In reply to Chicken Brian.

The greywater question is an exercise in semantics. "Greywater" in the US is wastewater that doesn't contain sewage. Normally, it comes from bathtubs, sinks, and laundry and can be used in certain restricted applications under most plumbing codes: irrigation, toilet tanks, things like that. "Rainwater" is not "greywater", as it's not ever been used, though they are mostly handled in the same way. The pila is a rainwater system.

The entire discussion is mostly moot in Guatemala, though. NO municipal water system in Guatemala is safe by American standards, due to a lack of plumbing code and enforcement as I mentioned in this article. Drinking straight form the tap here is just as dangerous as drinking straight from a river, or straight from collected rainwater. By this relative measure, these pilas are "clean water", and they do drink from them.

Karen's comment is right on; the wastewater doesn't go into the tank, it bypasses it. If they are incautious, with vigorous washing they could spill some soapy water over the side of the basin and into the tank. That sort of contamination is small-time compared to most of the other available options for drinking water. Part of our health lecture is stressing the importance of boiling all water before you drink it. That applies to water from the pila. Sadly, only about one person in a hundred listens to us.