Microbiological water quality in a large in-building, water recycling facility.

The Thames Water recycling plant at the Millennium Dome, London, reclaimed three sources of water: greywater from the washbasins, rainwater from the Dome roof and groundwater from a borehole on site. These were pre-treated separately, and the mixed stream filtered using ultrafiltration and reverse osmosis membranes. Monitoring for indicator microorganisms was undertaken throughout the plant and in the reclaimed water distribution system, as well as ad-hoc monitoring for the presence of pathogens in the raw waters. Treatment to the level of ultrafiltration was more than adequate to produce a water quality meeting existing worldwide reclaimed water guidelines for toilet flushing. Owing to the excellent quality of the water leaving the plant, no significant microbiological growth was observed in the reclaimed water distribution system during the year. The raw greywater exhibited a higher faecal bacterial load than the rainwater and groundwater, as predicted from more human contact (i.e. hand washing). Environmental strains of Legionella were observed in the three raw greywater samples analysed for pathogens, as was Cryptosporidium, Giardia and faecal enterococci. The rainwater had relatively high levels of faecal bacteria, probably of avian origin. Giardia was detected in one rainwater sample confirming the potential for this water source to contain pathogens.