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 determine the correct sequence of events in the secondary treatment of sewage, we need to understand the processes involved. Here’s a step-by-step breakdown of the secondary treatment of sewage:

### Step 1: Floc Formation
The first step in the secondary treatment of sewage is floc formation. During this process, fine particulates in the sewage clump together to form larger aggregates known as flocs. This is typically facilitated by the action of microorganisms that produce extracellular polymeric substances (EPS), helping to bind the particles together.

**Hint:** Remember that floc formation is crucial as it prepares the sewage for further treatment by creating larger particles that can be more easily removed.

### Step 2: Aerobic Digestion
Once flocs have formed, the next step is aerobic digestion. In this stage, microorganisms break down the organic matter present in the sewage in the presence of oxygen. This process helps to reduce the biochemical oxygen demand (BOD) of the sewage, making it less harmful to the environment.

**Hint:** Aerobic digestion requires oxygen, which is why aeration tanks are often used in this stage to ensure that sufficient oxygen is available for the microorganisms.

### Step 3: Settling
After aerobic digestion, the mixture undergoes settling. In this step, the flocs, which now contain digested organic material, settle to the bottom of the treatment tank. This process separates the treated water from the solid waste, allowing for the removal of the sludge that forms at the bottom.

**Hint:** Settling is essential for clarifying the treated water, as it helps to remove the remaining solids before further treatment or discharge.

### Step 4: Anaerobic Digestion
The final step in the secondary treatment process is anaerobic digestion. In this stage, any remaining organic matter is broken down by microorganisms in the absence of oxygen. This process further reduces the volume of sludge and can also produce biogas, which can be captured and used as an energy source.

**Hint:** Anaerobic digestion is important for the final breakdown of organic material and is typically done in a separate tank designed to maintain an oxygen-free environment.

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

Thus, the correct answer is **Option 1**.