Sewage Treatment Plant For Removing Pollutants From Wastewater
Author: Pinakin Sagar
Sewage comprising of human wastes and toxins from industrial discharge poses severe threat to the water bodies where it is eventually released. Presence of harmful bacteria, suspended solids, toxic ions, etc. in the sewage can lead to high levels of water pollution. Therefore, this wastewater is subjected to a series of physical and biological treatments before it reaches a natural water body. These processes take place inside a Sewage Treatment Plant and render sewage both biologically and chemically harmless.
The sewage water received at Sewage Treatment Plants is generally grayish in color with an unpleasant smell. The whole treatment comprises of four stages: preliminary, primary, secondary and tertiary.
Preliminary treatment – In order to make sewage suitable for the main treatment process, it is subjected to preliminary treatment. This includes screening, removal of grit, grease and oil. In the stage called screening, wastewater is passed through screens in order to remove solid impurities that can damage the machinery as well as block pipes. In grit removal stage, grit and dirt are removed by a settlement process. The grit settles down which is disposed later to landfills. Another stage of pretreatment process removes oil and grease content from the sewage.
Primary treatment- In this process, the sewage flows into large water tanks. The heavier material settles down at the bottom of the tank which is then swept by a scraper blade to a submerged outlet. Here most of the solid waste is removed from the water. The slurry known as sewage sludge is dealt separately. The settled sewage moves on to next stage of treatment.
Secondary treatment (biological treatment) – In this treatment dissolved organic matter is removed from the sewage. It is done by two methods, first is biological filtration and second is activated sludge. In the first process, settled sewage is distributed through holes in a deep rectangular bed of stones. While in the second process, settled sewage is mixed with micro- organisms and bacteria known as activated sludge. This is then aerated by large blowers in tank.
Tertiary treatment – Sand or gravel filters are used to clean the water discharged to water bodies. Bathing waters are disinfected by ultraviolet rays and disinfectants.
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