Let's look at the components of an immersed membrane bioreactor, or iMBR.
Clearly, you can’t have a membrane process without a membrane. The membrane sits in a tank on top of some coarse bubble diffusers, fed by the membrane blowers. Those diffusers deliver relatively large bubbles of air to the membrane, and the prime function of those bubbles is to scour the membrane and keep it clean as it filters the sludge. The sludge is delivered to the tank, and the permeate extracted via the membrane is clarified and largely disinfected. It’s not fit for drinking without further treatment, but it’s clean enough to be fed straight to the RO.
The membrane tank is fed from the biological (or process) tank where biological degradation of the incoming wastewater takes place. The tank is usually divided into two zones: an anoxic zone and an aerobic zone - and there are two different biochemical functions going on in those two zones. The aerobic zone is fitted with fine bubble diffusers, fed by the process air blowers. These diffusers deliver small bubbles of air, designed to provide oxygen to the bugs in the tank. All those micro-organisms need to survive is oxygen and the organic matter from the sewage: they’re pretty easy to please. It takes them a while to so this – the residence time in the process tank is normally 6-12 hours, so large tanks are needed to accommodate this long residence time. But microbiology (or biomass – the microbiologically-active part of the sludge) is very effective at degrading organic matter. The tanks are stirred to keep the biomass suspended.
The sludge from the membrane tank is most often returned to the anoxic zone, and a portion of it is discharged to maintain the sludge solids concentration at some set value – normally between 8 and 10 g/L. The sewage is fed to the process tank via some fine screens, which remove any large particles which may clog the membrane channels. And that’s basically the immersed MBR process.