Microbial communities across water treatment plants : fate of coliforms and other bacteria

Abstract

The production of clean drinking water is an essential process that greatly affects the health and prosperity of a community. Microbial content is a key parameter of water quality: the treatment process must remove all potentially harmful microorganisms. However, there is still much unknown about the microbial communities present in the water treatment process and their effect on the end quality of drinking water. In water treatment, coliform bacteria are used as ‘indicator’ organisms: the detection of these microorganisms signifies the water supply has been contaminated by faecal matter and other pathogens are likely to be present. Despite the proven efficacy of chlorine as a water disinfectant, treatment failures (i.e. detection of coliforms in final water samples) still occur at water treatment plants (WTPs). The possibility of chlorine resistance or tolerance in coliforms as a cause of treatment failures was explored and the chlorine tolerance of Escherichia coli isolated from different environments compared. Although chlorine tolerance was found to be higher in E. coli from a WTP environment than lab strain E. coli, coliform bacteria were found to be very sensitive to chlorine and no evidence of genetic resistance or tolerance was observed. In order to expand current knowledge of the overall microbial community of WTPs, a detailed sampling survey of two working WTPs was carried out and the biofilm and bulk water community was analysed across time, treatment stage and source water type using methods including next generation sequencing and quantitative PCR based on the 16S rRNA gene. Source water type was found to be the main determining factor of bulk water community composition, while treatment stage had greater influence on biofilm community composition.