Membrane Bioreactor (MBR) - Process Basics

Illustrated on the bottom is a simplified process diagram. The MBR system is comprised of four major treatment steps:
  • Screening
  • Anoxic Tank
  • Pre-Aeration Tank
  • Membrane Tank
The raw sewage (Q) passes through a fine screen, <3 mm (1/8-inch), into the anoxic tank via gravity. In the anoxic tank, the raw sewage is mixed with the recycled activated sludge from the membrane tank (QM). The activated sludge flows by gravity from the membrane tank to an area of the anoxic tank separated by a baffle wall. This baffle minimizes the oxygen carryover from the membrane tank and increases the denitrification rate. Therefore, recycled activated sludge first drops to the bottom of the anoxic tank before it is mixed.

The mixed liquor (QD1) is pumped from the anoxic to the pre-aeration tank at a recycle rate of 4 – 5 Q. Fine-bubble diffusers are installed at the bottom of the pre-aeration tank. From the pre-aeration tank, mixed liquor (QN) flows by gravity into the membrane tank. The treated effluent or permeate (QP) is forced through the membranes by permeate pumps. The permeate will leave the plant at a rate of approximately Q. Periodically, sludge (QWS) will be wasted directly from the membrane tank into a sludge tank or sewer trucks for smaller plants.

Influent and effluent criteria, climate, and other parameters may change the treatment process illustrated below. For example, additional anaerobic tanks (for biological phosphorus elimination) or anoxic tanks (to further reduce nitrates) may be required.




Simplified MBR process diagram
blocks_image