Bacterial Water Quality of Rainwater Fed Domestic Hotwater Systems

Rainwater harvesting within the urban environment is being increasingly recognised for its important role in reducing pressures on mains water supply and reducing stormwater runoff. The extent to which rainwater tanks can improve our management of the urban water cycle depends on the level of usage of the rain-harvested water. By incorporating the hotwater service into the suite of uses of harvested rainwater, substantial systemic benefits can be realised. This paper discusses some of the water quality implications of using harvested rainwater through hotwater systems. Field sampling of hot and cold water taps supplied by rainwater tanks was conducted over a 12 month period, together with laboratory conducted thermal destruction experiments on enriched rainwater samples. The field results showed substantial reductions (85-100%) of E. coli, total coliform, Pseudomonas, and Heterotrophic Plate Count concentrations in harvested rainwater passing through hotwater systems maintained between 50-70°C. The results of the laboratory thermal destruction experiments indicated that the time required to reduce a heterogeneous bacterial population at 55°C over each of the first three log reductions were 10, 190, and 400 seconds, respectively. The lengthening of times required for inactivation demonstrated a pronounced shift in population. The hygienic quality of harvested rainwater was greatly improved after passing through hotwater systems maintained at adequately high temperatures.