During the first part of the 1970s, large amounts of money were invested in Sweden in new plants for biological-chemical treatment and in complementary treatment steps at existing plants for municipal wastewater treatment. The post-precipitation process constituted the normal process combination, with the activated sludge process as the most common biological treatment process. Practical experiences over the last ten years have shown that post-precipitation plants can be operated in several operational modes, such as two-stage precipitation with different precipitation agents, different methods for recirculation of sludges, nitrification and denitrification, and biological phosphorus reduction. This development is due to more stringent effluent requirements and a desire to achieve lower costs and lower energy and chemical consumptions.

During the rapid development of advanced sewage works in Sweden in the 1970's interest was primarily focused on the construction of treatment works. Practical experience from ten years of operation have shown that it may be advantageous to operate the plants in ways other than those they originally were planned for, for instance using two-stage precipitation or different techniques for recirculation of chemical and biological sludges. In order to facilitate the introduction of more efficient operational modes a joint project was started in 1980 between the Swedish Water and Waste Water Works Association (VAV) and the National Swedish Environment Protection Board (SNV). The purpose of the project, Sewage Works Evaluation Project (SWEP), is to evaluate and optimize biological-chemical treatment of municipal wastewater. Results from this project will be reported in this paper with special emphasis on the sewage works at Boras, Eskilstuna, Hässleholm, Landskrona, and Link ö ping. Within the SWEP, a lot of information is collected including physical and chemical data (analytical values from the laboratory), operational data (measurement values from the sewage works) and plant data. Methods of handling and evaluating these data will also be discussed in this paper.