Separation of CTMP mill-activated sludge with ceramic membranes

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 µm 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 140 l/(m²h) could be achieved for 23 h with plate membrane (0.1 µm). 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.

Activated sludge of a chemi-thermo-mechanical pulp mill could successfully be treated with crossflow ceramic microfiltration. Optimum operational conditions for the crossflow filtration with the PREC pilot utilizing plate membranes (0.1 µm) proved to be a transmembrane pressure of 1 bar and a crossflow velocity of 6 m/s, which was the highest velocity tested. In cake filtration with the bigger pore size, cylindrical membranes in the Certus-filter, a similar pressure range seemed to be appropriate. Backflushing in the PREC pilot at a pressure of 2 bar for 1 s every 120 s increased the flux and reduced the pure water flux reduction from 14.6% to 8.1%. By increasing the experimental temperature, the permeate flux increased as expected according to changes in viscosity and density of water. At optimum conditions, it was possible to achieve a stable flux higher than 140 l/(m²h) for 23 h. Discharges of suspended solids, turbidity, BOD7 and CODCr could be considerably reduced — more than 98, 80, 45 and 35%, respectively, compared to those of the final clarifier in the CTMP mill-activated sludge plant. Only concentrations of total phosphorous were higher in permeates compared to those of the in the activated sludge clarifier. Industrial alumina membrane of 1.7 µm pores without backflushing could achieve stable flux of less than one-fourth of that with the 0.1 µm membrane. Permeate quality of the membranes did not differ much. Cake filtration (Certus-filter) could reach flux of 25.4 l/(m²/h) in 3 h of filtration. The permeate flux was fairly independent of transmembrane pressure (0.75–2 bar) and membrane pore size (0.25 or 1 µm). The permeate quality was better than that with the PREC-unit. Ultrasound was a very efficient way to clean the membranes in the Certus-filter after the equipment was rinsed properly. The chemical washing of membranes for 20 min with HNO3 at 0.05 mol/l was able to restore the pure water flux to the level it was before the filtration in both tested units. If constantly high-quality effluent from activated sludge process is needed, direct separation of normal activated sludge is possible with the type of crossflow ceramic membrane system used in this study.