Biological treatment methods are required to remove organic contamination. The mostly used methods are Constructed wetlands, Sequence Bach Reactors, membrane bioreactors and biological aerated filter. grey water treatment with vertical Constructed wetlands with sizes of 2 m² per inhabitant in the Flintenbreite settlement, Luebeck has shown good performance (figure 32).

Constructed wetlands are cheap in construction and operation. Therefore, it has been widely used in Europe and transferred successfully to some developing countries. However, because of space scarcity, it is not always appropriate for densely populated urban areas. For these areas, sequence batch reactor (SBR), membrane bioreactors and biological aerated filter can be suitable. Treatment of grey water in small scale showed that SBR can greatly reduce organic matter, nutrients and turbidity SBR (Li et al., 2001). In other investigations Shin et al. (1998) have found the performance of SBR on treating grey water satisfactory. Biological aerated filter (BAF) followed by membranes are installed at the Millennium Dome in London for the treatment and reuse of hand basin grey water (Hills et al., 2001).

Figure32: grey water treatment in Flintenbreite-Luebeck

The biological process must be followed by a physical process in order to retain active biomass and to prevent the passage of solids into the effluent, if the effluent is for reuse purposes. Treatment with the membrane-bioreactors (MBR) will probably be the choice of the future, especially if reuse is intended. The MBR is the amalgamation of a suspended growth reactor and membrane filtration device into a single unit process. The process represents an intensification of traditional biological processes with the added advantage that the membrane retains particles including bacteria and viruses. Effluent from MBR treated grey water is typically solid-free, low in organic pollution and contains non-detectable levels of coliforms (Stephenson et al., 2000).