The supply of good quality of water is necessary for big and small cities. All the towns and cities also have means of disposal of the waste water. Thus the sewer system, sewage disposal and the sewage treatment plants are necessary to protect the health of people in the community.
Most of the towns and cities in our country have a government managed sewer system for disposal of sanitary sewage, domestic sewage and effluents from industries, etc. Liquid waste from industries contains organic and inorganic substances. In general, the sewage treatment procedures consist of three consecutive steps:
1. Primary treatment or mechanical treatment,
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2. Secondary treatment or biological treatment,
3. Tertiary treatment or combination of mechanical, biological and chemical treatments.
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1. Primary Treatment:
At this stage the solid waste is separated from the water stream. This helps in removing fats, oils, greases, solids like rags, cans, fruits, etc. These are collected in a dumpster and later on disposed in a landfill.
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When the sewage water passes through sedimentation tanks, the sludge settles down and the floating materials (oil and grease) are then removed. The sludge is processed separately. If the heavy metals and toxic substances are removed from the sludge, then it can be used as fertilizer for plants.
2. Secondary Treatment:
In this process organic solid matter in the sewage water is removed with the help of microorganisms. The biodegraded material is converted into biomass, which settles down. The left over water is filtered and further treated depending upon its use.
This water has low level of organic materials and suspended matter. Generally the water is chlorinated to destroy the harmful pathogenic bacteria.
3. Tertiary Treatment:
This involves further purification of waste water by removing salts, such as nitrates and phosphates, dissolved organic matter, metals, pathogens and other pollutants by advanced biological, chemical and physical processes. Now the waste water is clean enough to be used for agricultural and industrial purposes.