Welcome to the forum! What kind of detergent? If you're sure not to use it for drinking or consumption of any kind, including washing dishes (unless it was already dish detergent), I think that would work. It might be more practical to find different ways of doing those things that don't require water, such as a composting toilet instead of keeping water for flushing. For drinking water, I save all of our vinegar and -- when we used to drink it -- juice jugs, but not the flimsier water bottles or lightweight plastics.