During secondary treatment of the primary effluent how does the significant decrease in BOD occurs?

Explanation

### Step-by-Step Solution:

1. **Understanding BOD**:
- Biological Oxygen Demand (BOD) is defined as the amount of oxygen required by microorganisms to decompose organic matter in a specific volume of water (usually measured in 1 liter) over a set period (typically 5 days at 20°C).
- A high BOD indicates a high level of organic pollution in the water.

2. **Primary Effluent**:
- Primary effluent is the wastewater that has undergone preliminary treatment to remove large solids and debris but still contains dissolved organic matter and nutrients.
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