Rainwater through Hotwater Services – Microbial Risk Assessment

This research was prompted by concerns that rooftop-harvested rainwater fed into household hot water services might expose the public to harmful pathogens such as salmonella. Householders can be exposed to untreated heated rainwater while washing and when this water is used to wash or prepare food. Water heated to 60°C and maintained at this temperature for several minutes’ kills most, but not all infectious pathogens. The problem is that many Australian hot water services operate at lower temperatures that do not kill enteric pathogens. In this study fewer than 5% of water samples collected from unheated rainwater tanks contained pathogens, but this relatively low frequency still poses a risk and is part of the reason that heated rainwater systems fail to meet existing guidelines for safety. Modifications that would improve safety include installing a device that prevents water cooler than 60°C leaving the storage tank and using cold potable water instead of cold rainwater for tempering or in mixer taps. UV disinfection devices attached to rainwater systems are an effective way to reduce health hazards to acceptable levels.