Stormwater harvesting for non-potable-water purpose: The first permanent membrane-treatment installation in a multifamily building in Sweden

Stormwater management is essential for preventing urban flooding and protecting water quality in receiving bodies. Traditionally viewed as a nuisance, stormwater is now being re-evaluated as an alternative resource for non-potable uses such as toilet flushing and irrigation through rainwater harvesting systems (RWHS). Within the European Union's Horizon 2020 project REWAISE, pilot studies with ceramic membranes have confirmed stormwater's potential as a source that meets both European reclaimed water quality standards and US EPA requirements for heavy metals (Hey et al. 2023). Building on these findings, a permanent stormwater treatment system employing membrane technology was installed in the basement of 'Röda Oasen', a multifamily building in Malmö, Sweden. This system supplies 22 toilets and 4 washing machines with treated stormwater, potentially saving approximately 475,000 liters of drinking water annually. To ensure year-round availability of treated stormwater, from a Swedish perspective an unique collaboration with the local water utility, VA SYD, was established. The stormwater catch basin was innovatively designed to prioritize storage over traditional functions such as hydraulic delay, including the installation of a pump to supply stormwater to the treatment facility. The treatment system consists of two 500-liter tanks for stormwater and building drainage, and two parallel membrane tanks equipped with submerged flat-sheet ceramic membranes, each with a nominal pore size of 0.2 μm and a total surface area of 6.9 m². The system began operating in early February 2024 and already has supplied more than 100,000 liters of water by the end of April 2024, equivalent to an average daily replacement of 1,100 liters of drinking water. As this installation is unprecedented in Sweden, local authorities mandated water quality monitoring of at least 55 different quality parameters including: nutrients (phosphate phosphorus, ammonium nitrogen, nitrite+nitrate nitrogen, total nitrogen), organic compounds, e.g TOC, SS, COD, heavy metals, e.g. As, Cd, Cr, Fe, Hg, Ni, Pb, Cu, Zn), PAHs (benzo(a)anthracene, chrysene, benzo(b,k)fluoranthene, benzo(a)pyrene, indeno(1,2,3-cd)pyrene, dibenz(a,h)anthracene, total carcinogenic pahs, naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benzo(g,h,i)perylene), microbiological parameters, e.g. clostridium perfringens, coliform bacteria, E. coli), and others such as alkalinity and conductivity. The performance of the system suggests that the treated stormwater is not only suitable for toilet flushing and laundry but also for irrigation, potentially reducing the demand for drinking water even further. Given the promising results of this initial installation, plans for additional implementations are currently in development.FundingThis project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 869496.ReferencesHey T., Takman M., Lipnizki F., Norlander H. , Aspegren H., Jephson T., Membrane filtration of collected stormwater for water harvesting, Presented at the Nordic IWA conference 2023, Göteborg, Sweden, 5-7 September 2023.