Secondary treatment in sewage treatment is

Explanation

Municipal waste-water is also called sewage. It contains large amount of organic matter and microbes, many of which are pathogenic. This sewage cannot be discharged into natural water bodies before disposal, sewage is treated in sewage treatment plants to make it less polluting treatment of waste water is done by heterotrophic microbes naturally present in the sewage. this treatment is carried out in two stages.
(1). Primary treatment : It involves physical removal of particles from the sewage through filteration and sedimentationi. floating debris is removed by seanential filteration. then the grit (soil & small pebbles) is removed by sedimentations.
All solids that settle form the primary sludge and the supernatant forms the effluent the effluent is taken for the secondary treatment.
(2). Secondary treatment or Biological treatment : The primary effluent is taken into large tanks where it is constantly agitated mechanically and air is pumped into it. this allows vigorous growth of aerobic microbes into flocs. while growing, these microbes consume the major of the orgnanic matter in the effluent. this significantly reduces the BOD of the effluent. the sewage water is treated till the BOD is reduced. now the effluent is passed into a settling tank where the bacterial flocs are allowed to sediment. this sediment is called activated sludge. this activated sludge is pumped into aeration tanks (small amount) and anaerobic sludge digesters (large amount). here other kind of bacteria which grow anaerobically, digest the bacteria & fungi in the sludge. during this digestion bacteria produces a mixture of gases such as methane, `H_(2)S&CO_(2)`. The effluent from the secondary treatment plant is released into natural water bodies like rivers and streams.