Reverse Osmosis (RO) works by filtering water through a semi-permeable membrane that won't allow most contaminents to pass through. They generally use a process called cross flow, which ensures that they are constantly cleaning themselves. You end up with pure water. You use this water for cleaning windows. The down-side to RO is that typically, it will only recover between 5% and 15% of the water entering the system, the rest is waste. Hope this helps.