Proceedings

Abstract:

At the Käppala wastewater treatment plant (WWTP) in Stockholm, Sweden, a novel approach for dewatering of sewage sludge has been developed. As in many wastewater treatment plants the dewatering is a costly part of the treatment process. The sludge produced at the plant must be hauled to be used on farmlands and dewatering greatly reduces the volumes and transportation costs. By combining a potent chemical conditioning of the sludge, the so-called KemiCond® process, with a powerful mechanical dewatering, the HPS 5007 Bucher hydraulic filter press, conventional belt filter presses have been replaced and the total solids (TS) concentration increased from 19 % to 40 %. Moreover, the conditioning also functions as a potent pathogen control and reduces foul smells such as ammonia and mercaptans. Preliminary results have also shown that pharmaceutical residues in the sludge may reduced by the treatment. In this paper full scale results since November 2008 as well as laboratory tests are presented and the economical and environmental benefits are discussed.

See also Thunberg (2010) for a previous paper on the same issue.

Introduction:

Activated sludge has been widely applied for wastewater treatment, and especially municipal wastewater. In the course of using activated sludge a large amount of excess sludge is produced. The sludge contains metabolic products of bacteria including polysaccharides, protein and DNA all which are known to impair dewatering. At the Käppala WWTP the situation is no different and the handling and disposal of excess sludge incur a large expenditure in the wastewater treatment. As the load to the plant increases the need for high TS concentration of the sludge is becoming increasingly important. Chemical conditioning has been widely used to improve the dewatering (e.g. polyacrylamide, ferric chloride or calcium oxide) but TS concentrations above 20- 30 % are uncommon. Since November 2008 a novel approach for dewatering of digested municipal sludge has been in full operation at the Käppala WWTP, the KemiCond process in combination with the HPS 5007 Bucher hydraulic filter press. The method combines a potent chemical conditioning with at powerful mechanical dewatering resulting in a TS concentration of 40 % as a one year mean value and peaks above 50 %. With the previously used belt filter presses the TS concentration stayed at 19 % as a one year mean value. The amounts of sludge that needs to be transported have thereby decreased by more than 50 %. The chemical process has been in full operation since April 2006 treating all sludge produced at the plant. The hydraulic filter presses have been in operation since November 2008.

The Käppala WWTP treats wastewater from eleven municipalities in the Stockholm area, Sweden. The capacity of the plant is approximately 700 000 people equivalents (p e) and the current load is 550 000 p e. The treatment comprises of mechanical, biological, chemical and filtration steps. The biological step is an activated sludge process (ASP) with a pre-denitrification configuration. Effluent total nitrogen is kept below 9 mg/l as a one year mean value and effluent phosphorous below 0.2 mg/l by performing simultaneous precipitation with ferrous iron sulphate. Two mesophilic digesters treat the sludge anaerobically and produce approximately 320 000 m3 of digested sludge each year that needs to be dewatered.