I actually work at a water plant as a water quality specialist so I will tell you what I know:
Our water supply in the United States is regulated by the Department of Natural Resources. In order to be a provider of water you have to treat that water to specific regulations deemed safe by the D.N.R. Because of this, there are rarely issues in our drinking water supply. That being said, sometimes there are areas of distribution systems that lose pressure and cannot guaruntee safe quality. In that event, boil orders are issued until it can be demonstrated that the problem is fixed and the water is safe again.
Chlorine: Chlorine is used to disinfect the water once most turbidity is removed during the treatment process. There are many types of bacteria that cause concern but the main type is Coliform Bacteria(E-Coli are a subset of this). We are concerned with this, because it is an indicator organism for other types of pathogens in water. Although large quantities of chlorine are not safe for humans, the small amount in the water is safe for humans, while being a high enough level to kill any pathogens.
Fluoride: This is kind of a hot topic. Some people think fluoride should not be added to the water while others argue that is promotes good health. Fluoride can be beneficial to strengthen bones and teeth. However, some believe there are detrimental effects of fluoride as well. Local governments decide the level of fluoride to be included in the water supply, and water producers must follow these guidelines.
I hope this answers your question, I tried not to get too in depth. If you have any other questions about drinking water feel free to let me know.
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