Why don't people know how toilets work? The water comes from the cistern (and it appears that it's only even used for heat exchange). The article author mentions how "the murky water creeps [him] out" and then links to a photo of a tube with completely clean water.

The cistern contains drinkable water. Sure, it's not clean enough to drink out of, but it's tap water. It's not where the poo goes. The water is not murky, it's tap water.