Pressure on water resources, along with a need to reduce the risk of surface water flooding, have led to an increased interest in the adoption of a variety of sustainable water systems. This paper looks at the microbiology of rainwater harvesting in a UK context.

While harvested rainwater is not generally used as drinking water in the UK, it is used to replace water (e.g. for toilet flushing) which is currently of potable quality, thus it is important to assess the potential infection risks incurred by residents of properties using such systems.

A review of the literature suggests that there may, on occasion, be a number of pathogens present in harvested rainwater. This finding was confirmed in a small-scale empirical study of three rainwater harvesting tanks located in England and Wales, with Campylobacter spp. being isolated from up to 50% of the samples, with a maximum concentration of 91 per litre.

An exploratory quantitative microbial risk assessment, based on exposure scenarios derived from the empirical data suggests that if consistently high levels of Campylobacter are present in the harvested rainwater there could be an increase in the number of cases of campylobacteriosis in population. However, a more conservative estimate of Campylobacter concentration and frequency of contamination (based on median figures) suggests a virtually negligible risk of infection.