yes, RO filters work on pressure passing pure water through a membrane. What isn't pure is disposed of, and tends to be 25-50% more contaminated than the water going in. Most use 1.5-2 gallons of water to get 1 gallon of pure. When I got my RO/DI unit I ran the figures. Including waste water I end up paying around $0.02-$0.03 per gallon, roughly 1/10th the price from the local fish store ($0.20). Of course it depends on the cost of your local water, but I suspect you'd find that even accounting for filter changes yearly (and at his usage more like every 3-5 years), it is less than half the cost of paying someone else to filter it. I figured out that mine will pay for itself (including the first membrane change) in roughly 6 months (of course it depends on how much water you use, and I am using more than 15g per week).