Decentralized greywater treatment and reuse for non-potable purposes: Toward sustainable water management with experiences from Groningen, The Netherlands

Conventional centralised sewerage systems are increasingly challenged by climate change, population growth and sustainability goals. Decentralised wastewater treatment systems (DWTS) offer an alternative by reducing hydraulic loads, enabling local reuse and supporting resource recovery. This study analyses long-term performance and lessons learned from decentralised wastewater treatment in the Groningen neighbourhoods of Drielanden and Reitdiep.
The study is based on two residential case studies: Drielanden (166 dwellings, ~400 residents) and Reitdiep (180 dwellings). In both areas, grey water is treated locally using constructed wetlands, while black water is collected separately. In Reitdiep, black water is transported via a vacuum sewer system and stored in buffer tanks for future treatment options. The systems combine vertical-flow and horizontal-flow constructed wetlands, with total treatment areas of approximately 6,300 m² in Drielanden and 6,500 m² in Reitdiep. Hydraulic retention times are approximately one day in vertical-flow filters and up to 30 days in horizontal-flow systems. Monitoring is conducted through regular sampling of influent and effluent water quality.
Results show that the systems operate reliably and comply with discharge requirements without additional treatment. The integration of willows in Reitdiep enables biomass production and evapotranspiration, reducing effluent discharge. Decentralised systems are embedded in climate-adaptive urban design, including surface-based stormwater management through elevation differences and infiltration zones.
The study concludes that decentralised wastewater treatment using constructed wetlands is technically feasible and effective at neighbourhood scale. Key benefits include reduced pressure on central infrastructure, opportunities for resource recovery, and contributions to climate adaptation. Successful implementation requires careful system design, ongoing monitoring and active stakeholder engagement to ensure long-term performance and acceptance.