Ozekin, L.R. Valentine, Monochloramine decay in model and distribution system waters, Water Res. 35 (2001) 1766–1776. [38] S. Srinivasan, G.W. Harrington, Biostability analysis for drinking water distribution systems, Water Res. 41 (10) (2007) 2127–2138. [39] D.C. Sarker, A. Sathasivan, A.J. Cynthia, A. Heitz, Modelling temperature effects on ammonia-oxidising bacterial biostability in chloraminated systems, Sci. Total Environ. 454–455 (2013) 88–98.
Fig. 5. Inhibitory effects of metal on nitrification in terms of BRC.
3.5. Implication for nitrification control and residual management Nitrification plays an important role in causing chloramine decay in distribution systems where chloramine is used as a disinfectant and water utilities usually increase operational monitoring to determine causes and methods of control of nitrification. The traditional approaches to overcome nitrification and chloramine decay by break point chlorination, adjusting chlorine to ammonia ratio or re-chloramination have been relatively ineffective. In chloraminated samples, nitrification inhibition by metal addition has been shown to have some positive effects for maintaining chloramine residual although this depends on various factors such as pH, availability of natural organic matter and their complexing ability with metal, and substrate concentration (free ammonia). Although, a preventive approach to avoid triggers of nitrification is needed for long term control, addition of metal before or just upon the onset of nitrification could assist in better control of chloramine residual. While the experimental results in this study showed that co-inhibition of copper and zinc reduce chloramine decay, further research is needed to demonstrate its impact on other mechanisms in the nitrification process. 4. Conclusion For better understanding of how addition of the metals zinc and copper to chloraminated water would act as an inhibitor, experiments were performed on mild nitrifying chloraminated bulk water samples. Though zinc alone is able to inhibit nitrification and control chloramine decay, the co-inhibition effects of zinc and copper reduced the chloramine decay rate from 0.0072 h−1 to 0.0004 h−1. Therefore, zinc alone inhibited nitrification and thus reduced the chloramine decay rate but the combination of zinc with copper strongly reduced chloramine decay in chloraminated nitrifying bulk waters. Water utilities can take advantage of these findings if they opt to dose zinc and copper during warmer months when there is higher likelihood of chloramine concentrations decreasing below the BRC values. References [1] K.C. Bal Krishna, A. Sathasivan, Does an unknown mechanism accelerate chemical chloramine decay in nitrifying waters? J. Am. Water Works Assoc. 102 (10) (2010) 82–90. [2] Nitrification. Office of Ground Water and Drinking Water: Distribution System Issue Paper, (2002). [3] C.T. Jafvert, R.L. Valentine, Dichloramine decomposition in the presence of excess ammonia, Water Res. 21 (1987) 967–973. [4] Q. Zhang, E.G.R. Davies, J. Bolton, Y. Liu, Monochloramine loss mechanisms in tap water, Water Environ. Res. (2017) 1999–2005. [5] R.L. Valentine, C.T. Jafvert, General acid catalysis of monochloramine disproportionation, Environ. Sci. Technol. 22 (1988) 691–696. [6] R.L. Valentine, G.G. Wilber, R.L. Jolley, L.W. Condie, J.D. Johnson, S. Katz, R.A. Minear, J.S. Mattice, V.A. Jacobs (Eds.), Water Chlorination Chemistry: Environmental Impact and Health Effects, vol. 6, Lewis Publishers, Chelsea, MI,
2943