What is Biochemical Oxygen Demand (BOD)?

BOD determines the quality of water. It is measured by the amount of oxygen required by bacteria and other microorganisms to oxidize the organic matter present in a water sample over a period of 5 days.

The BOD of drinking water should be less than 1. BOD of raw sewage may rise to several hundred.

What is Primary Treatment?

It is a simple and ineffective method of sewage treatment. The process involves the undissolved solids in raw sewage to settle out of the suspension, forming sludge. It removes only one-third of the BOD and none of the dissolved minerals.

Due to the contamination by toxic chemicals present in the industrial waste, efforts made to use the sludge as a fertilizer has been hampered.

What is Secondary Treatment?

In this process, the waste is brought in contact with oxygen and aerobic microorganisms (organisms that can survive and grow in an oxygenated environment). This results in the breakdown of most of the organic matter to harmless substances such as carbon dioxide.

Many treatment plants pass the waste from primary treatment to secondary treatment.

Primary and Secondary treatment together can remove up to 90% of the BOD. Chlorination removes the bacteria from the secondary treatment waste. This is then discharged to surface water.

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