Dual water supply systems were installed in several newly built housing estates in the Netherlands in the late1990s. These residential homes were provided with both drinking water and separately with so-called household water for toilet flushing, laundry and the garden tap. Household water was produced by limited treatment from a variety of sources and had a lower quality than drinking water. No legislation for (the quality of) this type of water was present at the time and the Dutch government appointed six of these estates as pilot projects. Four pilot projects were intensively monitored for toxicological and microbiological safety as well as microbiological stability during a period of almost 16 months.

Specific incidents such as cross connections between drinking water and household water and observations of viruses and pathogenic protozoa in treated water demonstrated that some of these systems were microbiologically unsafe. Furthermore certain household waters had a relatively high biofilm formation potential leading to growth of Legionella sp. and Aeromonas and complaints from customers about the smell and colour of the household water. In nearly all cases concentrations of heavy metals and organic pollutants were below drinking water standards, hence the toxicological risk caused by chemical substances was not significant.

Based on the results of this study the Dutch government decided to discourage the production and distribution of household water on a large scale. At present all projects owned by water companies in the Netherlands have been terminated by replacing household water with drinking water.

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