Effects of Environmental Factors on Nitrification Occurrence in Model Drinking Water Distribution Systems. Ng M.Y.

Transcription

1 Effects of Environmental Factors on Nitrification Occurrence in Model Drinking Water Distribution Systems Ng M.Y. Division of Environmental Science and Engineering, National University of Singapore ABSTRACT Nitrification is a microbial process whereby ammonia is sequentially oxidized to nitrite and nitrate ions and is mediated by autotrophic bacteria. By using chloramines as a water distribution system disinfectant that tends to release free ammonia, it encourages the growth of both ammonia-oxidizing bacteria (AOB) and nitriteoxidizing bacteria (NOB). This water degradation may lead to biologically unstable water that is thus unsafe for human consumption. Despite its drawbacks, it is a better water distribution system disinfectant than chlorine as it is longer lasting and minimizes the formation of regulated disinfection by-products (DBPs) such as Trihalomethanes (THMs) and Haloacetic acids (HAAs). Therefore, it is pertinent to devise control measures to minimize nitrification occurrence in the distribution system. INTRODUCTION Over the years, chloramination has been integrated into the water distribution system as distribution system disinfection. This is due to its potential in minimizing microbial contamination and the ability to comply with the Federal regulations. Although chloramines are not as reactive as chlorine, it serves as a longer lasting disinfectant that is able to reach out to remote areas and minimize a bad taste and odor produced (AWWA, 2003). Most importantly, it forms much lower levels of regulated 1

2 DBPs than chlorine. These DBPs such as THMs and HAAs are known to be potentially carcinogenic and are harmful to the public health. On the other hand, chloramines pose a number of problems. It encourages the growth of both AOB and NOB via releasing free ammonia for nitrification to occur. As a result, there is a potential risk of nitrification process taking place which will affect the operation and water quality aspects. Nitrification is a microbial process whereby ammonia is sequentially oxidized to nitrite and nitrate ions and is mediated by autotrophic bacteria (Andrzej., 1996). Nitrification generally causes a decrease in chloramines residual, alkalinity, and an increase in nitrates and nitrites and, microbial counts. This degradation of water quality may lead to violation of the Safe Drinking Water Act. Thus it is pertinent to prevent nitrification so as to provide a more stable chloramines residual in the distribution system and biologically stable water which is safe for human consumption. Application of free chlorine before free ammonia addition is the most frequently used approach for chloramine formation.due to the risk of encouraging nitrification within the treatment plant process (sedimentation basin, filtration). Hence, this study seeks to review briefly the relationship between operational parameters and nitrification in lab-scale distribution system thereby, formulating potential control strategies and proposing possible ways of predicting nitrification potential for the local water distribution system. EXPERIMENTAL DESIGN AND SET-UP Biofilm reactors (Model CBR 90-1, Biosurface Technologies) are employed in the experiments, with a retention time of 1 day and a fixed rotational speed of 50 rpm. It is operated in parallel and its temperature is in a range of o c. Polycarbonate coupons were used as the biofilm support media inside the reactor. 2

3 Three sets of experiments were set up to examine the effects of influent water ph, chlorine to ammonia-n ratio, and monochloramine residual concentration on the nitrification occurrence. Table 1. Tabulated parameters to be investigated Experiment No.: Parameters: Influent ph Cl 2 :N ratio Monochloramine conc. Values to be investigated: 7.5 3:01 < 0.3 mg/l 8.2 4: mg/l 9 5: mg/l In the first set of experiment, tap water was used as the influent water and, HCl or NaOH solutions are added to achieve the targeted ph levels. Lower ph values were not examined due to corrosion control. In the second set of experiment, ammonia-n dose is varied at fixed chlorine dose to achieve the desired ratio for analysis. Ammonium chloride (NH 4 Cl, 36 mm as NH 3 -N) and sodium hypochlorite (0.5% as Cl 2 ) solutions were used for monochloramine formation. The monochloramine level was set at mg/l. In the third set of experiment, 0.5% (as Cl 2 ) NaOCl and 36 mm (as NH 3 -N) NH 4 Cl solutions are added to a dechlorinated tap water at a 5:1 of chlorine-to-ammonia-nitrogen weight ratio. For each set of experiments, both Most Probable Number (MPN) technique-based AOB enumeration data and heterotrophic plate counts (HPC) levels were taken. For the MPN technique, the nitrosomonas cells are enumerated by the 3-tube MPN procedure. Three sets of 3 tubes were used (10mL of solution in each tube) and inoculated with 1, 0.1 and 0.01mL of samples to be tested. After innoculation, tubes were incubated in the dark for 21 days at 28 o c. At the end of incubation period, cultures were tested for presence of nitrite. A positive result is obtained when a deep reddish color is observed (AWWA, 2003). 3

4 For the HPC method, a R2A agar is used. The agar is inoculated with a sample and incubated for 7 days at 28 o c. At the end of the incubation period, colonies of bacteria are counted. RESULTS AND DISCUSSION Various results were obtained from the experiment and tabulated in a graphical format as shown in Fig Changes in total organic carbon (TOC), ammonia-n, nitrite-n, nitrate-n concentrations, and both AOB levels and HPC levels in the reactors operated under the varied parameters were recorded. Free ammonia is known to promote the growth of AOB thus when ammonia levels are lower than normal, it is indicative of nitrification occurrence and that nutrient levels are being depleted (Hill, N.A.). Nitrite and nitrate are products of nitrification, hence it is the best indicators of nitrification. An increase in HPC R2A counts is usually observed when nitrification takes place due to the presence of organic-rich products released by AOB, which serves as a nutrient source for HPC bacteria. As such, TOC is recorded. Lastly, AOB levels are taken into account as it mediates the process of nitrification. Note that both bulk and biofilm solution are tested for AOB levels and HPC levels. It is discovered that biofilms are able to encourage nitrifiation as it is able to shield the nitrifying bacteria from disinfectant residuals (Skadesen., N.A.) Experiments to evaluate the effect of influent ph on nitrification For all reactor runs, ph level measured in the reactor effluents were consistently lower than those in the influent water. These decreases in ph may not be indicative of nitrification as it may be caused by low alkalinity and buffering capacity of water, which could be associated with accelerated loss of monochloramine residual. From the data collected, a slight but noticeable decrease in ammonia-n and increase in nitrite-n and 4

5 nitrate-n concentrations were observed in the reactor effluents as shown in Fig. 1b-d, implying nitrification occurrence. Fluctuations were observed due to different conditions, including both chemical and biological consumption. For the MPN technique-based AOB enumeration, the nitrification potential is favored at ph 9.0 for the bulk solution whereas for biofilm solution, it is the least favored. On the other hand, HPC was observed to be at a high magnitude whereby bulk HPC levels remained as 10 4 CFU/mL while biofilm HPC remained as 10 2 CFU/cm 2. Change in TOC (mg/l) (a) ph 7.5 ph 8.2 ph 9.0 Change in NH 3 -N (mg/l) (b) ph 7.5 ph 8.2 ph Time (weeks) Time (weeks) Change in NO 2 -N (mg/l) (c) ph 7.5 ph 8.2 ph 9.0 Change in NO 3 -N (mg/l) (d) ph 7.5 ph 8.2 ph Time (weeks) Time (weeks) Fig. 1. Changes in (a) TOC, (b) ammonia-n, (c) nitrite-n, and (d) nitrate-n concentrations in the reactors operated under different influent water ph. Taking into account the results obtained, there is no observed differences in nitrification potential between the three influent water ph levels. This can be explained by the consistent drop in water ph to less than 7.6 within the reactor, thus 5

6 mask the effect of influent ph on nitrification potential. HPC growth can be further inferred as a loss of monochloramine residual and biologically unstable water quality. Experiments to evaluate the effect of Cl 2 :N ratio on nitrification For all three reactors, it is observed that the measured monochloramine residual remained consistently low and there is a change in the nitrogen species concentration. This change can be interpreted as nitrification occurrence. A decrease in TOC concentrations were also observed in the reactor effluent. For the MPN techniquebased AOB enumeration as shown in Fig. 2, it can be distinctly deduced that nitrification potential is best at a 4:1 Cl 2 :N ratio. Also, both bulk and biofilm HPC growth levels are relatively similar in all three reactors. Fig. 2. (a) Bulk and (b) biofilm AOB levels in the reactors receiving the chloraminated water produced with chlorine to ammonia-n weight ratios of 3:1, 4:1 and 5:1. For the prevention of nitrification, it is pertinent to have proper control of ammonia and chlorine dosage. This can be achieved by limiting the residual free ammonia available for AOB to feed on. However, it is essential to note that a ratio beyond 5:1 will lead to the formation of dichloramine thereby leading to customer complaints about taste and odor. On the other hand, if the ratio is below 4:1, excess free ammonia is able to enter the system leading to nitrification. Experiments to evaluate the effect of monochloramine concentration on nitrification 6

7 A significant loss of monochloramine residual is observed regardless of the initial amount of monochloramine residual present in the influent water. However, the influent water with the lower monochloramine residual exhibits a comparable change in ammonia-n, nitrite-n and AOB levels. In addition, there is not much deviation in the HPC levels between the high residual reactor and the low residual reactor as shown in Fig. 3. As such, it can only be inferred that a lower amount of monochloramine residual present in the influent water may enhance nitrification. Fig. 3. (a) Bulk and (b) biofilm HPC levels in the reactors operated with the influent water having different monochloramine concentrations. Raising the chloramine residual is effective at controlling nitrification. However, in doing so, there will be a risk of AOB growth exceeding the AOB inactivation rate, resulting in nitrification (Skadesen., N.A.). Furthermore, chloramines decay and losses which is dependent on the water quality characteristic, distribution systems demand and residence time, have to be considered too. A system with longer residence time will experience greater decrease in chlorine residual, and be more vulnerable to nitrification. CONCLUSION 7

8 From the experiments conducted, it can be deduced that nitrification potential is favored at chlorine to ammonia-n ratio of 4:1 and low influent monochloramine residual. The optimal ph is unknown due to the difficulty in distinguishing the differences in nitrification potential among the three influent water ph levels. Therefore, more investigation is required to determine how locally applicable is the results and whether it will be effective as a parameter to predict and control nitrification. As nitrification is a biological phenomenon and can occur very rapidly, but not instantaneously (Smith, N.A.), trend graphs are used to obtain a proper interpretation of nitrification monitoring parameters. REFERENCE Andrzej. W., Joseph. G.J., Joseph. P.M., Lee. H.O. and Gregory. J.K. (1996), Occurrence of nitrification in chloraminated distribution systems, AWWA AWWA Research Foundation. (2003), Ammonia From Chloramine Decay: Effects on Distribution System Nitrification, Chapter 5, pages Hill. P. H., (N.A.), Assessment and Operational Responses to Nitrification Episodes, AWWA Manual M56, 1 st Ed., Chapter 9, page Smith. C.D., (N.A.), Monitoring for Nitrification Prevention and Control, AWWA Manual M56, 1 st Ed., Chapter 7, Skadesen. J. and Cohen. Y.K., (N.A.), Operational and Treatment Practices to Prevent Nitrification, AWWA Manual M56, 1 st Ed., Chapter 8, page