Before extension, the wastewater treatment plant at Fiskeby Board Paper Mill consisted of a selector activated sludge process followed by a final clarifier. For short periods, especially in the summer, the floc settling characteristics deteriorate dramatically, leading to excessive sludge losses from the final clarifier. This, in turn, results in SS concentrations up to 1 000 mg/l in the effluent.
In order to improve the removal of particles, the treatment plant was extended with a DAFRapide® as a final polishing step after the clarifier.During the observation period of 25 months beginning September 2002, typical loadings were 20 – 30 m/h. Most of the time, the SS content in the inlet water was low, 10 mg/l, which normally did not require any further treatment. However, during 8 percent of the observation period, the inlet SS content was > 100 mg/l and could sometimes reach values as high as 900 mg/l. These occasions with sludge carry-over from the sedimentation represented 80 % of the total sludge amount that was removed during the observation period. Sludge carry-over from the sedimentation process occurs very suddenly, requiring immediate action. Before the extension, the operators had to start the equipment for dosing of chemicals and increase the surface sludge removal capacity. Therefore, continuous monitoring of suspended solids in the incoming water was installed, controlling all necessary measures to be taken.