Anaerobic treatment of kraft pulp-mill waste activated-sludge: Sludge dewaterability and filtrate quality

Kraft pulp-mill waste activated-sludge (WAS) was anaerobically digested at pilot-scale under various organic loading conditions to assess the effects of digestion on subsequent sludge solid/liquid separation in a belt filter press and the quality of the resulting filtrate. Anaerobic digestion resulted in deteriorated dewaterability of WAS, as indicated by lower cake solids, lower solids capture, increased chemical usage and unstable or lower capacity of the belt filter press with digested sludge (DS) than with WAS. The dewaterability of WAS and DS improved in mixtures with primary sludge (PS). DS dewatering performance improved with increasing feed solids concentration. Dewatering of DS and WAS produced filtrates of similar quality in terms of total solids. The concentrations of soluble organic matter (COD S, BOD 7, VA) in filtrates of DS were lower than of WAS. In the mill where experiments were conducted the DS plus PS dewatering filtrate would contain 13–47% of the soluble nitrogen and up to 10% of the soluble phosphorus of the composite wastewater, indicating that DS filtrate is a potential source of nutrients for the treatment of phosphorus- and nitrogen-deficient kraft pulp-mill wastewater.