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Purpose - The paper seeks to review some of the main operational problems of wastewater treatment plants designed for nutrient removal. It aims to pay particular attention to bulking and foaming due to massive growth of filamentous organisms, failure or non-optimal operation of secondary settling tanks, and compliance with more stringent effluent requirements. Design/methodology/approach - The discussion of the major operational problems is based on a thorough review of the available literature and own experiences with more than 80 wastewater treatment plants. Findings - To suppress the undesirable growth of filamentous organisms and to achieve an all-year round stable operation of secondary settling tanks several design options for improvement have been developed based on the experience gained so far. The most desirable option for improving the operation of a single treatment plant is to a large extent site specific. Beside adopting the design procedures also operational measures are possible. Practical implications - The paper recommends several practical options for optimising the performance of wastewater treatment plants with regard to the aforementioned problems. These recommendations are based on proved practical experience and, therefore, may act as a flexible toolbox for an individually tailored design or operation of the plant in order to avoid and/or combat the problems during operation. Originality/value - The described options for improvement the operation are based on long lasting practical experience with large scale wastewater treatment plants. They can be used by any operator of such plants for solving some of the major problems of modern wastewater treatment plants. [ABSTRACT FROM AUTHOR]

The Ruhrverband, acting as a water association responsible for integrated water resources management within the entire natural river basin of the Ruhr, operates a network of 83 wastewater treatment plants (WWTPs) and connected sludge disposal facilities. According to German regulations, the disposal of sewage sludge containing more than 5% of organic dry solids will be prohibited as of 1 June 2005. In Germany, the only future alternative to incineration will be the agricultural utilization of sludge. However, this way of sludge disposal is presently the subject of critical discussions in Germany because of the organic and inorganic toxic substances, which may be contained in sewage sludge, despite the fact that very stringent standards are to be met by agricultural uses. On the other hand, application of sewage sludge to agricultural land is explicitly supported by the European Sewage Sludge Directive 86/278/EEC. In the face of this controversial situation the Ruhrverband has initiated, in 2000, the development of a comprehensive and sustainable sludge and waste disposal concept for all wastewater facilities it operates in the entire Ruhr River Basin. The concept includes de-central sludge digestion and dewatering and subsequent transport to two central sludge incineration plants. It is expected that in future not more than 5% of all sludges produced in Ruhrverband's WWTPs will be used in agriculture. That means, the major part of 95% will have to be incinerated.