Before reaching our homes, most tap water has gone through treatment plants where it is filtered and disinfected. The process is designed to make the water safe. But, research shows that the most common way to treat water adds chemicals that can get turned into cancer-causing agents. The Environmental Protection Agency is taking steps to reduce this danger, but some health experts say the agency isn’t doing enough. Daniel Grossman has the second in our series, “The Thirst for Safe Water.”