Used water can be returned to the tap

Water that is used to wash hands is barely contaminated but goes straight down the drain. It is now possible to reuse this greywater, thanks to a grid-free treatment system developed by researchers at the Swiss Federal Institute of Technology in Zurich (ETH) and the Swiss water research institute Eawag as part of the Blue Diversion AUTARKY project. The greywater goes through several treatment stages on-site and ultimately has a bacterial count lower than that of Zurich tap water, the ETH reported in a press release.

The key component of the system is a fine-pored plastic membrane, which retains pathogenic organisms that break down the faecal and urinary contaminants in waste water. However, nutrient concentrations in handwashing water are too low for the bacteria. Researchers solved this problem by adding nutrients – such as nitrogen and phosphorus – to the soap. “The bacteria perform very effectively, with a removal rate of almost 100 per cent,” explained ETH Professor Eberhard Morgenroth. After this process, an activated carbon filter removes any traces of organic matter remaining in the water. In the final step, chlorine is produced from dissolved salt by means of an electrolytic cell to disinfect the water during storage.

The ETH points out that systems do already exist for treating greywater on-site for use in toilet flushing, but this recycled water does not meet the required quality standards to be used for other purposes. The researchers have now achieved this extra purity.

A prototype of this handwashing system was deployed on a communal green space within the city of Zurich this summer. From January, it will be tested in an informal settlement in South Africa. ETH has above all designed this system for use in underdeveloped regions, but it might also be applied in train toilets in future, which means water would no longer have to be constantly replenished by railway staff. “Rail operators and train equipment suppliers are very interested in the system we’ve developed,” said Morgenroth.