RO membrane does most of the heavy work in the system. It removes over 96% of total dissolved solids (i.e. salts, minerals, metals), microorganisms and organic substances in water. Membrane divides the water flow into two streams. Filtered water from the membrane goes to the storage tank. Reject water goes into the drain. Most standard RO systems produce around 4 gallons of waste water per 1 gallon purified (4:1) if you're on a city water supply above 70 PSI water pressure. This ratio is controlled by the flow restrictor on the drain line. WECO VGRO systems only waste 1 gallon for every gallon purified since they employ water saving GRO membrane technology.