Primary treatment – Primary treatment of sewags is done to remove large objects like polythene bags and other floating objects . The sewage is passed through through 2-8 cm screening bars and wire meshes of reducing pores . The sewage is passed through grit chamber and then it is allowed to go to the primary setting tank , the suspended particles settle down in two to three hours , it is called sludge . the rest flows for secondary treatment.
Secondary treatment – It is also called biological treatment . It consists of large aeration tanks , which agitate the primary effluent and help microbial growth.
It reduces the biochemical oxygen demand of the effuent This is passed into settling tank where it sediments as activated sludge . The sludge is passed into anaerobic Sludge digesters.
Here , the bacteria digest the sludge anaerobically and release gases like methane , hydrogen sulphide and carbon dioxide . After this the effuent is released into the water bodies.
This treatment is essential to control pollution in natural water bodies , to check water borne diseases/ pathogenic organism .