Question

The scientists from a research institute collected samples of water from sewage pipes of two different cities, A and B. On analysis, the BOD value of the sample from city A was found to be 500 mg/L. The BOD value of the sample from city B was 200 mg/L.

1. Which one of the two cities needs a sewage treatment plant?
2. Briefly discuss the steps involved in the treatment of sewage.
3. What will be the effect of sewage treatment on the value of BOD?

Answer 1

1. BOD measures the amount of organic matter in water. As a result, a low BOD indicates high-quality water, whereas a high BOD indicates contaminated water. So, city-A requires a sewage treatment plant.
2. The steps involved in the treatment of sewage are:
   1. Primary treatment - In this procedure, the effluent or sewage moves through the primary sedimentation tanks. In these tanks, sludge settles at the bottom, and grease and oil rise to the surface, where they are eliminated. The tanks are equipped with scrapers that consistently push the accumulated sludge toward the hopper located at the tank's base.
   2. Secondary treatment - This process effectively degrades biological waste, including human, food, soap, and detergent. Municipal sewage treatment plants typically treat settled sewage through an aerobic process. After primary treatment, bacterial and waterborne aerobic microorganisms absorb pollutants to convert effluent into clean water. This process produces clean effluents that can be discharged into rivers.
   3. Tertiary treatment - This is the final stage of filtration, where effluents are treated to their highest quality before discharge to the environment. The tertiary treatment must meet extremely high standards for treating effluents so that they are cleaner than stream or river water. It reduces phosphorus and nitrogen.
3. As the majority of the organic matter is separated during sewage treatment, the value of BOD decreases.