Ultrafiltration is one of the techniques currently used for disinfecting water – viruses and bacteria are reliably retained by a membrane with extremely small pores. For more than ten years, Eawag has successfully been carrying out research to determine how this method can function using the effect of gravity on water instead of high pressure, cleaning and chemicals. These new discoveries are being applied in increasing numbers of ways. In addition to decentralised drinking water purification, Eawag is now researching uses in areas such as greywater recycling and pre-treatment of seawater for desalination.
Three schools in the vicinity of Lake Victoria in Uganda have, for the last few years, not only been learning centres, but have also been operating water kiosks where villagers can buy treated drinking water. Simple membrane filtration systems, developed at Eawag and installed together with the University of Applied Sciences and Arts Northwestern Switzerland, remove turbid material, bacteria and viruses from the water, thus ensuring that significantly fewer people are drinking polluted water. As a result, diarrhoea in children under five years of age has decreased by 69%, and in adults by an astounding 78%. Maintenance of the membrane filters is very easy; they function without permanent addition of chemicals and require no energy input – except for pumping the water from Lake Victoria to the station. “Whereas conventional ultrafiltration systems function with high pressure and have to be regularly cleaned out, our filters work by gravity alone”, explains Regula Meierhofer from Eawag’s Sandec department. “Cleaning is not necessary.”
Filtration using gravity
When Eawag began its research into membrane filtration over ten years ago, no one expected that it would work under these conditions. “As part of an EU project, we investigated how one could simplify membrane filtration for decentralised water purification in such a way that no cleaning was necessary”, recalls Wouter Pronk from the Process Engineering Department. “We assumed that we would have to treat the water before filtering so as not to clog the pores.” But then Eawag researcher Maryna Peter realised something surprising in her work for her dissertation: if the filters function using only the effect of gravity effect on the water, although the permeability of the membrane decreases during the initial five days, it remains stable at a low level thereafter – even after several months of use. River water, for instance, flows at a stable 4 to 5 l/m2 per hour. Thus, gravity-driven membrane (GDM) filtration was born.