Membranes for polishing biologically-treated wastewater
Membrane modules for polishing – hollow fibre and capillary tube configurations
Membrane modules used for 'polishing' (i.e. filtering residual solids downstream of the biotreatment process) are the same type as those employed in potable and pure water filtration. The membranes themselves are cylindrical in geometry, i.e. filaments, normally between 0.5 and 2 mm in outside diameter. Flow through the membrane can either be out-to-in, or 'shell-side to lumen-side', as with a hollow fibre (HF), or in-to-out ('lumen-side to shell-side'). In-to-out flow through a filament is representative of the 'capillary tube' (CT) configuration, but CT modules are often referred to as 'hollow fibre' by the suppliers.
HF membrane modules used for polishing can be immersed in a tank and operated under suction, like those used in membrane bioreactors, but are more usually stand-alone – or 'pumped'. Unlike the MBR modules, immersed modules used for polishing are not necessarily air-scoured.
For polishing applications, the solids concentration onto the membrane is considerably lower than for an MBR immersed hollow fibre (iHF) membrane module: <50 mg/L compared with 8–12 g/L for the MBR. Consequently, the membranes can be much more tightly packed in the module (Fig. 1). Volumetric packing densities may exceed 2000 m2 membrane area per m3 internal module volume, compared with generally less than ten times this packing density for the iHF MBR modules. The lower solids loading also permits higher fluxes, requiring a smaller membrane area.
Although membranes are still employed for polishing in a number of large municipal wastewater reuse plants worldwide (Raffin et al, 2013), it is widely acknowledged that this is a more expensive option than integrating the membrane separation step into an MBR. This is because the conventional activated sludge (CAS) process upstream of the membrane polishing step includes a sedimentation tank which adds substantially to the investment cost.