Effect of filtration conditions and backflushing on ceramic membrane ultrafiltration of board industry wastewaters

A biologically treated combined wastewater from a board mill was ultrafilte red with ceramic membranes. The effect of membrane modification, cross-flow velocity, filtration pressure, and backflushing on ultrafiltration was stu died. 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 prope rties of the membranes. The cross-flow velocities were from 0.8 to 8 m/s an d the filtration pressures from 0.7 bar to 2.3 bar. The backflushing was ma de 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 for med 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 becam e clear. The unmodified alumina membrane had the highest permeability, 129 1/(m(2) h bar) and the magnesia-modified membrane had the lowest permeabili ty, 107 1/(m(2) h bar), after 2 h of filtration. The permeability of the ti tania-modified membrane was close to that of the unmodified membrane. The r esults showed that when the cross-flow velocity was increased the permeate flux increased. However. it seemed that there existed some optimum cross-fl ow velocity above which the increase in flux was not significant or the flu x began to decrease. Moreover. the increase in filtration pressure increase d the permeate flux. There exists an optimum pressure above which the incre ase of pressure does not increase the flux significantly. With a higher cro ss-flow velocity the optimum pressure was higher. The backflushing decrease d 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 redu ced 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, a nd the substances absorbing at a wavelength of 280 nm. (C) 2001 Elsevier Sc ience B.V. All rights reserved.