Question

explain the back washing and filtration process in a water treatment plant

Answer 1

The water from the sedimentation is somewhat free from turbidity and bacteria. It is further treated by filtration. The theory of filtration can be summarised under the following heading.

1. Mechanical straining : As the water passes through the filter, particles greater than the pore size of the filter get trapped forming a layer above the filter media which inturns acts as a filter media.

2. Flocculation and sedimentation : the voids of the filter media acts as the sedimentation tank. The particles flocculates and gets deposited in the pores.

3. Biological Metabolism : water contains microorganisms which recides in the void. In the water, organic impurities are present which get used up by the microorganisms as food and metabolise the organic impurities and converts them into harmless form. Thus, water inturns get free from the harmful organic impurities.

4. Electrolytic charge : the sand grains of the filter media has charge which interacts with impurities in water. Thus, as interaction between sand and impurities takes place, the water gets neutralised and become purer.

Generally backwashing is employed in case of rapid sand filter. Cleaning of filter media is done when the head loss exceeds critical value. Backwashing is necessary because the filter media particle size is much more because of which the impurities travels a larger penetration. Thus, water at high pressure and compressed air is applied from the bottom. The applied pressure results in increase in the depth of filter media and impurities are released in backwater trough. Backwashing is required in interval of 24 to 48 hrs and amount of water required ranges from 2 to 5 % of total water to be filtered. Generally, pressure head used in backwashing is 4m.