The primary reason for treating our sewage/wastewater is to remove the organic carbon constituents from the wastewater. Why is it important to do that?

The heterotrophic microbes present in surface water bodies like lakes and rivers can use this organic carbon for their primary metabolism. The organic carbon compounds act as electron donors, while the oxygen dissolved in the water is taken up as an electron acceptor. As the microbes take up dissolved oxygen, it depletes the oxygen levels in the water body (Biological Oxygen Demand). The drop/absence of dissolved oxygen causes an unavailability of oxygen for fishes and other aerobic fauna in the water body.

So how do we solve this problem in a typical wastewater treatment plant?

We mimic the process that happens in the water body, in a wastewater treatment plant.

The bioreactor has a rich and diverse microbial community which can degrade the organic carbon and oxygen is introduced into the bioreactors using mechanical aerators. Thus the effluent from the treatment is safe to be discharged into a nearby water body. 

Wait, we 're not over there yet!!

In similar fashion, the nitrogen and phosphorus (requires aerobic and anaerobic treatment steps) are removed stringently followed in the Western countries. This stops eutrophication of water bodies and prevents algal bloom events.

I got you and you were right, removal of phosphorus also can stop foaming of Bellandur lakes!