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Science Saturday Recycling system saves – and cleans – handwashing water

recycling plant for slightly contaminated wastewater

Recyling plant for slightly contaminated wastewater in the field test in summer 2018 in Zurich

(Eva Reynaert/Eawag)

Swiss scientists have developed a system that allows water used in handwashing to be recycled. This should save water and help boost sanitation in developing countries. Train toilets are another potential use.

A group of environmental engineers at the Swiss Federal Institute of Technology ETH Zurichexternal link led by professor Eberhard Morgenrothexternal link has come up with a grid-free (non-connected) treatment systemexternal link that allows relatively clean wastewater from showering, bathing or handwashing – also known as greywater – to be repeatedly recycled and sanitised.

There are already commercial systems available that enable greywater to be treated on-site for toilet flushing, but the resulting water does not meet quality standards for other uses, Morgenroth said in a statementexternal link.

But this is not true of the system developed over the past seven years by Morgenroth, who is also head of Process Engineering at Swiss Federal Institute of Aquatic Science and Technology (Eawagexternal link), and his team of collaborators.

After several treatment stages, the system produces odour-free and colourless greywater with “a bacterial count lower than that of Zurich tap water”.

How it works

A key component is ultrafiltration via a fine-pored plastic membrane, which retains pathogenic organisms. The microbial film that develops on the membrane breaks down the faecal and urinary contaminants in wastewater

“If we add nutrients – such as nitrogen and phosphorus – to the handwashing soap, the bacteria perform very effectively, with a removal rate of almost 100%,” Morgenroth explained.

After ultrafiltration, any traces of organic matter remaining in the wastewater are removed by an activated carbon filter. In a final stage, an electrolytic cell is used to produce chlorine from dissolved salt, to disinfect the water during storage.

The team described their results in a studyexternal link published in the journal Water Research.

Train toilets and tests

The system is primarily designed for use in developing countries, where sanitation is lacking, but the researchers are looking into other additional applications like passenger train toilets.

Morgenroth is convinced that water recycling will become more prevalent in the future – in Switzerland as well as in other countries – since droughts are likely to become more frequent in the future.

“Increasing numbers of regions will then no longer be able to afford the luxury of using clean drinking water for every application,” he commented.

A prototype system was field tested in a Zurich park this summer and was able to successfully support peak usage rates of over 100 people a day.

The next two-month field test will take place in an informal settlement in Durban (South Africa), from January 2019.

ETH Zurich/ilj

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