Question

what are sewer systems ? how does it prevent us to drink polluted water?

Answer 1

Sewer system is a network of pipes which carries sewage and disposes the sewage water. This sewer system disposes sewage water to treatment plants which is further treated and purified by waste water treatment plants.
It prevents us to drink polluted water as this system transports dirty sewage water to treatment plants where it is purified and made reusable.