Bioreactors (MBR) combine conventional Suspended Growth biological treatment processes with membrane filtration system to provide an advanced level of organic and suspended solids removal. Organic contents (BOD) will be biologically degraded by Activated sludge, whereas the Suspended solids are removed by the Membrane.
In an MBR system, the membranes are submerged in an aerated biological reactor. Thus MBR tank performs both functions of aerobic treatment and solids / liquid separation. The membranes have porosities ranging from 0.035 microns to 0.4 microns, which is considered between micro and ultrafiltration. This level of filtration allows for high quality effluent to be drawn through the membranes and eliminates the filtration processes. The MLSS (Mixed Liquor Suspended Solid) concentration in the MBR System is 8 to 20 g/L compared to 3 to 4 g/L in conventional Activated Sludge Systems, thus the retention time required is only 30% of conventional system. This dramatically reduces the process tank volume.
The membrane modules comprises two sections – the top section containing membrane cartridges fixed into a stainless steel housing constructed with the lower section containing coarse bubble diffusers. The bubbles released by the lower diffuser section generate an upward sludge bubble flow over the membrane surface. This bubble flow is able to minimize fouling and allow low pressure suction filtration of the treated effluent into the inner compartment of membrane cartridge and thence to the collecting manifold.
The filtration takes place by means of suction pump which delivers the treated water in overhead back-pulse tank. The product water flows by gravity into Product water storage tank. After a fixed time of service cycle, the membranes are subjected to relaxation of 1 minute. Back pulse takes place after every 4 hours for a period of 15 minutes. In this step, product water from the overhead tank flows by gravity into the membrane module and dislodges the impurities from the membrane surface. Air scouring continues during filtration, rest and back pulse period. A cleaning tank is required separately for maintenance and recovery cleaning requirement.
Two MBR configurations exist: internal/submerged, where the membranes are immersed in and integral to the biological reactor; and external/sidestream, where membranes are a separate unit process requiring an intermediate pumping step