After the primary treatment, the primary effluent is taken for secondary treatment where it is passed into large aeration tanks and is constantly agitated and air is pumped into it. This leads to the vigorous growth of useful aerobic microbes into flocs. Flocs are masses of bacteria associated with fungal filaments to form mesh-like structures. These microbes in the flocs consume the major part of the organic matter in the effluent and reduce the BOD (biological oxygen demand) of the effluent. Less BOD means that the wastewater is less polluted. After this, the effluent is passed into a settling tank where the flocs are allowed to settle and this sediment is called activated sludge. A small part of the activated sludge is pumped back to serve as the inoculum. Inoculum is the small amount of a material containing microbes that can start a new culture. This way same microbial culture is recycled and reused to start a new culture for the treatment.
