To understand how reverse osmosis purifies water, you must first understand the process of osmosis.

Osmosis is a physical force. It is the natural tendency of water with a low concentration of dissolved particles to move across a semi-permeable membrane to an area of water with a high concentration of dissolved particles.

The water will try to reach an equilibrium on both sides. i.e. both sides of the semi-permeable membrane will have the same concentration of dissolved particles. This is how plants absorb nutrients from the soil.

Picture a tea bag placed in a mug of hot water. (The tea bag is the semi-permeable membrane). At first, the water is free of tea. However, with time, the tea will appear to seep from the tea bag into the mug. This is the process of osmosis. If you were to leave the tea bag in the mug for long enough, the concentration of tea inside the teabag would equal the concentration of tea outside the teabag.

Now for reverse osmosis

The process of reverse osmosis requires that the water be forced through a semi-permeable membrane (the tea bag from the previous example) in the opposite direction of the natural osmotic flow; leaving the dissolved particles in the more highly concentrated solution.

In order for reverse osmosis to occur, the amount of force or pressure applied must exceed the osmotic pressure. Raw water enters a module housing the membrane system. The water is forced against the semi-permeable membrane and only clean water molecules pass through the pores in the membrane. Impurities are rejected and flushed away leaving you with the purist water possible.