How Reverse Osmosis (RO) works

Osmosis is the natural tendency of two fluids, separated by a membrane, to equalize salt concentrations. This is the mechanism that plants use to extract water from surrounding soils. It is also how water passes from the intestine into the human body. Osmosis depends on having a membrane that is semi-permeable, allowing some substances, such as water molecules, to pass through while blocking others. Blocked substances include chemicals that are dissolved in water. When two mixtures of chemicals are separated by a membrane that is permeable to water, but doesn't allow chemicals to pass through, water flows from the side that has the lowest concentration of chemicals to the side that has the highest concentration.

To illustrate this concept imagine a leather bota bag filled with salt water in a swimming pool filled with fresh water.Leather is a semi-permeable membrane. Oils pass through to lubricate the skin, but other fluids are retained. Water will slowly flow from the swimming pool into the bag to try to equalize the concentrations of dissolved solids on both sides of the leather. As this occurs the water pressure will rise in the leather bag, to the point where the bag bursts or flow stops because the difference in pressure between the two sides of the membrane can no longer drive flow into the bag. The pressure caused by differences in chemical concentrations is called osmotic pressure. The strength of pressure depends on the difference in concentration of chemicals on either side of a semi permeable membrane.

Reverse osmosis relies on a semi permeable membrane and pressure to remove chemicals dissolved in water. Using the example of the leather bag, if a few bricks are placed on top of the bag, the pressure inside increases until it reaches the osmotic pressure created by difference in chemical concentrations. Because of the pressure, fresh water will flow from the bag through the leather leaving the salts inside the bag. This is essentially what happens with reverse osmosis units that are available for home use. High concentrations of chemicals remain trapped inside a permeable membrane that has pores small enough to allow water to pass through.  Chemicals are trapped inside the membrane and flushed away as waste water. Pressure comes from the water pressure that is part of normal operation of household water supplies—whether from a well or from a public water supply system that pipes water to a home.Reverse osmosis represents a reverse of normal osmotic processes because pressure has been applied on the side of the membrane where chemical concentrations are highest.