RO system uses pressure which forces the external supply of water to flow through the membrane and passes out of the faucet after undergoing the different stages of filtration. All the impurities that were blocked from passing through the semi-membrane are usually discarded and wasted away to drain. Find more at How Does Reverse Osmosis System Work?  

Source: https://www.quora.com/How-does-Reverse-Osmoses-waste-system-work