After primary treatment the settled sewage water enters a specially designed reactor where it is mixed to a dense microbial population under aerobic conditions. Micro organisms , mainly bacteria convert soluble and colloidal organic matter into new cells . Now all that is required is to separate the dense microbial biomass from the purified wastewater . This is done by clarifier ( tube settler , lamella clarifier , upflow filter ) . The sludge from the clarifier is very different from the primary sludge , being composed mainly of biological cells rather than gross faecal solids . There are many different biological technologies available for sewage treatment including fixed film reactors , activated sludge systems , stabilization ponds eg . SAFF , FAB STP , Moving Bed Bioreactor MBBR based Sewage Treatment Plant , MBR , SBR Membrane Biological Reactor Sewage Treatment Plant