In order to meet future standards of 8 mg/l total-nitrogen, 0.3 mg/l total-phosphorous and 10 mg/l BOD7, the Klagshamn wastewater treatment plant in Malmö has to be upgraded. A concept where the standards could be met within the existing activated sludge plant has been found.
Primary precipitation is followed by a single sludge post-denitrifying process where methanol is added as external carbon source. The system is high rate with only 7 hours retention time in the biological reactor. In order to improve the process stability and make a future increase in load possible within the existing plant, handling of digester supernatant has been examined.
The normal operation of the sludge handling system leads to discharge of a supernatant with a high content of ammonium. The load from the supernatant made about 15% of the total-nitrogen load on the plant on an average basis but could reach up to 65% during working hours. This extra load could be recognized as peaks of ammonium and nitrate in the effluent. The digester supernatant and its impact on daily variations in ammonium and nitrate have been examined in detail and the examination has formed the basis for a mathematical modelling of the system in order to test strategies for optimized handling.
Equalization of the supernatant flow, separate nitrification of the supernatant, and completely separate treatment of the supernatant have shown to be able to improve process stability. Flow equalization seems to be as effective as the two other more expensive and complicated solutions.