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MBR processes can be configured as immersed or sidestream, and the membranes as flat sheet, hollow fiber or multi-tube. Membranes can also be employed as a 'polishing' step downstream of the biological process, rather than integrated with it as an MBR.
Of the key membrane-side parameters − flux, TMP, permeability and shear − the most important for MBR process design is the flux. The flux is partly determined by the membrane material, but mainly by operational factors.
Membrane technology used for removing solids in wastewater treatment is usually based on ultrafiltration or microfiltration. The membranes can be introduced either as a separate unit operation downstream of the biological step, or integrated into a biological process as an MBR.
A number of different polymeric and ceramic materials are used to form membranes.
Membranes generally comprise a thin surface layer which provides the required perm-selectivity on top of a more open, thicker porous support which provides mechanical stability.
The key MBR operating parameters on the membrane side comprise transmembrane pressure, flux and, for immersed membranes, specific aeration demand. The mode and frequency of cleaning is also important.
The MBR process configuration refers to way in which the membrane is associated with the biological treatment. There are two process configurations: immersed and sidestream.
The MBR membrane configuration refers to the geometry of the membrane (flat sheet, hollow fibre and multi-tube) and the direction of the flow ('inside to out' or vice versa).
Hybrid MBR processes are those which combine the conventional MBR technology with some other process. Examples include adding media to the biological process tank, or using a dense membrane in place of the usual ultrafiltration or microfiltration membrane.
Membrane modules used for polishing are the same type as those employed in potable and pure water filtration – usually hollow fibre and capillary tube configurations. These are immersed in a tank and operated under suction or stand-alone and 'pumped'.
Two of the emerging membrane technologies are forward osmosis − based on the principle of the naturally-occurring phenomenon of osmosis − and membrane distillation, which uses a micro-porous hydrophobic membrane as a semi-permeable barrier between two solutions at different temperatures.
