Explain why some times foaming is seen in river water near the place where sewage water is poured after treatment?

This type of foaming is caused due to non-biodegradable detergents present in the water even after the sewage treatment. Detergents can be classified as biodegradable and non-biodegradable detergents. A detergent is a water-soluble cleansing agent which combines with impurities and dirt to make them more soluble.