The correct sequence starts with the physical removal of floating debris , followed by the sedimentation of grit . Next, the primary effluent is aerated to promote the growth of aerobic microbes . After the reduction in BOD, the bacterial 'flocs' sediment in a settling tank (E). A part of the activated sludge is pumped back into the aeration tanks as inoculum while the majority is sent to anaerobic digesters , where gases like methane are produced (F). Finally, the treated effluent is released into natural water bodies .

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