Quantitatively understanding the performance of membrane bioreactors

The membrane bioreactor (MBR) is a special form of activated sludge in which a membrane separator allows perfect solids retention. This offers obvious benefits for effluent COD and attaining a large ratio of solids retention time to hydraulic retention time (SRT/HRT). However, these benefits come with trade-offs. This work explores the trade-offs with a mechanistic model based on the unified theory for the biomass and soluble components in microbiological processes and adapted for the special features of MBRs. In particular, only large biomass-associated products (BAPL) are retained by the membrane, while a high concentration of mixed liquor suspended solids (MLSS) lowers the oxygen-transfer rate and the critical trans-membrane flux. According to the model results, effluent COD is sensitive to the influent COD and to the ability of the membrane to retain BAP_L. While the ability of an MBR to achieve high MLSS and volumetric loading has cost benefits, high MLSS increases the required aeration power and decreases the trans-membrane flux. These strong trends point out the areas in which MBR research ought to yield a large benefit.