Sewage Treatment by Activated Sludge Process-

Since water pollution is caused by domestic, industrial & agricultural wastes. It is used for sewage treatment. it is aerobic. biological method. It is very fast & takes place in presence of oxygen. It contains mineral, organic matter, inorganic matter & also contains harmful living organism.

Sewage- Water containing domestic or Industrial waste is called sewage. Strength of sewage is express in terms of BOD & COD. The treatment of sewage has to be carried out as follows,

Preliminary treatment-(Screening)

In this treatment floating suspended impurities like bag, wood, metal, plastics, solids, grits, oil greases etc are removed.

Primary treatment-(Sedimentation)

In which used sedimentation process to remove suspended organic solids. Sometimes the chemicals like alum & hydrated lime ate added in this treatment to assist the removal of finally divided & colloidal solids or precipitate.

Secondary or Biological treatment- It is essentially an aerobic chemical oxidation which includes filtration or activated sludge processes. Filtration is done through specially designed filters like contact beds or intermittent sand filter or trickling filter. So that aerobic conditions are maintained all the times. It removes finally divided suspended matter.

Trickling filter or bio filter- It is commonly used for the biological oxidation of sewage. It is circular or rectangular in shape & about 2-5 m deep. They are filled with porous materials Eg. Crushed rock or hard coke or broken bricks. In filtrate (trickled sewage) the aerobic bacteria grow using the organic matter in the sewage as their food. Therefore it is necessary to maintain highly aerobic condition. Waste products are $CO_{2}$, $NH_{3}$ & $PO_{4}$

Activated Sludge Process- It is a biological waste water treatment process which spends up waste decomposition. Apart of the digested (activated) sludge is added to the raw sewage together with oxygen, which promotes coagulation of the suspended & colloidal matter. Here suspended & dissolved organic matter is oxidized effectively by the aerobic bacteria. After aeration, the effluent is sent to sedimentation tank, where sludge is deposited & clean water is drawn off.

Tertiary Treatment-

It is applied to decrease the nitrogen & phosphorous from effluents.

Precipitation:

Effluent + line+ phosphorous---$\rightarrow$ Calcium phosphate

Nitrogen Stripping

It is carried out to remove ammonia gas,

Disinfection:

It is chlorinated to kill bacteria which may remain in the effluent of sewage.