Not all wastewater is created equal – it may not be drinkable, but some of what goes down the drain is perfectly fine for watering the garden or flushing toilets. Water used for hand-washing is relatively easy to treat for reuse, and now engineers at ETH Zurich have built and tested a standalone hand-washing station for use in public places and developing countries that can do just that.

The system is called a biologically activated membrane bioreactor (BAMBi), which runs water through a three-step filtration process before the next user washes their hands in it. The key ingredient is an ultrafiltration membrane that's designed to allow for bacteria to build up as a biofilm. That might sound counterintuitive, but the idea is that these bugs catch and break down dangerous contaminants in the water.

Through tests the team realized there was a problem with that first step though – the water wasn't nutritious enough for the bacteria to survive very long. To remedy that, the team added nutrients like nitrogen and phosphorus to the soap on hand (pun intended) at the station. That kept the biofilm strong and well-fed enough to remove almost 100 percent of contaminants.

After that stage, the water passes through an activated carbon filter to remove any remaining traces of organic matter. And finally, salt is dissolved in the stored water and zapped with an electrolytic cell to produce chlorine. The end result is clean, odorless, colorless water that, the team says, has less bacteria in it than Zurich's tap water.

To test BAMBi, a prototype of the self-contained system was set up in a communal green space in Zurich for 100 days. In that time the system reportedly serviced over 100 people on some days, and remained operational throughout the whole test.

Following that success, the next field test will be conducted in South Africa, beginning in January 2019. Since it can operate off-grid for long periods of time, BAMBi is primarily designed for use in developing countries, but the team says it could also find use in public bathrooms on passenger trains or the like, removing the need for staff to regularly top up the clean water supply.

If all goes to plan, this could be part of a whole suite of sustainable, self-contained amenities for developing countries. The BAMBi could join the NEWgenerator, a mini wastewater treatment plant that recovers greywater, energy and fertilizer from sewage, and a urinal that uses "pee power" to run the lights at night.

A paper describing the test run of the BAMBi system was published in the journal Water Research.

Source: ETH Zurich