2. Membrane bioreactor – Overview

In the conventional activated sludge process, the biomass/treated water separation stage is based on gravity settling downstream of the bioreactor. This separation stage has two essential roles: to minimize the suspended solids content in the treated water, and to enable concentrated sludge to be recycled back to the bioreactor, where it maintains a biomass concentration suited to the expected purification process.

The reliability of this stage is therefore decisive not only for the quality of the treated water, but also for the control of biological processes by preventing any "leaching" of the biological reactor (gradual decrease in biomass concentration and therefore activity in the reactor due to a growth rate of purifying species lower than the specific extraction flux of the bioreactor).