The performance and characteristics of a membrane bioreactor (MBR) and a conventional activated-sludge system (AS) fed wastewater containing casein and starch were compared. Except different solids retention times (20 days for AS, 30 days for MBR), the systems were operated under identical
conditions. Approximately 99.0% chemical oxygen demand and 96.9% dissolved organic carbon removal were achieved in the MBR compared to 94.5 and 92.7% in the AS. Both systems showed effective nitrification and phosphorus utilization. The MBR sludge was composed of small flocs; free swimming
bacteria; and a small number of filamentous organisms, nematodes, and ciliates. The AS sludge was composed of larger flocs and higher amounts of filamentous organism and nematodes. The biomass in the MBR had a higher viable fraction than the AS. Enzymatic activity tests showed that (1) overall
activity was consistently higher in the MBR, (2) both systems produced enzymes specific for the degradation of feed chemicals, (3) no enzymes were detected in the MBR effluent compared to significant amounts in the AS, and (4) the MBR contained more enzymes in the soluble phase than the AS.
Particle size distribution tests verified that the AS sludge contained large flocs, while the MBR sludge contained primarily small flocs. The specific resistance of the MBR sludge was three orders of magnitude higher than the AS. The AS sludge showed better settleability than the MBR.
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