SWEDISH FULL-SCALE EXPERIMENTS ON MODIFIED OPERATIONAL MODES IN REMOVAL OF NUTRIENTS

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.