You might disagree, but hear me out on this. In most areas of the United States, water comes into the house from a public utility. It is typically stored at a reservoir and piped to a treatment facility, many of which use a reverse osmosis water filter as one step of their complete system. One of the problems is that in order to provide "safe" drinking water, treatment facilities must disinfect water that has been sitting in their tanks or reservoirs. The most commonly used disinfectant is chlorine. Chlorine is also added to the water to protect the membrane or the reverse osmosis filter from rot.