The specific process through which this occurs is known as ion exclusion, where a concentration of ions at the membrane surface form a barrier that allows other water molecules to pass through while excluding other substances. Semipermeable membranes have come a long way from the natural pig bladders used in early osmosis experiments. Before the 1960s these membranes were too inefficient, expensive, and unreliable for practical applications. Modern advances in synthetic materials have generally solved these problems, allowing membranes to be highly efficient at rejecting contaminants while making them tough enough to withstand the greater pressures necessary for efficient operation. RO systems typically require a carbon prefilter for chlorine reduction, which can damage the RO membrane, and a sediment prefilter to to ensure that fine suspended particles in the water won't clog the membrane.
Hardness reduction is also desirable for efficient operation.
These are the typical countertop or undersink systems that rely primarily on the natural water pressure to make the reverse osmosis process function. Undersink units typically utilize a pressurized storage tank where water pressure tends to increase as the tank fills. This provides sufficient pressure to get the processed water to the spigot at the sink. Low pressure units typically provide between 2 and 20 gallons of water per day. Numbers range from one to ten gallons of waste water per gallon of treated water.