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I assumed the walk in freezer was the latrine. I suppose they could get a bit more water if they recycled the urine in some way.
At first, I was going to have a problem with the amount of water available in the canned foods but I have rethought that. Assume a prison population of around 1500 and about 5 days supply of food for the prison. That's 7,500 man-days of food. We had 5 prisoners sharing that food so we are back to a supply that would last them for 1,500 days. Of course, the majority would be dehydrated. Maybe 25%-40% would be canned with a liquid that would be good as a water substitute ( not too salty). They should still have a ton of dry goods that they will never be able to use.