Question

Study the figure given below and answer the questions that follow.

• Identify the figure and state its importance.
• Why is air diffused into the aerator tank? Explain.
• • [(i)] What changes take place in the settling tank? 
    OR
  • [(ii)] How is BOD related to organic matter present in the water?

Hint:

The secondary treatment in sewage involves aerobic microbes that degrade organic waste. High BOD means more organic pollution in water.

Answer 1

1. The figure represents the Secondary Treatment or Biological Treatment in a Sewage Treatment Plant.
   Importance: This stage helps in removing the biodegradable organic matter from the wastewater using aerobic microorganisms.
2. Air is diffused into the aerator tank to provide oxygen to aerobic bacteria and microbes. These microbes utilize the oxygen to break down organic matter in the sewage efficiently, reducing the Biochemical Oxygen Demand (BOD) of the water.
3.  
   • [(i)] In the settling tank, the activated sludge settles down at the bottom due to gravity. The clear water or effluent is then separated from the top. A portion of the activated sludge is returned to the aeration tank (Return Activated Sludge - RAS), while the rest is removed as Waste Activated Sludge (WAS). OR
   • [(ii)] BOD (Biochemical Oxygen Demand) is directly proportional to the amount of organic matter present in water. The more the organic content, the more oxygen is required by aerobic bacteria to break it down, leading to a higher BOD value.