Every city has water plants for purification and filtering before it is used as potable water. Most cities also have plants for cleaning the water before it is sent back into the natural water reservoirs, like the ocean. It is important to have both, I think, to help prevent disease among humans, as well as to protect the environment. https://www.thinkwaterfiltration.com/en/water_filtration_system.html
Every city has water plants for purification and filtering before it is used as potable water. Most cities also have plants for cleaning the water before it is sent back into the natural water reservoirs, like the ocean. It is important to have both, I think, to help prevent disease among humans, as well as to protect the environment. https://www.thinkwaterfiltration.com/en/water_filtration_system.html