Secondary treatment or biological treatment: The primary effluent is passed into large tanks where air is pumped into it. This allows vigorous growth of aerobic microbes. While growing, these microbes consume the major part of the organic matter in the effluent. This significantly change the BOD (biochemical oxygen demand) of the effluent. 

The effluent in then passed into a tank a small part of the sludge is pumped back into the aeration tank to serve as the inoculum. The remaining major part of the sludge is pumped into large tanks called sludge digesters. Here, other kinds of bacteria grow anaerobically in the sludge. The biogas produced can be used source of energy as it is inflammable. The effluent from the secondary treatment plant is generally released into natural water bodies like rivers and streams.

The greater the BOD of water

More is its pollution level

The correct answer is Option (3) → More is its pollution level

BOD (Biochemical Oxygen Demand) is a measure of the amount of oxygen that microorganisms require to decompose organic matter in water. The higher the BOD, the more organic matter is present, which means more oxygen is consumed by bacteria and other microorganisms to break it down.

Key Points:

• Higher BOD: Indicates a higher level of organic pollution in the water. This is because more organic waste (like sewage, industrial waste, or decomposing plants and animals) is present, leading to more microbial activity and therefore more oxygen consumption.
• Lower BOD: Suggests lower levels of organic pollution, meaning the water is cleaner and requires less oxygen for microbial decomposition.

So, when the BOD is high, it means the water is more polluted because the microorganisms are consuming a large amount of oxygen to break down the excess organic matter. This can lead to oxygen depletion in the water, which is harmful to aquatic life.