Coliforms, biofilms, microbial diversity and the quality of roof-harvested rainwater

Previous studies of the bacterial quality of roof-harvested rainwater have been confined to either detection of specific pathogens and faecal indicator organisms, or epidemiological studies of health effects associated with consumption of stored rainwater. As yet however, a general consensus on tank-water quality has not been reached. In this paper it is proposed that achieving reliable microbial quality assessment will require a more detailed understanding of the true microbial diversity of rainwater harvesting systems and microbial processes at work within the tank environment. The paper details the preliminary findings of an investigation into the microbial diversity of rainwater harvesting systems currently underway in eastern Australia. The study includes rainwater tanks at both urban and rural locations, from which a wide variety of micro-organisms have been cultured and identified. The results have indicated considerable variation in the level of contamination and bacterial composition between sites, and over time. In all cases free living environmental species were predominant, and evidence of faecal contamination was minimal by comparison. The key outcomes of a parallel pilot study into the formation of biofilms in rainwater tanks, and their potential impact on water quality, is also presented, revealing a possible relationship between the extent of biofilm development and the quality of water delivered. Further support for the bio-remedial capacity of biofilms was provided by in-vitro experiments on biofilms which confirm the capacity of an established biofilm to remove lead from the main water body. It was concluded that a greater understanding of the microbial ecology, and the factors influencing microbial composition of the rainwater tank, will be critical to the optimum utilization of domestic rainwater harvesting as part of an integrated solution to water supply crises faced by many of Australia’s major urban centres.