After the primary sewage treatment, primary effluent is taken for secondary or biological treatment into aeration tanks. A large number of aerobic heterotrophic microbes grow in the aeration tank. They form flocs which are masses of bacteria held together by slime and fungal filaments to form mesh-like structures. The microbes in flocs digest a lot of organic matter, converting it into microbial biomass and releasing lot of minerals. This decrease the Biochemical Oxygen Demand of effluent. Now, the effluent is passed into settling tank where the bacterial flocs are allowed to sediment. The sediment of settling tank is called activated sludge. A part of it is further used as inoculum in aeration tank to start a new batch of secondary sewage treatment and the remaining is passed into a large tank called anaerobic sludge digesters. Here, the aerobic microbes get killed but the anaerobic microbes produce marsh gas (biogas) which is used as a source of energy.