Part of the water that flows into a reverse osmosis (RO) system flows through the membrane and comes out as treated water. The rest is used to wash away the rejected contaminants down the drain as waste.
If not properly designed, RO systems can use large quantities of water to produce little treated water or use small quantities of water and produce low quality treated water while shortening the membrane life. Most domestic RO systems are designed for a 20-30% recovery up to 50% recovery, i.e. 2-5 litres of water produced for every 10 litres of water used. This can also be expressed as a ratio of 1:4 down to 1:1. Higher recovery rates can be obtained but doing so will shorten the membrane life.
RO membranes are easily fouled if contaminants or concentrated impurities are not washed away quickly enough. The higher the contaminant or TDS level the lower the recovery rate of treated water or the more wastewater required.