The purpose of this study was to compare the performance of a pilot-scale membrane bioreactor (MBR) in the treatment of municipal strength wastewater at solid retention times (SRTs) ranging from 30 days to two days. Cumulative nitrogen and phosphorus mass balances resulted in closures exceeding 90% at each steady state period. Biomass production rate and biomass viability generally increased with decreasing SRT, whereas overall enzymatic activity did not change significantly at most SRTs, but was highest at the two day SRT. Nitrification decreased at two day SRT but did not fail completely. At higher SRTs, nitrification was not noticeably affected by the sludge age. Phospholipid fatty acid (PLFA) analysis showed substantially diverse biomass in the sludge at different SRTs. Different ratios of gram positive bacteria, eukaryotic organisms, and yeast cells were observed in the mixed liquor at varying SRTs. On the other hand, BIOLOG analysis indicated that the overall capacity of the biomass to degrade different carbon substrates did not change significantly at different SRTs. The concentration of metals in the MBR mixed liquor declined steadily with decreasing SRT. The MBR effluent contained negligible amounts of Fe, Zn, Mn, and Co at each condition, indicating the retention of these metals regardless of the SRT.

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