Background Wastewater treatment plants can be affected by problems relating to environmental efficiency and the cost/benefit ratio of their operation. Many existing plants do not meet quality requirements and have to be renovated or replaced by new ones. Large plants often cause high local recipient load and eutrophication. Smaller plants may suffer from poor efficiency, high operational costs and investment needs. If wastewater treatment is centralised, piping in large areas requires additional costs. In many cases, industrial and municipal wastewater treatment function separately. The removal of nitrogen from wastewater can be expensive: the use of synthetic carbon sources adds extra costs and bypass removal consists of just basic sieving, which results in an average or low quality of wastewater purification. The inefficient removal of phosphorus can result in eutrophication, especially in ecologically sensitive areas. Objectives The project aimed to find a cost-effective and environmentally sustainable method for the treatment of wastewater in mid- to large-size wastewater treatment plants by combining industrial and municipal wastewater. It planned to demonstrate that by reasonable investments and innovative treatment solutions old middle-sized wastewater treatment plants can be renovated to meet the tightest requirements of quality and even exceed those set by Directive 91/271/EEC on urban waste water treatment. Such improvements in quality allow savings in the investment of new (and often larger and more centralised) wastewater treatment plants and maintain optimal operational costs. Finally, employment would be generated on a local scale in the interests of industry and the local authorities. The project aimed to implement the treatment of biologically treated effluents and by-pass effluent in a single process unit to make better use of by-pass effluent for treatment purposes and decrease the phosphorus discharge by 50% to below 0.3mg/l. It also planned to use an advanced bio-film process and industrial carbon waste instead of synthetic carbon sources to minimise biological nitrogen removal to less than 6 hours, and save costs in the use and production utilities of synthetic carbon sources. A comprehensive process analysis and validation period was scheduled and a manual was planned for use on an EU-wide scale. Results The project achieved its goal of renovating the wastewater treatment process in spite of mechanical problems that affected basic equipment. The bio-film process to remove nutrients has been shown to work. However, the capacity of the process seems to greatly depend on the pH in the incoming water, which varies with temperature. To stabilise the pH of the water, more chemicals were needed than had been anticipated. The beneficiary has used the industrial wastewater to good advantage, and the carbon-rich water had the expected impact on nutrient removal. Sludge-rich wastewater (without any pre-treatment), however, was shown to create problems. As a result, difficulties were also encountered at the tertiary treatment and microfloation stages. A feasible and economically viable solution has yet to be found.