Does an unknown mechanism accelerate chemical chloramine decay ...

disinfection Accelerated chloramine decay has been observed after the onset of nitrification in water utilities. The reason for such rapid loss is not known. To investigate the role of chemical parameters in accelerated chloramine decay, mildly and severely nitrified bulk water samples were collected from laboratory-scale reactors. After samples were filtered through a 0.2-µm polycarbonate membrane filter, chloramine decay tests were conducted by adjusting similar initial chloramine residuals. Accelerated chloramine decay and significant nitrite and total ammoniacal nitrogen loss were observed in severely nitrified water. To discern whether increased nitrite levels and pH were responsible, chemical decay coefficients were determined by adjusting initial chloramine residual, nitrite, pH, and temperature in the mildly nitrified sample. The chemical decay coefficient determined in a severely nitrified sample was three to four times higher than that in a mildly nitrified sample, indicating that changes in adjusted parameters that occurred during nitrification could not explain the rapid chemical chloramine loss in severely nitrified water.

Does an unknown mechanism accelerate chemical chloramine decay in nitrifying waters?

W K.C. BAL KRISHNA AND ARUMUGAM SATHASIVAN

ater quality degrades in the distribution system because of indigenous microbes and their regrowth. Since the early 1900s, water utilities have used chlorine and chloramine as a secondary disinfectant to eliminate microbes and suppress their regrowth. Increased concerns over chlorinated disinfectant by-products—especially trihalomethanes and haloacetic acids— have prompted many water utilities to switch from chlorine to chloramine (Cotruvo, 1981; Brodtmann et al, 1979). Compared with chlorine, chloramine offers the advantages of higher stability, ability to penetrate biofilm, and minimal objectionable taste and odor. Chloramination is usually carried out by adding chlorine followed by ammonia (NH3) or by adding chlorine and ammonia simultaneously. Initially, chloramine decay occurs as a result of autodecomposition and reaction with other chloramine-demanding agents (including natural organic matter [NOM] and microbes). The decay process releases free ammonia, which serves as an energy source for nitrifiers and results in nitrification. Nitrification is a two-step microbial process in which ammonia is oxidized to nitrite (NO2) by ammonia-oxidizing bacteria (AOB) and nitrite is oxidized to nitrate (NO3) by nitrite-oxidizing bacteria. Nitrification is thought to be the primary cause of rapid chloramine loss. Nitrification has been observed more frequently at low chloramine residual

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concentrations (Odell et al, 1996; Wilczak et al, 1996; Lieu et al, 1993; Skadsen, 1993). Chloramine concentrations used for potable water (1.0–2.0 mg/L) should be sufficient to eliminate nitrifiers (Wolfe et al, 1990). Once severe nitrification sets in, however, it is difficult to control it, even by increasing the chloramine residual up to 8.0 mg/L (Skadsen, 1993). Nitrification deteriorates water quality and enhances chloramine demand.

Compared with chlorine, chloramine offers the advantages of higher stability, ability to penetrate biofilm, and minimal objectionable taste and odor.

The bacterial chloramine decay mechanism has yet to be clearly defined. It has been hypothesized that rapid chloramine decay is the effect of chloramine reaction with nitrite produced by AOB. It has also been suggested that nitrite could accelerate chloramine decay, particularly in the presence of bromide (Valentine, 1985), or that ammonia oxidation could shift the equilibrium of monochloramine formation so that free ammonia is metabolized and subsequently monochloramine is hydrolyzed (Wolfe et al, 1988). Other researchers have proposed a possible mechanism of direct co-metabolism of chloramine in the presence of nitrifiers (Woolschlager et al, 2001). To understand the basic mechanism of chloramine decay, it is necessary to separate the role of microbes (including nitrifiers) and other chemical reactions. Previous research developed a simple approach that uses a microbial decay factor Fm, which is the ratio between microbial and chemical decay coefficients (Sathasivan et al, 2005). The Fm method separates microbial and chemical decay and quantifies their respective roles in chloramine decay. Generally, the chemical decay coefficient kc is affected by chloramine residual, total chlorine-to-ammonia ratio, temperature, total organic carbon (TOC), pH, nitrite, and other agents present in water (Harrington et al, 2002; Vikesland et al, 2001; Jafvert & Valentine, 1992), whereas the microbial decay coefficient k m depends on microbial activities. Sathasivan and colleagues (2010; 2008) analyzed the behavior of bulk water samples collected from the Sydney, Australia, water distribution system by incubating samples at 20–25oC. They showed that there were two distinct behaviors occurring in two different stages of chloramine decay or when nitrifying bacterial activity was present. The stage occurring first was termed the mildly nitrifying stage, and the second was termed the severely nitrifying stage. In the mildly nitrifying stage,

chloramine decay was reasonably stable, and the nitrite level was < 0.010 mg/L N. However, in the severely nitrifying stage, chloramine residuals dropped below 0.5 mg/L, nitrite reached high levels (> 0.10 mg/L N), and chloramine decay excessively accelerated (total decay was about one order higher than that observed in the mildly nitrifying stage). For example, in one sample, the total decay in the severely nitrifying stage was 0.040 h–1, compared with 0.0045 h–1 in the mildly nitrifying stage. For purposes of this article, the severely nitrified sample was defined as in the Sydney water distribution system, i.e., as the sample with a nitrite level > 0.1 mg/L N. Despite many efforts to investigate the chloramine decay mechanism, a question remains as to what accelerates chloramine decay in severely nitrifying conditions. The objective of the research described here was to investigate chemical decay in both mild and severe nitrification stages and quantify the role of chemical parameters that changed during different nitrification stages in a laboratory-scale distribution system.

MATERIALS AND METHODS Reactor startup and operation. Two systems of reactors—each containing five different reactors connected in series (Figure 1)—were set up at the Department of Civil Engineering laboratory at Curtin University of Technology in Perth, Australia. The reactors were constructed of high-density polyethylene (HDPE) tanks, and the volume of each reactor was 25 L. Each reactor was connected by HDPE pipes. The feedwater tank was also made of HDPE and was closed by a lid. The maximum volume of the feedwater tank was 25 L. Both reactor systems were facilitated with automatic water flow and

FIGURE 1

Reactor setup

Feedwater tank

Power supply and control units

Water level control sensors Reactor 1

Reactor 2

Heating plates

Reactor 3

Reactor 4

Reactor 5

Outlets

2010 © American Water Works Association BAL KRISHNA & SATHASIVAN | 102:10 • JOURNAL AWWA | PEER-REVIEWED | OCTOBER 2010

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temperature-control devices. The water flow rate was controlled using water level sensors and control valves between the feeding tank and reactor 1. The gravity flow rate was maintained by adjusting the pipe level between reactors 1 and 5 (Figure 1). Water temperature was raised using heating plates installed at the bottom of each reactor. Temperature was controlled using stainless-steel sensors inserted inside the reactors and programmed logical control units. A raw water sample from Mundaring Weir, near Perth, was collected in a 1.0-m3 HDPE tank and stored at room temperature before it was fed into the systems. Feedwater was prepared by adding chlorine followed by ammonia addition after ~4 h. Analytical-grade sodium hypochlorite (12.5% weight/volume) and ammonium chloride were used to maintain chlorine and ammonia residuals, respectively. The 4.5:1 ratio of total chlorine (Cl) to total ammoniacal nitrogen (TAN) was maintained in the feedwater tank. TAN is the summation of NH3-N, ammonium ion (NH4+-N) and monochloramine (NH2Cl-N). During the startup period, a chloramine concentration of ~1.0 mg/L (Cl-to-TAN ratio of 4.5:1) was maintained in each reactor. In order to expedite nitrification and obtain distribution system–specific inoculums, chloraminated water samples collected from the Goldfields and Agricultural Water Supply System, Western Australia, had been placed as seed microorganisms in every reactor except reactor 1. About 20 L/d of water was continuously fed through the feeding tank, maintaining a constant hydraulic retention time of about 20±2.0 h in each reactor. Water temperature was maintained at 20.0±2.0oC in the first three reactors (reactors 1, 2, and 3), whereas 23.0±2.0oC was maintained in the last two reactors (reactors 4 and 5) to achieve higher microbial activities. Although pH values varied from 7.5 to 8.3 in sample water, the pH was adjusted to 8.0±0.1 in the feedwater tank. Feedwater dissolved organic carbon (DOC) was 2.7±0.4 mg/L during the experimental period. The chloramine concentration was gradually increased to 2.5 mg/L in the feeding tank, maintaining the same Cl-to-TAN ratio and pH as during the startup period. The various nitrification conditions (from no nitrification to severe nitrification) that occur in real distribution systems were created along the reactor by varying hydraulic retention time, temperature, and chloramine residual. TABLE 1

Experimental design. Sample bottles (500-mL polyethylene terephthalate) were cleaned by dipping them into a 10% sodium hypochlorite solution for 24 h, followed by rinsing with deionized water until the bottles were free of chlorine. All sample collection glassware, filtration units, and filter papers were autoclaved. Stock solutions for all chemicals were prepared in purified water.1 Monochloramine solution was prepared using stock solutions of ammonium chloride (500 mg/L N) and sodium hypochlorite (500 mg/L Cl2). For adjustment of nitrite concentration, 500-mg/L N nitrite stock solution was prepared using sodium nitrite. The pH was adjusted using 1 M hydrochloric acid and 1 M sodium hydroxide. All chemicals used in this experiment were analytical grade. Experiments were divided into two sets. The first set was designed to elucidate the chemical decay characteristics in mildly and severely nitrified bulk water samples. In the first set of experiments, samples were collected from reactors 2 and 4, and nitrification surrogate parameters (total chlorine, TAN, nitrite, nitrate, and pH) were measured. Measured chemical parameters are shown in Table 1. As defined by Sathasivan and co-workers (2008), reactors 2 and 4 were mildly and severely nitrifying, respectively. Therefore, sufficient samples were collected from these two reactors for further experiments. All samples were filtered through a 0.2-µm polycarbonate membrane filter to remove microbes. Initial chloramine residual was adjusted to 2.0 mg/L, maintaining a Cl-toTAN ratio of 3.8:1 in each sample. Samples in duplicate were incubated in a dark water bath at a constant temperature (20oC). The samples taken from reactors 2 and 4 were identified as MNS (mildly nitrified sample) and SNS (severely nitrified sample), respectively. For each sample, nitrification surrogate parameters were monitored over time, and the chemical decay coefficient kc was estimated using exponential regression. As shown in Table 1, the MNS and SNS differed in the parameters of chloramine residual, Cl-to-TAN ratio, pH, nitrite, and nitrate. In the first set of experiments, the initial chloramine residual, Cl-to-TAN ratio, and temperature were adjusted to the same values in both nitrification conditions. The remaining chemical parameters were nitrite and pH, which are known to affect chloramine degradation.

Chemical parameters of the samples before experiments were conducted

Sample Origin

Sample Name

Total Chlorine mg/L

TAN mg/L

Nitrite mg/L N

Nitrate mg/L N

NOx mg/L N

DOC mg/L

pH

Reactor 2

MNS

1.89±0.03

0.52±0.008

0.007±0.001

0.048±0.001

0.055±0.001

2.80±0.14

8.05±0.1

Reactor 4

SNS

0.38±0.03

0.22±0.003

0.218±0.003

0.150±0.003

0.368±0.007

2.76±0.14

7.70±0.1

DOC—dissolved organic carbon, MNS—mildly nitrified sample, NOx—nitrite + nitrate, SNS—severely nitrified sample, TAN—total ammoniacal nitrogen



The second set of experiments was designed to determine the effect of pH and nitrite level on chloramine loss. In this experimental set, samples were collected from the mildly nitrified reactor (reactor 1) and filtered as described previously for the initial experimental set. Chemical parameters were measured before the chloramine decay test was performed. Total chlorine, TAN, nitrite, and nitrate were 2.0 mg/L, 0.45 mg/L N, 0.003 mg/L N, and 0.050 mg/L N, respectively. The pH was 8.0. Filtered samples were divided into four subsamples. The pH and nitrite levels were adjusted as shown in Table 2 to elucidate the possible influence of nitrite and pH on chloramine decay in the mildly nitrifying water samples. The subsamples were identified as SA, SB, SC, and SD (Table 2). All subsamples were prepared in duplicate and incubated in a dark water bath at 20oC, and chlorine decay testing was carried out. Analytical procedures. Total chlorine, TAN, nitrite, nitrate, and DOC were measured immediately after the samples were collected from reactors and from sample bottles in batch experiments. A high-precision wet chemistry automated analyzer2 was used to measure TAN, nitrite, and nitrite + nitrate (NOx) concentrations. TAN contained in the water samples reacted with hypochlorite ions generated by the alkaline hydrolysis of sodium dichloroisocyanurate to form monochloramine. Monochloramine reacts with salicylate ions in the presence of sodium nitroprusside at about pH 12.6 to form a blue compound, which was measured spectrophotometrically at 660-nm wavelength (USEPA, 1981). Nitrite was measured using sulfanilamide method 4500-NO2– B (Standard Methods, 1998). NOx was measured by catalytic reduction to nitrite by the nitrate-reductive enzyme in the presence of reduced nicotinamide dinucleotide (USEPA, 1981). The nitrite produced reacted with reagent to form a strong azo dye that was measured spectrophotometrically at 540 nm or 520 nm. Before NOx was measured, the chloramine residual was reduced stoichiometrically using 0.5% sodium thiosulfate stock solution. The analyzer had a high detection limit for TAN and nitrite and was able to detect NOx to a level of 0.002 mg/L N. Standard curves for TAN, nitrite, and NOx were calibrated in the range of 0.0 to 1.0 mg/L N using standard solution prepared from ammonium chloride, sodium nitrite, and sodium nitrate, respectively. The experimental error was calculated by measuring standard solutions of TAN, nitrite, and NOx and adding standard solution in known concentrations to the samples taken from the reactors. The measurement error was found to be ±1.5% (95% confidence level). The maximum error measured at any time was ±2.0%. The NOx measurement included the additional experimental error during reduction of total chlorine using sodium thiosulfate; the NOx measurement error was found to be ±2.0% (95% confidence interval). Nitrate was calculated by deducting the nitrite from the measured NOx. Total chlorine residuals were measured by DPD colorimetric method using a pocket

colorimeter.3 Averages of two readings from duplicate samples were reported. At pH values above 7.5 and a Clto-TAN mass ratio of ~4.5, more than 99% of chloramine present was in the form of monochloramine (Valentine, 2007); therefore, a pH 8 total chlorine is predominantly monochloramine residual. The total chlorine measurement had an experimental error of ±0.03 mg/L. DOC was measured using a TOC analyzer4; the experimental error for DOC was ±5%. A portable pH meter5 was used to measure pH values; the measurement error was ±0.1.

RESULTS AND DISCUSSION General characteristics of mildly and severely nitrifying bulk water samples collected from the reactors. Water samples were collected from reactors 2 and 4. Table 1 shows the analyzed chemical parameters of the samples before adjustment for experiments. Total chlorine residuals of 1.89 mg/L measured in reactor 2 had dropped to 0.38 mg/L by the time flow reached reactor 4. Simultaneously, nitrogenous species (TAN, nitrite, and nitrate) also demonstrated changes from reactor 2 to reactor 4 (Table 1). As water flowed from reactor 2 to reactor 4, the pH dropped from 8.05 to 7.70. The changes in pH, nitrogenous species, and chlorine residuals were signs of the nitrifying bacterial activities in reactor 4 (Vikesland et al, 2001; Wolfe et al, 1990). Furthermore, the difference in NOx level (0.313 mg/L N) between reactors 2 and 4 and the low chloramine residuals (< 0.5 mg/L) demonstrated the presence of severe nitrification in reactor 4. DOC levels were the same in both reactors when the experimental error (±5%) was considered. Nitrogenous species profile in mildly and severely nitrified bulk water samples during incubation. In order to understand the chemical chloramine decay characteristics and quantify the role of chemical reactions in mildly and severely nitrified samples, an experiment was conducted as defined in the section on materials and methods for the design of the first experimental set. The nitrogenous species were regularly monitored in the bulk water samples to understand how they changed

TABLE 2

Sample Origin Reactor 1

Details of adjusted parameters in MNS before the decay test was performed

Total Sample Chlorine Name mg/L

TAN mg/L

Nitrite mg/L N

pH

SA

2.0±0.03

0.45±0.007

0.003±0.001

8.0±0.1

SB

2.0±0.03

0.45±0.007

0.003±0.001

7.7±0.1

SC

2.0±0.03

0.45±0.007

0.285±0.004

8.0±0.1

SD

2.0±0.03

0.45±0.007

0.285±0.004

7.7±0.1

MNS—mildly nitrified sample, TAN—total ammoniacal nitrogen


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with chloramine residuals. Figure 2 shows the profiles of the measured nitrogenous species. Accelerated TAN and nitrite losses were observed in the first few days of incubation, i.e., when chloramine residual was high (Figures 2 and 3). The initially adjusted TAN was 0.52±0.008 mg/L for both samples MNS and SNS. Within 24 h of incubation, TAN dropped to 0.47±0.007 and 0.44±0.007 mg/L in MNS and SNS, respectively (Figure 2, part A). However, TAN decreased slowly on the days that followed. At the end of the experiments (35 and seven days of incubation for MNS and SNS, respectively), measured TAN losses were 0.085±0.0015 and 0.123±0.014 mg/L in MNS and SNS, respectively (Table 3). Although TAN levels were stable in SNS after seven days (168 h) of incubation, TAN was monitored continuously in SNS up to about 25 days

Nitrogenous species profile for TAN (A), nitrite (B), and nitrate (C)

FIGURE 2

MNS SNS

A 0.6

TAN—mg/L

0.5 0.4 0.3 0.2 0.1 0.0 0

200

400

600

800

1,000

800

1,000

800

1,000

Time—h

B

Nitrite—mg/L N

0.3

0.2

0.1

0.0 0

400

600

Time—h

C

0.5 Nitrite—mg/L N

200

0.4 0.3 0.2 0.1 0.0 0

200

400

600

Time—h MNS—mildly nitrified sample, SNS—severely nitrified sample, TAN—total ammoniacal nitrogen

(590 h) to confirm whether filtration was effective for eliminating nitrifying bacteria in the bulk water. Accelerated TAN loss was observed in SNS but not in MNS (Table 3; Figure 2, part A). As with the TAN levels, a significant drop in nitrite level was observed (from 0.218 ± 0.003 to 0.157 ± 0.002 mg/L N) within the first 24 h of SNS incubation. After this initial drop, a progressive but slow decrease in the nitrite level was observed (Figure 2, part B). The nitrite level remained stable in MNS because of the low nitrite level (0.007 mg/L N) at the initial stage. Furthermore, both the TAN and nitrite concentrations remained constant after the chloramine residual had dropped to 0 mg/L N in SNS. The nitrate profile is shown in Figure 2, part C. At the initial incubation period, nitrate increased significantly in SNS but not in MNS. This increase could be attributable to the high nitrite present in SNS. At the final stage (after seven days of incubation), SNS showed a nitrate increase of 0.137±0.021 mg/L N and a nitrite loss of 0.118±0.005 mg/L N, indicating that nitrite loss and nitrate gain were equivalent (Table 3). As with TAN and nitrite, changes in the nitrate level were observed only in the presence of chloramine. However, in MNS, the nitrite level was low (0.007 mg/L N), and the nitrate level was stable throughout the experimental period. To confirm whether filtration is effective for eliminating nitrifiers, NOx values were monitored throughout the experiments in both MNS and SNS. After experimental errors were considered, no differences were observed in NOx values between the start and end of the experiments (Table 3), implying that bulk water samples were free from nitrifiers. Furthermore, SNS was incubated an additional 422 h, and NOx was monitored after the chloramine had dropped to 0 mg/L. If nitrifiers had been present, NOx values would have increased. Therefore, it can be concluded that filtration was effective for eliminating nitrifiers. Total inorganic nitrogen (TIN) is the summation of TAN and NOx-N. TIN mass balance was carried out in both samples at the final stage of incubation. TIN losses were 0.11±0.03 and 0.08±0.02 mg/L, in SNS and MNS, respectively, whereas TAN losses were 0.123±0.014 and 0.085±0.015 mg/L in SNS and MNS, respectively. Therefore, in each sample, TIN and TAN losses were the same, indicating that any change in TIN was attributable to TAN loss. Chloramine decay profiles in mildly and severely nitrified bulk water samples. Figure 3, part A, shows chloramine residual profiles measured as total chlorine. Rapid chloramine loss was observed in SNS compared with MNS. For SNS, chloramine loss was significantly high in the initial days, with the chloramine level dropping to 0 mg/L within six days of incubation. The chloramine level in MNS took more than 30 days to drop below 0.4 mg/L. To quantify the role of chemicals in chloramine loss, kc values were calculated using exponential regression for both samples. The kc values determined in MNS and SNS were 0.0015±0.0002 h–1 and 0.0210±0.003 h–1,



respectively (Figure 3, part B). The error presented was the difference between the upper and lower 95% confidence level. At a high nitrite level, total chlorine decay did not properly follow the first-order decay; therefore, the error in estimation of the decay coefficient was higher in SNS. The kc value measured in MNS was similar to the mildly nitrified bulk water samples collected from the Sydney water distribution system (Sathasivan et al, 2008; 2005). A significantly higher kc value was measured in SNS compared with MNS. The experiments were repeated to verify whether accelerated decay was present in all of the samples. On all occasions, accelerated decay (i.e., kc values in the range of 0.021±0.003 to 0.031±0.005 h–1) was observed in the samples termed SNS. Chemical parameters known to affect chloramine decay include chloramine residual, Cl-to-TAN ratio, nitrite, temperature, and characteristics and value of DOC and pH (Harrington et al, 2002; Vikesland et al, 2001; Jafvert et al, 1992); nitrate concentration does not affect chloramine decay. Initial chlorine residuals, Cl-toTAN ratio, and incubation temperature were similarly maintained in both samples. DOC values in both samples were within experimental error. The only expected differences in chemical parameters between samples that could affect chloramine decay characteristics were pH, nitrite, and possibly DOC characteristics. In the autodecomposition of chloramine, ammonia in monochloramine oxidizes to nitrogen gas with a smaller quantity of nitrate and other undefined products (Valentine et al, 1987). In the current experiment, TAN levels decreased only in the presence of chloramine (Figure 2, part A; Figure 3), and it remained stable over the experimental period once chloramine dropped to 0 mg/L. Moreover, because the Cl-to-TAN ratio was less than 5:1, there was no possibility of dichloramine formation. It was previously noted that TAN and TIN were the same, indicating that any loss in TIN could be attributed to TAN loss. Therefore, it is possible that TAN loss may be associated with nitrogen gas production, with autodecomposition as the mechanism. As in the TAN profile, the nitrite level decreased in SNS only in the presence of chloramine (Figure 2, part B; Figure 3). The decrease in nitrite was equal to the gain in nitrate (Table 3), indicating that nitrite was oxidized to nitrate in the presence of chloramine and the mechanism was nitrite oxidation. Therefore, the profiles of TAN, nitrite, nitrate, and total chlorine (Figures 2 and 3; Table 3) indicated that autodecomposition and nitrite oxidations are two possible mechanisms for chloramine loss in SNS and there was no possibility that nitrifying bacteria were present to convert ammonia to nitrite. Role of pH and nitrite in chemical decay of chloramine. In previous sections, it was shown that chloramine decayed significantly faster in SNS than in MNS. The observed differences that could explain the acceleration in SNS were high nitrite and low pH. To understand whether these fac-

tors did indeed explain the SNS acceleration, experiments were conducted in which the pH and nitrite level in MNS were adjusted as described in the materials and methods section for the second set of experiments (Table 2). Figure 4 shows chloramine residual profiles of subsamples adjusted for pH and nitrite. The chlorine residual profiles for the pH-adjusted subsamples (SA and SB) did not differ significantly. Total chlorine residuals measured at the end of the incubation period (19 days) were 1.0±0.03 and 0.7±0.03 mg/L for SA and SB, respectively. Furthermore, determined kc values for SA and SB were 0.0015±0.0002 and 0.0021±0.0003 h–1, respectively (Table 4). From this experimental result, it was concluded that such a small pH difference (0.3) could not explain the acceleration of chloramine decay observed in SNS. However, a significant effect on chloramine decay was observed when either nitrite or nitrite and pH together were modified (Figure 4). After 19 days of incubation, chloramine residuals dropped to approximately 0.3±0.03 in subsample SC (nitrite adjustment) and 0.2±0.03 mg/L in subsample SD (combined nitrite and pH adjustment). In addition, the kc values determined were 0.0045±0.001 and 0.006±0.0015 h –1 in SC and SD, respectively (Table 4). As shown in the table, these values were about three to four times higher than those in SA. Although significant accelerated decay was observed after adjustment of nitrite in SC and combined nitrite and pH in SD, the decay coefficient was still much higher (0.0210±0.003 h–1) in SNS (Figure 3, part B; Table 4). The observed decay coefficient is four to five times higher in SNS than in SC or SD. The initial Cl-to-TAN ratio (3.8:1) and

TABLE 3

Details of nitrogenous species at the beginning and end of the experiments Sample Name

Parameter

MNS*

SNS†

Initial TAN—mg/L

0.520±0.008

0.520±0.008

Final TAN—mg/L

0.435±0.007

0.397±0.006

TAN loss—mg/L

0.085±0.015

0.123±0.014

Initial nitrite—mg/L N

0.007±0.001

0.218±0.003

Final nitrite—mg/L N

0.005±0.001

0.100±0.002

Nitrite loss—mg/L N

0.001±0.002

0.118±0.005

Initial nitrate—mg/L N

0.039±0.003

0.158±0.011

Final nitrate—mg/L N

0.045±0.003

0.295±0.010

Nitrate gain—mg/L N

0.006±0.006

0.137±0.021

Initial NOx—mg/L N

0.046±0.002

0.376±0.008

Final NOx—mg/L N

0.050±0.002

0.389±0.008

NOx loss—mg/L N

0.004±0.004

0.013±0.016

MNS—mildly nitrified sample, NOx—nitrite + nitrate, SNS—severely nitrified sample, TAN—total ammoniacal nitrogen *Presented values are after 35 days of sample incubation †Presented values are after seven days of sample incubation (after chloramine residual had dropped to 0 mg/L)


87

FIGURE 3

Total chlorine decay profiles (A) and kc (B) in MNS and SNS MNS SNS

A Total Chlorine—mg/L

2.5 2.0 1.5 1.0 0.5 0.0 0

150

300

450

600

750

900

Time—h

B 0.030 0.025

kc—h

–1

0.020 0.015 0.010 0.005 0.000 MNS

SNS Nitrification Stages

kc —chemical decay coefficient, MNS—mildly nitrified sample, SNS—severely nitrified sample

FIGURE 4

SA

2.5 Total Chlorine Residual—mg/L

Chloramine residual profiles at different pH and nitrite concentrations SB

SC

SD

2.0

1.5

1.0

0.5

0.0 0

100

200

300

Time—h

400

500

nitrite level (0.218 mg/L N) were lower in SNS than in SC and SD. Therefore, the decay coefficient should have been lower in SNS, because low Cl-to-TAN ratio and low nitrite level have been reported to show a lower decay coefficient (Vikesland et al, 2001). If accelerated chemical decay can be explained only by chemical parameters (total chlorine, Cl-to-TAN ratio, nitrite level, and pH), the kc value should be lower than 0.006±0.0015 h–1 in SNS. In order to verify the rapid acceleration observed in SNS, decay tests were conducted repeatedly by collecting samples from the severely nitrified reactor (reactor 4). The initial chloramine residuals, Cl-to-TAN ratio, and temperature were the same in all of the decay tests, but pH values were in the range of 7.7 to 8.0. Different nitrite levels were observed in each sampling time. The kc values were determined at the different nitrite levels and are shown in Figure 5. In each repeated test, accelerated decay was observed (kc values in the range of 0.021±0.003 to 0.031±0.005 h–1), and kc values increased with increasing nitrite levels. As mentioned previously, at high nitrite levels, total chlorine decay did not properly follow first-order decay; therefore, error determined at the 95% confidence level is high. It was concluded from the analysis that chemical decay was accelerated once severe nitrification was triggered. For comparison purposes, the authors determined additional kc values at different nitrite levels but similar initial total chloramine residuals, Cl-to-TAN, pH, and temperature; these are shown in Figure 5. Even after adjustment of the high nitrite level (0.315 mg/L N) in the mildly nitrified sample, the kc value was still three to four times lower than that in the severely nitrified samples (Figure 5). Other researchers (Vikesland et al, 2001; Hao et al, 1994) have reported that pH values of 8.3 and 7.6 do not significantly affect chloramine loss, but lower pH significantly accelerates the decay rate. The pH values adjusted in all samples were between 8.00 and 7.70; these were very close if the pH measurement error (±0.1) is considered. The monochloramine decay model developed by Vikesland and colleagues (2001) showed that nitrite alone could not accelerate monochloramine decay even in the presence of 0.5 mg/L N. They also reported that nitrite accelerates monochloramine decay only in the presence of a catalyst (0.5 mg/L bromide). After comparing chemical decay in the literature (Vikesland et al, 2001; Hao et al, 1994), the authors concluded that pH and nitrite alone could not explain such a rapid chloramine loss in a severely nitrified sample. Therefore, another component or reaction must account for the severely nitrifying sample collected from reactor 4. In addition, all the reactors were made of HDPE and the sensors inserted inside the reactors were made of stainless steel; therefore there is no possibility that any compounds were released from the materials used in the construction of the reactor. Clearly, further experiments and analysis are needed to understand the cause of the



TABLE 4

kc values at different conditions

Origin of Sample

Sample Name

Total Chlorine—mg/L

Reactor 1

SA

2.0±0.03

0.45±0.007

0.003±0.001

8.0±0.1

0.0015±0.0002

Reactor 1

SB

2.0±0.03

0.45±0.007

0.003±0.001

7.7±0.1

0.0021±0.0003

TAN—mg/L

Nitrite—mg/L N

pH

kc—h–1

Reactor 1

SC

2.0±0.03

0.45±0.007

0.285±0.004

8.0±0.1

0.0045±0.0010

Reactor 1

SD

2.0±0.03

0.45±0.007

0.285±0.004

7.7±0.1

0.0060±0.0015

Reactor 4

SNS

2.1±0.03

0.52±0.008

0.218±0.003

7.7±0.1

0.0210±0.0030

kc—chemical decay coefficient, SNS—severely nitrified sample, TAN—total ammoniacal nitrogen

rapid acceleration of the decay rate. However, the authors suggest two possible causes: first, soluble microbial products released by microbial activities or microbial breakdown of NOM and second, other chloramine-demanding compounds produced by microbial action. It has been widely reported that nitrifying bacterial activity can release soluble microbial products (Wilczak et al, 1996; Rittmann et al, 1994; Furumai & Rittmann, 1992; Wolfe et al, 1990). In addition, Noguera and coworkers (2009) found that soluble microbial products produced under nitrifying conditions can help heterotrophic bacterial growth and thus accelerate chloramine decay. Therefore, soluble microbial products may play a role in accelerating chloramine decay in severely nitrifying samples. Furthermore, microbial action on NOM to form breakdown products that eventually accelerate chloramine decay cannot be ruled out. Additional experiments are needed to elucidate what compounds are involved and how they are accelerating chloramine decay in severely nitrifying samples beyond what can be explained by nitrite and pH.

• The decay coefficient calculated after adjusting the pH in mildly nitrified water to 7.7 (the pH observed in severely nitrified water) showed that such a small change in pH marginally increased chloramine decay. • When all of the parameters (chloramine residual, Cl-to-TAN ratio, nitrite, and pH) that are observed in severely nitrifying samples and that are known to have an affect on chloramine decay were adjusted in the mildly nitrified sample, the decay coefficient calculated was still three to four times lower than that observed in severely nitrifying water. • Given that the material used for reactor construction did not release any compound that would accelerate chloramine decay and because nitrification is known to produce soluble microbial products, it is suspected that the compounds accelerating chloramine decay could be soluble microbial products or microbial breakdown products of NOM.

FIGURE 5

kc at different pH and nitrite concentrations

CONCLUSIONS MNS (pH = 8.0) SNS (pH = 8.0) SNS (pH = 7.7)

0.04

–1

0.03

kc —h

To better understand chloramine decay in mildly and severely nitrified samples, experiments were conducted by collecting samples from laboratory-scale reactors. Traditional indicators (Cl-to-TAN ratio, nitrite, nitrate, pH, and chloramine residuals) were found to change as described in the literature. Chloramine decay observed in severely nitrifying samples and mildly nitrifying samples was found to be similar to that reported in the literature. To understand the effect of known parameters on chloramine decay, the indicators suspected to affect the residual decay were adjusted to similar levels in both the mildly nitrifying and severely nitrifying samples. These experiments yielded the following major conclusions: • Accelerated chloramine loss was observed on rechloramination of severely nitrified bulk waters. This was about 14 times that observed in mildly nitrified samples. Similar to chloramine loss, rapid nitrite and total ammoniacal nitrogen losses were observed in severely nitrified bulk waters.

0.02

0.01

0.00 0

0.1

0.2

0.3

0.4

Nitrite—mg/L N kc —chemical decay coefficient, MNS—mildly nitrified sample, SNS—severely nitrified sample


89

ACKNOWLEDGMENT The authors acknowledge the funding provided by Curtin University of Technology in Perth, Australia; the Australian Research Council (ARC Linkage project: LP0776766) in Canberra, Australia; and Water Corporation in Leederville, Australia. The authors would like to acknowledge George Kastl, who designed the system used for this experiment when he was an employee of Sydney (Australia) Water Corporation. Acknowledgment is also due to Sydney Water Corporation for permission to use the design. The experimental help provided by Dipok Sarker is also acknowledged.

from the Asian Institute of Technology in Bangkok, Thailand. Arumugam Sathasivan (to whom correspondence should be addressed) is an associate professor in the Department of Civil and Construction Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia; [email protected]. Date of submission: 03/12/10 Date of acceptance: 07/30/10

FOOTNOTES

ABOUT THE AUTHORS

K.C. Bal Krishna is a PhD candidate at Curtin University in Perth, Australia. He has a bachelor’s degree in civil engineering from Pulchowk Engineering College in Lalitpur, Nepal, and a master’s degree in engineering

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