Stormwater harvesting with membrane technology: From an idea to apartment building installation

Introduction. The EU-Horizon 2020 project REWAISE - REsilient WAter Innovation for Smart water Economy – aims to reduce drinking water consumption by 30%. In case of Sweden, the average drinking water consumption is currently 140 litres/day and person [1] and thus should be reduced to approx. 100 litres per day and person. Approximately 45 litres/day and person are currently used for toilets and washing machines, uses for which drinking water quality is not essential. The idea is to replace drinking water with stormwater upgraded to a quality matching the needs of the purpose with membrane technology. Initial trails.The membrane concept for stormwater harvesting is based on submerged ceramic membranes in a membrane unit. The membranes initially selected were silicon carbide membranes (Liqtech, Denmark) with a nominal pore size of 0.2 micron and molecular weight cut-off (MWCO) of approximately 400 kDalton. Thus, microplastics and micropollutants with a size larger than 0.2 micron should be removed from the stormwater. The membrane unit can be operated with aeration to reduce membrane fouling and the membranes can be cleaned using backflushing as well as chemical cleaning. A containerised unit with approximately 5 m2 of total membrane area was installed at a local stormwater pond in the City of Lund (Sweden). The results showed that it was possible to efficiently reduce micropollutants and microplastics plus even to a certain degree heavy metals. The result showed that the polished water was able to be classified close to class ‘A’ according to the guideline EU 2020/741 allowing its use on agricultural products for human consumption. First full-scale installation.Based on the initial trails, a full-scale demonstration unit was designed and installed in the Röda Oasen apartment building in Malmö (Sweden). The unit has membrane area of 13.8 m2 of ceramic silicon carbide membranes (Cembrane, Denmark) with a nominal pore size of 0.1 micron and molecular weight cut-off (MWCO) of approximately 200 kDalton. The unit has been in operation since the beginning of February 2024 and produces ca. 1 300 l of water for 14 toilets and 4 washing machines in the building using stormwater from the roof of the house and an underground storage tank. The installation has been approved by the local authority including a control plan for the water quality. Based on the results so far, the property owner saves approximately 40% of his drinking water consumption corresponding to an annual drinking water saving of 475 000 litres.Outlook and conclusions. The membrane-based concept for stormwater harvesting is currently operating without any major problem and delivers the desired water quality but optimisation work regarding the cleaning sequences and monitoring of the long-term performance is still on-going. Furthermore, one additional system using roof water only has currently started at the Chemical Centre of Lund University (Sweden). AcknowledgmentsThe research is partly funded by the REWAISE ‘Resilient Water Innovation for Smart Economy’ project under the European Horizon 2020 programme, project No. 869496. References[1] Svenskt Vatten (2017). Värt att veta om vatten. https://www.svensktvatten.se/globalassets/fakta-om-vatten/dricksvattenfakta/vart-att-veta-om-vatten_2017.pdf