After significant reduction of BOD during secondary treatment of sewage, what further processes are carried out ? Mention any one important outcome of these processes. $\displaystyle \mathbf{1} \frac{\mathbf{1}}{\mathbf{2}}+\frac{\mathbf{1}}{\mathbf{2}}$

$\displaystyle \star$ The effluent from aeration tank is passed into settling tanks where flocs settle down forming activated sludge $\displaystyle \star$ A small part of activated sludge is pumped back into the aeration tank to serve as inoculum and major remaining part is pumped into large tanks called anaerobic sludge digesters $\displaystyle \star$ Anaerobic bacteria digest the sludge and produce a mixture of gases as methane hydrogen sulphide and carbon dioxide or biogas$\displaystyle \star$ Biogas produced can be used as a source of energy / Effluent from secondary treatment plant is generally released into natural water bodies like rivers and streams.