Sanitary sewer

Modern sewer systems often separate rainwater from polluted water. "Wastewater" (polluted water, also called sewage) is treated before it is released into a river or lake again. This treatment may not be needed for rainwater (also called stormwater). This idea of using separate sewer systems for wastewater and rainwater is called sanitary sewer.
A sanitary sewer or foul sewer is an underground pipe or tunnel system for transporting sewage from houses and commercial buildings to treatment facilities. Sewage is treated to control water pollution before discharge to surface waters. 
Cholera is a deadly disease caused by foul water. The system of treating foul water was first invented in London. There cholera had repeatedly caused disease, but in the middle of the 19th century the first successful water treatment plants were invented.