What is the procedure for Treatment of Wast Water?


environmental chemistry

The procedure used for treatment of wastewater depends on the source from which it has originated. The only way to check the pollution caused by various industries is that each industrial unit must process its own wastewater. The processed water can be reused in the same industry. Waste water, particular municipal wastes (including domestic and household wastes) is treated in the following three stages

  • Primary or preliminary treatment
  • Secondary treatment or biological oxidation process
  • Tertiary treatment or complete chemical treatment

In the Primary treatment, the raw sewage water is first passed through a series of screens to remove large floating material. All non-biodegradable and chemical non-degradable solids like rocks, sand, grit, plastic and metal parts are separated by this method. The removal of these solids allows the polluted water flow freely. The water is then passed through a chamber (called the grit chamber), which is packed with sand and small stones_ This is helpful in removing the dirt, which is disposed off. The resulting filtered water then enters a sedimentation tank, in which particulate matter settles down to form the sludge. Sometimes, it is necessary to add certain chemicals like alum, which help to hasten the settling process. The sludge is removed and used after processing as manure. The above primary treatment removes about 30-40 per cent of the pollutants in the form of suspended matter.

Sewage water treatment

Sewage water treatment

In the secondary treatment, the oxidize able biodegradable solids are removed with the aid of living organisms. In this procedure, the water from the primary treatment, viz. from the sedimentation tank is taken to an aeration tank in which air is pumped along with some sludge from the .final (second) sedimentation tank. The sludge contains aerobic (oxygen rich environment) bacteria that break down organic pollutants in the water. After several hours in the aeration tank, the water is taken to the second or final sedimentation tank, in which most of the sludge settles dam. The total sludge is subjected to microbial digestion, in which process methane gas is evolved, which is used as a fuel. The water from the final sedimentation tank is then disinfected in disinfection tanks usually by chlorine; this removes the disease-causing organisms_

Normally, primary (or preliminary treatment) and secondary treatment (or biological oxidation process) is the complete treatment of water (see given figure)

Tertiary treatment is the final step in the purification of water. It includes purification of water by treatment with chemicals. This is helpful to remove highly soluble salts as precipitates by the addition of alum or other flocculating agents. Such treated water still contains nitrates, nitrites, phosphates and other non-biodegradable chemicals (like herbicides and insecticides) and biological agents (like bacteria, virus etc). Such water is rendered alkaline by the addition of time, which precipitated the phosphates. Subsequently some metallic hydroxides are added. By this treatment phosphates are precipitated as insoluble phosphate salts.

Nitrates and nitrites, which are very soluble in water, are removed by electrodialysis. Chemical coagulants like alum, ferrous sulphate, ferric sulphate etc. also help to remove nitrates and nitrites. Aeration of resulting water helps to remove foul odour and tastes. Finally, water is treated with chlorine or ozone, both chlorine and ozone are toxic to bacteria and virus.

Complete removal of pollutants can be affected by advanced procedures like reverse osmosis, electrodialysis and vigorous agitation with air. This treatment does not require any chemical. UV rays are helpful to kill most of the bacteria in water.

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