Background Discharges from combined sewer overflows in many situations constitute a problem in relation to reaching "good status" in the quality of the surface waters of member countries, which is one of the requirements of the Water Framework Directive. Recent surveys in the County of Aarhus, Denmark, have demonstrated that 17 % of the overflows lead to unacceptable conditions in the receiving waters. Appropriate and cost-efficient technical methods for local treatment of the discharges were not available prior to the launch of the project, and standards for issuing permits for these discharges were also lacking. Objectives The objective of the LOTWATER project was to demonstrate a full-scale treatment technique for combined sewer overflows using a lamella separator. The aim was to provide a cost-efficient tool for achieving the objectives of improving the quality of surface waters, as laid down in the Water Framework Directive. More specifically, the project hoped to quantify and verify the efficiency of the concept for local treatment of combined sewage at three facilities, representing outlets to small watercourses, discharges to larger surface waters and discharges to marine coastal waters, including verifying the positive impact on the receiving waters. The project was to test the use of a known technology, the lamella separation treatment, under new conditions. Results The project was able to show that the treatment of overflow sewage waters could be achieved using several technical interventions depending on the expected overflow volumes, catchments and recipient water bodies. Additionally, the project was able to quantify treatment efficiencies regarding the most common polluting factors such as chemical oxygen demand (COD), suspended solids and Escherichia coli (e.coli). Three facilities were designed and constructed as anticipated. A detailed measurement programme was also designed and implemented, enabling assessment of the treatment efficiencies of the facilities themselves and the corresponding effect on the surface water receiving the discharged water. This part of the project was supervised by an independent auditor who was an internationally recognised expert in the field. The project established the knowledge needed for the implementation of the Water Framework Directive, including identification of cost-efficient means to achieve sufficient environmental quality in surface waters. The calculations of cost-efficiency indicate that two of the three demonstration facilities are cost-effective compared to traditional methods and led to improved water quality at the surface waters in question. The last of the demonstration facilities had a clear improvement of the water quality of the surface water in question. However, the measurements showed that some of the treatment steps are not needed in order to achieve the sufficient environmental standard anticipated to be sufficient in order to comply with the Water Framework Directive. As such, this demonstration facility was found to be not cost-effective, as a more simple method would have been sufficient. The cost-efficiency calculations show that many of the simple storage devices for combined sewer overflows used throughout Europe can be improved. Some of the technologies tested in the project are still not cost-effective for standard use. However, when space is scarce or the discharges occur near bathing waters, the very advanced technologies tested in the project can be recommended for large catchments. The general improvement of European water quality, e.g., through the implementation of national legislation and the water Directives, has mainly been brought about via treatment of continuous discharges of wastewater. However, storm water outflows from the combined sewer systems continue to have an adverse effect on fresh water and marine waters. In order to meet the objective of the Water Framework Directive that at least ‘good ecological status’ should be obtained in the majority of the community waters, discharges of combined sewer outflows must be further minimised. The only recommended solution to reduce discharges is the construction of large retention basins for storm waters. This solution unfortunately is quite expensive and deteriorates the performance of biological wastewater treatment plants. The solution provided by the project is more sustainable. In Denmark alone, the needed annual investment over the next 15 years, in order to minimise the effects of combined sewers, is estimated to be approximately €100 million. The method employed in the project would be on average 20% cheaper than traditional treatment methods. Furthermore, as the method is applicable in several other Member States as well, potential savings at Community level are substantial.