Effect of Mixed Liquor Suspended Solids on the Oxygen Transfer Rate in Full Scale Membrane Bioreactors

One of the key parameters for membrane bioreactor (MBR) design and operation is the air supply and the related energy consumption. The objective of this work was to investigate how the alpha factor varies with MLSS for both fine and coarse aeration systems in full scale MBR plants. Oxygen transfer rates were determined in two MBR plants with large MLSS variations using (i) an indirect method using mass balances to estimate the daily average OTR on a long term period; (ii) direct in situ oxygen transfer tests with 3 different methods to measure the instantaneous OTR. The various methods were used to verify and/or supplement each other. The results from both plants agree and suggest an alpha value for fine bubble between 0.32 and 0.41 at 10 g MLSS/L. It appears that MLSS impacts alpha of coarse and fine bubble in a similar way, even though the coarse bubble alpha values are higher than for fine bubble (e.g. 0.57 at 10 g/L).