Membrane separation is one potential technique to further unite industrial production and wastewater treatment. This is due to the low and constant level of solids in permeate thus facilitating specific water reuse. Membranes can be applied in primary, secondary and tertiary treatment. In this study microfiltration with ceramic membranes (0.1-1.7 mum pore size) was tried out for the activated sludge of a chemi-thermomechanical pulp (CTMP) mill. In the two different types of laboratory-scale pilots, flat-plate and cylindrical membranes were utilized. Transmembrane pressure, crossflow velocity, temperature and backflushing were variables in experiments of 3 to 23 h duration. Results were compared to the final effluent quality of the activated sludge clarifier of the mill. With optimum transmembrane pressure and crossflow velocity, a flux of more than 1401/(m(2)h) could be achieved for 23 h with plate membrane (0.1 mum). Elevated temperature and backflushing (1 s/120 s) improved the performance. Concentrations of permeate suspended solids and turbidity were dramatically lower than those of activated sludge clarifier effluent. Improvement in CODCr and in long-term experiments, also in BOD7, was considerable. Total phosphorus was the only parameter showing opposite behaviour. A plate membrane with a larger pore size without backflushing as well as cake filtration with cylindrical membranes resulted in dramatically reduced flux but better permeate quality. Ultrasound and nitric acid were found as effective cleaning methods in the cases studied.