Sludge characteristics of a submerged membrane bioreactor (MBR) and an activated sludge process (AS) were compared, during a first phase at the same operating conditions (low MLSS and conventional SRT) and in a second phase with a high sludge retention time (SRT) in the membrane bioreactor. During the first phase, a bimodal flocs size distribution was observed in the MBR with simultaneously a macro-flocs population (240μm) bigger than the flocs of activated sludge due to the absence of recirculating pump, and also more microflocs (1 to 15μm) and free suspended cells retained by the membrane. It is shown that the membrane leads to an accumulation of proteins and polysaccharides in the sludge supernatant which is probably responsible for the high fouling propensity of the sludge during the starting period of MBR. These compounds are partially degraded after 50 to 60 days of operation. In the first phase respirometric experiments didn't demonstrate a significant difference in the maximal removal rates of either MBR or AS biomass (with excess substrate), except in the dynamic period during which the membrane retention gave an advantage by increasing the biomass activity. On the other hand, the respirometry shows that the half saturation constant for nitrification was significantly higher in the MBR process, suggesting higher substrate transfer limitation. During the last phase, it is shown that an increase of SRT from 9 to 106 days leads to a diminution of average macro-flocs size in the MBR from about 240 to 70μm. With the SRT increase, modification in the organic compounds is also observed (proteins, polysaccharides and COD) in the sludge supernatant. Increasing the SRT from 9 to 40 days seems to slightly reduce the level of organic compounds (probable biodegradation), but the concentrations increased when SRT changes from 40 days to 106 days (probable accumulation of non biodegradable compounds).

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