Any material which cannot be used in the form in which it is produced is called a waste. According to their physical state, waste can be classified into
   3 categories – Solid waste, liquid waste & gaseous waste.
    The impurities present in waste water are called contaminant. This waste water is called sewage.


1. Domestic Waste Water :
     It includes all kinds of wastes like human excreta, food waste, soaps, detergents, oil, animal excreta, urine, etc.      
2. Agricultural Waste Water :
     The waste water generated from farms and agricultural fields contains harmful pesticides, weedicides and animal wastes.
3. Industrial Waste Water :
     The waste water generated from various industries contains harmful chemicals. The discharge of industrial waste water is called Industrial effluent.      
4. Petroleum Oil :
     The leakage of petroleum oil into the sea during drilling and shipping pollutes sea water. Oil spill is caused due to release of oil into rivers and oceans knowingly or unknowingly.      
5. Mining : 
     Waste water is also generated as a result of mining activities.      
6. Construction Activity : 
     Lot of waste water is generated during various stages of building houses, homes, malls, multiplexes, etc.  


    There are three processes which are involved in treating waste water before it is discharged into the water reservoirs. These processes are : 
    1. Primary treatment    2.Secondary treatment 
    3. Tertiary treatment

A. Primary Treatment :

  • It is the first stage and is called primary treatment. The waste water is passed through rotating screens to remove large objects.      
  • The water is then passed through a grit and sand tank to remove small stones and pebbles.      
  •  The soild wastes such as faeces settle down at the bottom of the tank and are removed with the help of a scraper. This is  a sludge     
  •  The sludge can be used for production of biogas to be used as fuel or for producing electricity. Light  materials float on top and are known as scum.
      The scum is removed with the help of a skimmer.      

B. Secondary Treatment :

  •  It is the second stage and is called secondary  treatment. The treatment at this stage involves pumping of air into the clarified water to help aerobic
    bacteria to grow. The bacteria decompose the suspended waste.      
  •  The activity of the bacteria produces decomposed organic material which settle down at the bottom of the tank as activated sludge. It is passed through
    sand drying beds to separate the solid material out of it. This solid waste can be        used as   m
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