What are membrane bioreactors?

Membrane bioreactor (MBR) is generally a term used to define wastewater treatment processes where a perm-selective membrane, eg microfiltration or ultrafiltration, is integrated with a biological process − specifically a suspended growth bioreactor.
MBRs differ from ‘polishing’ processes where the membrane is employed as a discrete tertiary treatment step with no return of the active biomass to the biological process.
Almost all commercial MBR processes available today use the membrane as a filter, rejecting the solid materials which are developed by the biological process, resulting in a clarified and disinfected product effluent. A membrane bioreactor is essentially a version of the conventional activated sludge (CAS) system.
While the CAS process uses a secondary clarifier or settlement tank for solid/liquid separation, an MBR uses a membrane for this function. This provides a number of advantages relating to process control and product water quality.


During MBR wastewater treatment, solid–liquid separation is achieved by Microfiltration (MF) or Ultrafiltration (UF) membranes. A membrane is simply a two-dimensional material used to separate components of fluids usually on the basis of their relative size or electrical charge. The capability of a membrane to allow transport of only specific compounds is called semi-permeability (sometimes also permselective). This is a physical process, where separated components remain chemically unchanged. Components that pass through membrane pores are called permeate, while rejected ones form concentrate or retentate.

There are five types of membrane configuration which are currently in operation

  • Hollow fibre (HF)
  • Spiral-wound
  • Plate-and-frame (i.e. flat sheet (FS)
  • Pleated filter cartridge
  • Tubular