Reverse osmosis uses a membrane that is semi-permeable, allowing pure water to pass through it, while rejecting the contaminants that are too large to pass through the tiny pores in the membrane.  As some of the fluid passes through the membrane the rest continues downstream, sweeping the rejected contaminants away from the membrane and down the drain.

Consequently the membrane has to be replaced every one to two years depending on the amount of water used and the quality of the incoming supply.

For further details of reverse osmosis please follow this link.