Understanding the role of heterotrophic bacteria in degrading chloramine residuals in distribution systems

Abstract

The maintenance of an adequate disinfectant residual until water reaches customers is required in order to ensure safe drinking water at the customer tap. Chloramine has been widely used for such purpose in Australia, because of its persistent characteristics and reduction in formation of carcinogenic disinfection by-products. However, chloramine can decay rapidly in water distribution systems due to microbial activities, particularly nitrification. Therefore, managing chloramine decay once nitrification has triggered is challenging and often a losing battle for water utilities. Many corrective strategies have been attempted to control or avoid nitrification. However, it has not yet been possible to resolve this problem, and almost all utilities expend considerable resources annually to maintain chloramine residuals in distribution systems. Recent reports have shown mildly nitrifying conditions occur before the onset of nitrification. Water undergoing mild nitrification tends to host heterotrophic bacteria in abundance and is characterised by substantial microbial chloramine decay which leads to conditions suitable for nitrification (lower chloramine residual and higher free ammonia residual). Therefore, the heterotrophic bacterial community and their influence on chloramine decay were studied to help water utilities formulate effective strategies to manage chloramine decay in drinking water distribution systems. Water was collected from 2 different water reservoirs from the Sydney distribution system in May and June 2012, and heterotrophic bacteria colonies identified by molecular methods. The capability of these microorganisms to degrade chloramine was examined. When water was plated on R2A medium, only five colonies grew and all five colonies were isolated. They were identified as Methylobacterium oryaze, Methylobacterium adhaesivum, Brevibacterium frigoritolerans and Bacillus cereus. Two of the five isolates were found to have the same genetic identity. These belong to Proteobacteria (first two), Actinobacteria, Firmicutes phyla, respectively. Results showed that all microorganisms were able to degrade chloramine without causing nitrification but the decay rates were different. The Bacillus cereus isolate exhibited the highest first order decay coefficient (0.687 hr-1 by 3.3 x 106 cells/mL culture) followed by Methylobacterium sp isolate (0.1225 hr-1 by 3.3 x 106 cells/mL culture) and Brevibacterium 3 frigoritolerans isolate (0.067 hr-1 by 3.3 x 106 cells/mL culture). As Methylobacterium sp. are reported to be highly abundant (5-20%) in chloraminated waters, further investigation to formulate control strategies are needed. This research is the first example of the role of heterotrophic bacteria in the chloramine decay process in distribution systems, which subsequently provides conditions suitable for the latter onset of nitrification.

Date of Award	2014
Original language	English