All the things that go down the drain and end up at the wastewater treatment plant are not removed there. Some of the industrial byproducts that end up in sewers, the agricultural chemicals that runoff farmland, and pharmaceuticals that pass through our bodies all can end up in our streams and lakes. It’s a soup of chemicals. They’re difficult to keep out of drinking water. Water treatment plants do a good job of killing the viruses and bacteria that cause waterborne diseases such as typhoid, cholera, and dysentery. There are some rare reports of pathogens such as cryptosporidium and legionella making people sick- or worse.