A hydrolic ram only pumps about 10% of what goes throu it, so out of 500 gallons you would only pump back 50 gallons, And out of the 50 gallons, only 5, and out of 5 gallons only a 1/2 gallon. So, in a very short time, you will only recover 55 1/2 gallons. Unless you have another source of water to refill the 500 gallon tank. It would not work for long. If you had a free way to refill the tank, I would build a dump valve to quickly empty the storage tank, and turn it into useable power. A simple dump vale could be made like the one in your toilet water tank, but a float instead of a handle.