RO filters work by forcing water under pressure against the membrane. The membranes allow the small water molecules to pass through while rejecting most of the larger contaminates. Like any filter, RO membranes will eventually clog and need to be replaced.  Prefilters are often placed in front of the membrane to help lengthen the lifetime. These filters commonly consist of a micron sediment filter and a carbon block filter. The micron filter removes large particles and the carbon filter removes chlorine, large organic molecules and some heavy metals. Of course, the use of prefilters makes initial unit cost more expensive but they should pay for themselves in longer membrane life. RO units are rated in terms of gallons per day of output with 10-50 gallon/day units typically available.