Sewage is the waste-water which contains both solid and liquid wastes released by homes, industries, hospitals, offices and other users. It also includes rainwater that has run down the streets during a storm or heavy rain. Sewage contains various contaminates including disease-causing microbes.

If untreated sewage is discharged into rivers or seas, then the water in the rivers or seas would also get contaminated and it would be harmful for aquatic animals and plants.

That is why; it is harmful to discharge untreated sewage into rivers or seas.