Class 12th Biology 2012 Set1 Delhi Board Paper Solution

Bacteria and fungi are two categories of naturally occurring microbes present in sewage. The bacteria along with the fungal mycelia form flocs. These flocs are utilised during the secondary treatment of sewage. The primary effluent after separation of the grit and debris is taken to the secondary treatment. Here, the effluent is passed to an aeration tank where it is constantly agitated and air is pumped into it. This leads to vigorous growth of bacteria and floc formation, the bacteria in these flocs consume of organic matter; thus decreasing the BOD of the sewage.