Microorganism-particle association in water supply systems

Availability of safe drinking water is fundamental to any community. This is achieved through disinfection processes including, but not limited to, filtration and chlorination. In Melbourne, Australia, the availability of high quality source water allows the majority of treatment to consist of chlorination without pre-filtration. In the absence of filtration, the effects of particles entering the distribution system are of interest owing to the potential of surface-associated microorganisms to resist disinfection. In order to evaluate this risk, assessment of resistance behaviours in the presence of surfaces, and an understanding of interactive behaviours, is required. Investigations into resistance behaviours relevant to Melbourne's drinking water supply were undertaken with specific regard to the Silvan surface water reservoir. This involved three different bacteria (Escherichia coli ATCC 25922 and Silvan water isolates Pseudomonas fluorescens and Serratia marcescens) and four different surfaces (goethite, Silvan Reservoir particles, modified Silvan particles and polystyrene). Initial investigations involved evaluation of surface characteristics where P. fluorescens demonstrated characteristics considered most suitable to adhesion. In accordance with surface characteristics evaluated, P. fluorescens was shown in batch assays to be the most adherent bacterium over all substrata investigated. Particle-mediated resistance to chlorination was investigated using activity assays combining a detachment method with sensitive epifluorescent detection. Consistent with adhesive behaviours, the bacterial strain found most resistive to chlorination in the presence of surfaces was the environmental strain of P. fluorescens. Resistance was found to vary with both bacteria and particle type and was found to correlate with adhesion (r2 ≥ 0.647). Biofilms also presented resistance to disinfection and pre-chlorination of bacteria was identified as a risk factor in regard to biofilm regrowth where nutrients were available. In terms of the Melbourne's drinking water supply, this work shows that indigenous bacteria and particles interact in a manner able to increase resistance of bacteria to chlorination. Particle-mediated resistance decreased with increased concentrations of chlorine and the bacteria-particle combinations examined were susceptible to treatment at values likely to be experienced in a treated drinking water environment. Thus, for the bacterial species and conditions evaluated, surface-mediated resistance to chlorination may be considered a potential risk to the quality of unfiltered drinking water supply, but one that may be controlled through appropriate treatment. The particular characteristics of indigenous bacteria and particles determined here were vital in evaluating this risk and in investigating the relationship between adhesion and resistance.