Well young lad, many people used the river as a dumping ground for their waste.
Father, why is the River Thames so dirty?
It used to be a great river, then the smell came along because people kept dumping their waste in the river. No one knew how to fix the problem, so nothing was done.
People kept drinking the water knowing that it wasn't good. 40,000 people died for Cholera in London alone and infant mortality rate was about 50%.
In the year 1858, A man by the name of Joseph Bazalgette designed and constructed 5 brick lined sewers. These helped keep the sewage flowing so it wouldn't sit in the river.
In modern days, water & sewage are pumped underground. The waste is transported and buried at a refuse facility. Waste is fermented into fertilizer after 30 days. Secondary Clarifiers then feed water through and is cleaned by bacteria. The water gets chemically treated with chlorine to ensure it is clean and then goes to the water supply.