Horizon DC watermaker with a built in flush unit. The amount of water needed to flush the system would depend on the volume of the membrane housing and hoses that you would have to clear. Ours activates once a week and runs for about 5 minutes. It pushes about a 1/4 inch stream out through the saltwater exhaust tube during that 5 minutes. I would expect it's about 3 gallons of water from one of the water tanks that passes through a charcoal filter to remove any chlorine that in the tank. It's suppose to be good for a week but I know it would be longer because once in a while I forget to set it to the flush setting and don't remember for a couple of weeks.