Question

what is sewage ? why is it harmful to discharge untreated sewage into rivers and lakes?

Answer 1

sewage is a waste material of humans and animals . It is come from gutter, and it is very harmful for river and lakes because it's produce some harmful gases .
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Answer 2

seawage is the waste water produced after using ,in the household or industries
it is very harmful to release it into rivers and lakes because it pollutes the river water
so they treat the seawage waste in the water treatment plants before releasing it in the water bodies
if seawage is released directly into the river bodies ,then the biodiversity may disturb