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 BAPL. 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.

Original languageEnglish (US)
Pages (from-to)1003-1013
Number of pages11
JournalSeparation Science and Technology
Issue number7
StatePublished - Apr 2010


  • Aeration power
  • Membrane bioreactor
  • Microbial products
  • Trans-membrane flux
  • Water quality

ASJC Scopus subject areas

  • General Chemistry
  • General Chemical Engineering
  • Process Chemistry and Technology
  • Filtration and Separation


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