Analysis of microfiltration performance with constant flux processing of secondary effluent

This study involves the microfiltration (MF) of secondary effluent from a sequencing batch reactor processing industrial waste. The MF unit was a hollow fibre module with gas backwash capability, and operated with pumped permeate (controlled flux) and dead-end, crossflow or intermittent feed. The results showed that crossflow had no effect on flux and that intermittent dead-end filtration was less productive than non-intermittent operation. For dead-end filtration the cycle-time between gas backwashes depends very significantly on the imposed flux (varying from about 100 min at 30 L/m2 h to about 5 min at 90 L/m2 h) and the feed solids content. Optimal operation has to balance operating (energy for backwash) costs and the capital (membrane area) costs. Cost analysis based on capital and energy costs indicates that for lower energy cost the unit needs to be operated at lower imposed flux but to minimise total cost it is necessary to operate the unit above 60 L/m2 h imposed flux depending on the maximum transmembrane pressure (TMP) allowed before back washing. Further analysis of TMP profiles showed that membrane resistance increased over time towards a maximum, which tended to increase with imposed flux. This implies more frequent chemical cleaning for high flux operation. Specific cake resistances were deduced from the profiles and indicated cake compression at higher flux and larger maximum TMP. Results of long-term trials are also reported. Water quality analysis shows consistent quality of permeate regardless of operating conditions.