Microbial factors accelerating chloramine decay in chloraminated systems

Abstract

Chloramine is the second most popular disinfectant behind chlorine. The main advantages of using chloramine are that it provides a longer lasting disinfectant residual and diminishes the formation of regulated disinfection by-products. However, at times microbial chloramine decay can overwhelm the stability. Nitrification has been found to be one of the primary factor for this instability and considerable resources have been spent to control and resolve this problem. Nitrification causes the problem in two ways: by producing nitrite which can subsequently react with and decay chloramine; and by diminishing the amount of ammonia available for re-dosing of chlorine to boost chloramine. In addition, soluble microbial products (SMPs) found to be playing a major role in accelerating chloramine decay in water distribution systems. SMPs are assumed to be unidentified proteins and/or enzymes secreted by microorganisms and sometimes found under severely nitrifying conditions. Several studies have been carried out to understand the role of nitrifies in distribution systems; nevertheless, there are no studies on SMPs to further elucidate the compounds or conditions under which they are formed. In order to develop control mechanisms for chloramine decay, it is necessary to identify SMPs and discover factors affecting the production of SMPs in the chloraminated systems. As dissolved organic carbon (DOC) impact on nitrification process, it could also be a controlling factor for the production of SMPs. To develop such an understanding, a lab scale reactor system has been maintained to initiate nitrification conditions suitable for different nitrifying conditions. The study had two primary objectives: understanding the suitable conditions for producing SMPs by varying DOC level. Second objective is the separation and identification of SMPs using different separation and identification techniques. The first step involved finding out the optimum DOC level for nitrification suitable for production of SMPs; and in order to achieve this, the lab scale reactor sets were treated with different DOC concentrations containing water (10-12, 7-8, 5-6, 3-4, and 0-1 mg-C/L DOC). Nitrification and production of SMPs could not be observed with the water containing 10-12 mg/L DOC. This could be due to the toxic nature or lost battle with heterotrophic bacteria with the presence of high DOC level such as 10-12 mg-C/L on nitrifiers. With the water containing 7-8 mg-C/L DOC level, nitrification conditions were achieved, but not the SMPs production. Both nitrification and SMPs production were observed with other DOC containing feed waters; which contribute to accelerated chloramine decay. When there were no nitrification/ mild nitrification, Ammonia oxidising microorganisms may not be responsible for the chloramine decay. BAC column which was maintained to reduce DOC level up to 0-1 mg-C/L also removed some heavy metals such as Cu, Pb, Zn and Mn present in the feed water. A modified biostability curve was needed to represent the onset of nitrification in inhibitory substances eliminated water systems. After the confirmation of SMPs existence in reactor water, they were separated for further identification using 30 kDa, 50 kDa and 100 kDa centrifugal filters. According to the chloramine decay test results obtained from the experiment, it was confirmed that 30 kDa centrifugal filters separate SMPs from bulk water samples. The NMR spectrometry could not identify the protein(s) as the protein is either bigger than the size NMR could detect or the concentration was too low for the detection with NMR. As a next step, gel electrophoresis followed by mass spectrometry was employed. More than 20 protein types specifically belonging to bacteria were identified. Among them 4 protein types were in acceptable range with high matching score value as they were between 30 kDa to 50 kDa range. Interestingly, there was a protein which belongs to Nitrosomonas spp; this needs to be further examined. Overall, the results for the first time provided the identities of the proteins which could be SMPs. Successful separation of SMPs from nitrified bulk water samples using 30 kDa centrifugal filters was also conducted for the first time. Moreover, this study highlights the effect of DOC concentration on the production of SMPs and how BAC column eliminates the metal inhibition on nitrification. The study also proved that usually supplied water naturally contains some inhibitory substances that need consideration in defining biostability.

Date of Award	2014
Original language	English