Select the correct sequence of events in secondary treatment of sewage.

Options

A. Floc formation `rarr` aerobic digestion `rarr` settling `rarr` anaerobic digestion
B. Aerobic digestion `rarr` floc formation `rarr` settling `rarr` anaerobic digestion
C. Floc formation `rarr` aerobic digestion `rarr` anaerobic digestion `rarr` settling
D. anaerobic digestion `rarr` floc formation `rarr` aerobic digestion `rarr` settling

Answer

To solve the question regarding the correct sequence of events in the secondary treatment of sewage, we can break down the process into clear steps:

### Step-by-Step Solution:

1. **Flock Formation**: The first step in the secondary treatment of sewage is the formation of flocks. In this process, fine particles in the sewage water clump together to form larger aggregates called flocks. This is crucial as it helps in the subsequent steps of treatment.

2. **Aerobic Digestion**: After flock formation, the next step is aerobic digestion. In this stage, microorganisms (aerobic bacteria) break down the organic matter present in the sewage using oxygen. This process converts the organic matter into carbon dioxide and water, significantly reducing the Biological Oxygen Demand (BOD) of the water.

3. **Settling**: Following aerobic digestion, the mixture undergoes settling. During this step, the heavier flocks settle down to the bottom of the treatment tank, separating the treated water from the solid waste materials.

4. **Anaerobic Digestion**: The final step involves anaerobic digestion, where the remaining organic matter is further broken down by anaerobic bacteria in the absence of oxygen. This step helps in reducing the volume of sludge and further purifying the sewage.

### Correct Sequence:
The correct sequence of events in the secondary treatment of sewage is:
1. Flock Formation
2. Aerobic Digestion
3. Settling
4. Anaerobic Digestion

### Conclusion:
Thus, the correct answer to the question is option A: Flock formation, aerobic digestion, settling, and anaerobic digestion.

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