Developments in the European Pulp and Paper Industry

Sludge from wastewater treatment in P&P industry contains large quantities of bound water, which is difficult to be dewatered. Combustion is a common disposal method for sludge and thus the mechanical dewatering is in key role in the treatment chain. One method to influence the dewaterability of sludge is the disintegration of sludge just before dewatering. In this study the modification of WAS from a pulp and paper mill using ultrasonic disintegration has been examined. Ultrasound changed WAS properties depending on the used energy input. Particle size and floc strength decreased while CST and cationic demand increased by ultrasonic treatment. Thus dewatering properties were in many respects deteriorated. However, ultrasound released bound water from living cells into aqueous phase of WAS. Solid part of fresh sludge could be decreased more than 20 % and DS increased 2 %-units at centrifugation when using one minute ultrasonic disintegration at the power intensity of 200 W/l. Bound water could be released especially from fragile, voluminous cells while the changes of sludge properties were still minor. Released water could be exploited in dewatering processes, such as settling, batch centrifugation, continuous belt filter pressing and chamber filter pressing. There is a good potential to improve dewatering of P&P WAS with ultrasound. However, both technically and economically it is better to treat the part of the sludge using ultrasound which will easily be disintegrated and has positive effect on dewatering.