29 January 2014

Water Treatment : Science

Water Treatment : CBSE / NCERT Notes

Waste water needs to be treated before it can be reused, or released into a water bodyWaste water has many impurities and these impurities should be removed by water treatment.
Waste water from households is treated at a treatment plant to remove the physical, biological and chemical matter
Water Treatment, CBSE / NCERT Notes, CTET Exam Notes

In the physical process, waste water is filtered to remove large impurities. 
For example
  1. Take a funnel. 
  2. Place a filter paper in the funnel and wet it with water.
  3. Then add some sand, fine gravel and medium gravel into the funnel. These are layers for the filtration of water.
  4. Now pour waste water into the funnel and collect clean water. 
  5. Repeat the same process several times till the collected water looks clean.
In water treatment plants, the layers are substituted by:

Bar Screens

Bar screens are screens that prevent the flow of large objects in waste water. These screens help to remove large objects like rags, plastic bags, cans, napkins and sticks from sewage. These bar screens are cleaned regularly either manually or with the help of the machines.

Grit and sand removal tank

The grit and sand removal tank may not remove some impurities, such as faeces.


clarifier is a tank with its central part inclined downwards so as to allow faeces to settle down. The inclined waste is then removed using a scraper. This waste is termed as sludge. 
Sludge is transferred to a separate tank where it is decomposed using bacteria. The gas produced is burned in an apparatus such as micro-turbine to generate electricity. Finally, a skimmer is used to remove the oils and grease.
Thus, if all the physical contaminates are separated from the sewage, then the sewage is termed to be clarified water.
Biological contaminates such as bacteria and microbes consume human waste, food waste, soaps and other wastes, and still remain in the clarified water. Now this clarified water is passed into an aerator in waste water treatment. An aerator pumps air into the water. After many hours, the bacteria settles at the bottom of the tank as activated sludge. The water present in the top is 95% clean and is let out into a water source, while the activated sludge is dried in a sand bed and is used as manure.
In the chemical process, take a chlorine tablet and put it in a beaker full of water. Observe that water is clean when the tablet dissolves. That is why chlorine tablets or ozone is used to disinfect water in treatment plants.
Different mechanisms were used to treat water, even in the olden days. The ancient Greek and Indian writings dating back to as early as 2000 BC recommended water treatment methods. People knew about water treatment procedures back in 2000 BC. The Egyptians discovered the process of coagulation for water treatment. Eucalyptus trees are planted along the sewage ponds in which water is released after water treatment processes. Eucalyptus trees absorb waste water from the pond and release pure water vapour into the atmosphere.
Oils and fats can harden and block water pipes. Fats clog the soil pores in an open drain, reducing the effectiveness of filtration of water. So avoid throwing tea leaves, solid food remains and napkins into the drain. These block the flow of oxygen to the microbes that help in water treatment. 
Chemicals like paints, medicines, motor oil and solvents kill the microbes that help in water treatment. People need to be made aware of the ill effects of throwing pollutants into the drain.

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