MEMBRANE BIOREACTOR PERFORMANCE AT SHORT MEAN CELL RESIDENCE TIMES

This study discusses the performance of a membrane bioreactor (MBR) and a completely mixed activated sludge (CMAS) system operated at short MCRT ranging from 0.25 to 5 d and hydraulic retention times of 3 and 6 h treating synthetic wastewater. Even at the shortest MCRT of 0.25 d, the MBR was able to produce excellent quality effluent free of suspended solids and total chemical oxygen demand (TCOD) of 10.7 ± 4.4 mg/L. The MBR removal efficiencies ranged from 97.0 – 98.4% (TCOD), significantly better than CMAS (77.5 – 93.5% TCOD removal and 94.1 – 97.0% SCOD removal). High concentration of dispersed microorganisms was observed in the MBR possibly contributing to better organics removal due to lower mass transfer resistance. The concentration of dispersed microorganisms in the MBR increased at shorter MCRT. The combination of decreasing exocellular polymeric substances (EPS) and increasing amount of dispersed microorganisms at shorter MCRT in both reactors contributed to deteriorating biomass settling properties. Nitrification completely ceased when MCRT was less than 2.5 d. The results showed that when nitrification is not a concern, it is feasible to operate an MBR at extremely short solids retention time producing excellent effluent qualities in terms of organics removal and free of suspended solids.