Imagine you have two glasses of water, one contains gasoline in its maximum contaminant level (MCL) and the other glass contains bacteria at MCL. Note, MCL is the legal limit of the allowed contamination that will not cause serious health damage. If you leave them for a week, which will get more contaminated? The answer is obvious — the one with bacteria, because they reproduce and produce waste products. Therefore, removing bacteria is the primary goal of drinking water purification.
The typical process of municipal tap water treatment starts with a pipe collecting water from a lake or river. Then a coagulant is added, usually an aluminum salt, which facilitates formation of larger clusters of colloid iron, humic acids and some other organic substances. The resulting suspension is filtered through a regenerable sand filter. Finally, water is treated with bactericide, chlorine in most cases.
After chlorination most plants supply water to consumers. Additionally, some plants employ UV-lamp disinfection, ozone treatment or even activated carbon filters, which improves tap water quality and rises its cost.
It is chlorine that protects us from such severe diseases like plague or typhus. No kidding, many Medieval outbreaks were caused by the use of contaminated water.
At the same time chlorine is toxic to us. It can damage our liver, lungs and other organs, create cancerogenic organic compounds, cause allergies and more. But it is well-studied and easily removed from water by most activated carbon filters.
Although some water plants around the world have experimented replacing chlorine with organic bactericides, these alternatives have yet to be thoroughly studied.
To sum up, municipal purification of tap water is crucial for our well-being and is a reliable safeguard against suffering the same problems as occurred during the middle ages. It comes at a price of exposure to chlorine which can be easily dealt with at home.