A biologically treated combined wastewater from a board mill was ultrafiltered with ceramic membranes. The effect of membrane modification, cross-flow velocity, filtration pressure, and backflushing on ultrafiltration was studied. The ceramic membranes were made of alpha -alumina and their pore size was 100 nm. Some of the membranes were modified with titania and some with magnesia. The modifications were made in order to change the surface properties of the membranes. The cross-flow velocities were from 0.8 to 8 m/s and the filtration pressures from 0.7 bar to 2.3 bar. The backflushing was made at 4 bar with a mixture of air and permeate for I s every I min. At low cross-flow velocities permeate flux was controlled by the fouling layer formed on the top of the membrane and the permeate fluxes of all the membrane modifications were similar. When the cross-flow velocity was increased the fouling layer decreased and the differences in membrane modifications became clear. The unmodified alumina membrane had the highest permeability, 129 1/(m(2) h bar) and the magnesia-modified membrane had the lowest permeability, 107 1/(m(2) h bar), after 2 h of filtration. The permeability of the titania-modified membrane was close to that of the unmodified membrane. The results showed that when the cross-flow velocity was increased the permeate flux increased. However. it seemed that there existed some optimum cross-flow velocity above which the increase in flux was not significant or the flux began to decrease. Moreover. the increase in filtration pressure increased the permeate flux. There exists an optimum pressure above which the increase of pressure does not increase the flux significantly. With a higher cross-flow velocity the optimum pressure was higher. The backflushing decreased the formation of fouling during filtration. From the biologically treated wastewater, suspended solids and turbidity could be removed almost totally. Moreover. iron, total phosphorous, and biological oxygen demand were reduced with over 60% and in some cases also manganese had retentions over 90%. The increase in permeate flux decreased slightly the retentions of color. iron, chemical and biological oxygen demand, total phosphorous, nitrogen, and the substances absorbing at a wavelength of 280 nm.