Answer:
Introduction of untreated sewage can lead to various ecological hazards such as:
Destruction of aquatic ecosystem: The sewage decreases the dissolved oxygen in the water which leads to the death of aquatic plants, fishes, and other creatures.
Health hazards: The untreated sewage leads to the accumulation of pathogenic microorganisms in the water bodies. It adversely affect the aquatic plants and animals.
The untreated sewage along with agricultural waste may lead to the accelerated eutrophication.