Performance of a membrane bioreactor and a completely mixed activated sludge system at short solids retention times

This study investigated the performance and biomass characteristics of a membrane bioreactor (MBR) and a completely mixed activated sludge (CMAS) system fed with synthetic wastewater and operated at short solids retention times (SRT) ranging from 0.25 to 5 d and hydraulic retention times of 3 and 6 h. For each experimental run, the reactors were inoculated with thawed activated sludge biomass previously frozen at −80°C with 15% (v/v) glycerol. Results showed that at SRT of 0.25 d, the MBR effluent total chemical oxygen demand (TCOD) was 10.7 ± 4.4 mg/L, compared to TCOD of 90.0 ± 9.2 mg/L and soluble COD (SCOD) of 23.7 ± 2.9 mg/L for the CMAS. Removal efficiencies were approximately 97.0–98.4% (TCOD) in the MBR, compared to 77.5–93.8% (TCOD) and 94.1–97.0% (SCOD) in the CMAS. Nitrification ceased when SRT was <2.5 days. The MBR biomass was composed of small, weak and uniform-sized flocs with large mass/number of short filamentous organisms and mainly dispersed microorganisms at SRT of 5 and 0.25 d, respectively. In contrast, the CMAS sludge was composed of large flocs with filamentous organisms as a backbone at SRT >2.5 d. The CMAS flocs were smaller and weaker at shorter SRT. Higher life forms were not observed in either system under any operating condition. The MBR sludge contained a much higher fraction of non-flocculating microorganisms. This fraction increased significantly with decreasing SRT. Biomass in the MBR had higher metabolic activity as determined by measurements of dehydrogenase activity and oxygen uptake rate. Higher activity and a significant presence of dispersed biomass and small flocs in MBR contributed to better reactor performance. However, increasing mass/number of non-flocculating microorganisms in both bioreactors contributed to poorer sludge settling. The fractionation of effluent suggested that more organic compounds with apparent molecular weight less than 500 daltons were produced at shorter SRT. The fraction of dissolved organic carbon in MBR effluent with molecular weights larger than 30,000 daltons decreased significantly with increasing SRT. BiOLOG™ analysis suggested that operating conditions and biomass separation methods affect community structures and rates of substrate utilization. The specific resistance to filtration is affected by a combination of the mixed liquor suspended solids concentration, the amount of non-flocculating microorganisms and the exocellular polymeric substances concentration. For filtration of sludges with high amount of non-flocculating microorganisms, a smaller pore size (0.8 μm) filter media and a pressurized filtration device are recommended.