Rainwater for residential hot water supply: managing microbial risks

Abstract There is growing interest in using alternative water sources such as rainwater harvesting and treatment systems to supplement traditional sources and secure a stable supply. For such systems, it is important to ensure adequate water quality, as microbial contamination can be a risk factor in rainwater. The primary objective of this study was to provide proof-of-concept for the microbial treatment capacity of a residential scale rain-to-hot-water treatment system to be installed in Melbourne, Australia. The system consists of a filtration, UV and heat-pump hot water unit, and disinfects roofwater prior to use of the hot water for bath, shower and laundry. The system’s efficiency was evaluated using long-term challenge tests investigating the full system and each of the separate components. The microbial treatment performance was assessed based on the systems’ ability to treat high levels of E. coli, E. faecalis, Campylobacter, Salmonella and MS2 phage under challenging conditions; with varying flow rates (10-40L/min) and microbe concentrations (104-105unit/L), and in scenarios of a power outage. Over a compressed year of operation, the full rain-to-hot-water treatment system was extremely efficient at reducing concentrations of E. coli, E. faecalis, Campylobacter, Salmonella and FRNA phages, with log reductions ranging from the lowest average of 2.1log reductions for Salmonella to a maximum of >5.1log for E. coli. Most of the treatment was provided by the UV system, and any remaining microbes present after this point were deactivated by the heat-pump system (provided that the water was given enough time to warm up). Additional modelling work showed that UV intensity, UV transmissivity and contact time (for the UV system) and temperature (for the heat-pump hot water system) could be used as parameters to predict microbial treatment performance of the system, indicating that these easily-measurable parameters could assist with ongoing operation optimisation and maintenance of such systems.