inorganic materials from the sewage through filtration and sedimentation. Floating debris is removed by sequential filtration. Then the grit (soil and small pebbles) are removed by sedimentation. All solids that settle form the primary sludge and the supernatant forms the effluent.
The effluent from the primary settling tank is taken for secondary treatment. Secondary treatment or biological treatment The primary effluent is passed into large aeration tanks where it is constantly agitated mechanically and air is pumped into it. This allows vigorous growth of useful aerobic microbes into floc (masses of bacteria associated with fungal filaments to form mesh like structures). While growing, these microbes consume the major part of the organic matter in the effluent.
This significantly reduces the BOD (Biochemical oxygen demand or Biological oxygen demand). BOD refers to the amount of the oxygen that would be consumed, if all the organic matter in one litre of water were oxidized by bacteria. The sewage water is treated till the BOD is reduced. The Pretreatment Waste water from cities and towns Remove coarse solids sand and grit Air - % Organic solids Recirculated to aeration tank Primary sludge Production of biogas Anaerobic sludge digester Secondary sludge Primary settling tank Aeration zone (air + liquid water) Secondary settling tank Disinfection zone Treated sewage for dispose/reuse Preliminary treatment Tertiary treatment Primary treatment Secondary or biological treatment Fig.
. Sewage treatment process XII Std Biology-Zoology Chapter- XII Std Biology-Zoology Chapter- Environmental Issues inactivates chlorine-resistant microorganisms like Cryptosporidium and Giardia. Case Study: Auroville, located in South India near Puducherry has been experimenting with natural wastewater recycling systems (Decentralized Waste Water Treatment System (DEWATS)) ( Fig. .7a ).
Such treatment plants have now also been implemented in Aravind Eye Hospital, Puducherry (Root Zone Wastewater Treatment (RZWT)) (Fig. . b) and the Chennai Mathematical Institute, Siruseri IT Park, Chennai.