Background Investigations undertaken by UPT (project partner) and others have shown that faecal and intestinal viruses in domestic wastewater cannot be completely removed through local purification plants that work with state-of-the-art technologies. Consequently, running waters (streams and rivers) and smaller lakes often exceed the bacteriological threshold values regarding their use for leisure and agricultural purposes (bathing and irrigation of agricultural, golf and sports fields etc.). Objectives The project aimed to construct a demonstration plant for the decentralised treatment of municipal wastewater at the location Ihn-Leidingen (650 population equivalents = PE) in the Saarland to produce service water (in compliance with the regulations in section 12 of the EU Directive 91/271 for municipal wastewater) based on the new LAGOON-MEMB concept. It would consist of two stages: Aerated lagoon/settling pond with rain-water storage function and partial BOD5-degradation Downstream activated sludge plant with newly developed inexpensive submerged mechanical filter (MF)-hollow fibre polymer membranes for biomass retention Other planned actions included: the verification of operational reliability and economic efficiency of the demonstration plant based on a comprehensive 16-month performance check; and the authorisation (certification) by the respective local authorities of the water treated by the demonstration plant for re-use as service water. Results The project constructed a prototype of a two-stage plant (aerated pond and activated sludge treatment with membrane filtration) in accordance with the LAGOON-MEMB concept. This prototype of a municipal wastewater treatment plant has a size of 700 PE and is located in the district of Ihn in the municipality of Wallerfangen in the Saarland. The plant combined the advantages of a pond treatment used in mixed wastewater treatment with those of a technical wastewater treatment plant. It can be used to demonstrate the operational reliability and economic efficiency of the above concept for the treatment of municipal sewage in rural areas to produce service water in long-term operation. The newly developed technological concept, LAGOON-MEMB, was demonstrated for the first time in real scale and tested under practical conditions. Operation results of the prototype proved the concept as an efficient solution for wastewater treatment for service water standard. All. objectives have been achieved. The test phase gave a treatment cost result of 28.5 ct/m³, including costs for electricity, recycling of residue material, chemical use and personnel. The water purification performance of the LAGOON-MEMB pilot plant was very good regarding both chemical and bacteriological parameter and far below the legal limit values. Neighbouring ground- and surface waters are exposed to 5-10fold less CSB, Nitrate and Phosphate compared to conventional treatment plants in that size. The treated water complies with the requirement of the bathing water directive (2006/7/EC) and with Article 12 of the EU Directive 91/271 (Urban Waste Water Treatment Directive) for municipal sewage, and was therefore also certified by the local authorities. Plants of the LAGOON-MEMB type are not only suitable for producing re-usable service water from municipal sewage, but they are also a viable solution for those types of applications in which requirements for wastewater treatment are more stringent, e.g. sensitive catchment areas (drinking water catchment areas of water conservation areas), discharge into rivers with poor degradability and rural areas without suitable rivers or streams for discharge. As a result, the technological concept, LAGOON-MEMB, can also be transferred to other areas of application where an efficient wastewater disposal concept is required. Further information on the project can be found in the project's layman report and After-LIFE Communication Plan (see "Read more" section).