Order of Steps in Sewage Treatment Plant:

The correct order of steps in a sewage treatment plant is (1) Filtration and sedimentation, (2) Anaerobic digestion, (3) Digestion of organic waste aerobically, and (4) Agitation in the aeration tank.

1. Filtration and Sedimentation:
Filtration and sedimentation is the initial step in sewage treatment. In this step, large solid particles and debris are removed from the wastewater through physical processes like screening and settling. The wastewater passes through screens to remove large objects and then enters sedimentation tanks where heavy particles settle to the bottom. This process helps in the removal of solid waste and reduces the organic load in the wastewater.

2. Anaerobic Digestion:
After filtration and sedimentation, the wastewater undergoes anaerobic digestion. In this step, the organic matter present in the wastewater is broken down by anaerobic bacteria in the absence of oxygen. This process produces biogas (primarily methane) and reduces the organic content of the wastewater. The biogas produced can be used as a renewable energy source, while the digested sludge can be further treated or used as fertilizer.

3. Digestion of Organic Waste Aerobically:
Next, the wastewater undergoes aerobic digestion, where the remaining organic waste is further broken down by aerobic bacteria in the presence of oxygen. This process helps in the removal of residual organic matter and reduces the concentration of pollutants in the wastewater. It also helps in the removal of pathogens, improving the quality of the treated wastewater.

4. Agitation in Aeration Tank:
The final step in the sewage treatment process is agitation in the aeration tank. In this step, the wastewater is vigorously mixed with air in the aeration tank to provide oxygen for the growth of aerobic bacteria. These bacteria further break down any remaining organic matter and nutrients present in the wastewater. Agitation helps in maintaining a homogeneous mixture and promotes the growth of beneficial microorganisms.

Conclusion:
The correct order of steps in a sewage treatment plant is filtration and sedimentation, anaerobic digestion, digestion of organic waste aerobically, and agitation in the aeration tank. This sequence ensures efficient removal of solid waste, breakdown of organic matter, and reduction of pollutants and pathogens in the wastewater, resulting in the production of treated water that is safe for discharge or reuse.