There should be no risk to using untreated water to flush the toilet. Presumably they are shipping two types of water- potable and non-potable. The non-potable

There should be no risk to using untreated water to flush the toilet. Presumably they are shipping two types of water- potable and non-potable. The non-potable is less difficult to produce and ship. You can flush a toilet by dumping a bucket of water into the bowl.