Monitoring the variations of OTR in a full scale membrane bioreactor using daily mass balances

Oxygen transfer in biological systems with high sludge concentration such as MBR is an important issue. The variation of α-factor vs. MLSS concentration was investigated in a full scale MBR plant under process conditions, using mass balances. Exhaustive data from SCADA and additional sensors (COD, DO, MLSS) were used to calculate the daily oxygen consumption (OC) using a non-steady state mass balance for COD and total N on a 24-h basis. To close the oxygen balance, OC has to match the total oxygen transfer rate (OTR(tot)) of the system, which is provided by fine bubble diffusers (FB) in the aeration tank and coarse bubbles (CB) in separate membrane tanks. Assessing first OTR(CB), then closing the balance OC-OTR(tot) when MLSS was stable allowed to calculate OTR(FB) and fit an exponential relationship between OTR(FB) and MLSS in the aeration tank. A comparison of an average value of the α-factor obtained by this balance method and by measurements with the offgas method on the same plant is presented and discussed.