What I do at home is keep potable and non-potable water on hand.

Potable water is for drinking, cooking, and body washing (via washrag).

Non-potable water can be purified, if necessary, but is used primarily for the porta potty and other cleaning purposes. When the porta potty is full, then use some additional water to make a clean flushing after dumping the porta potty down the toilet, if the sewage system is still functioning.