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The Process of Reverse Osmosis Water Purification

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by Jo Lynn, Staff Writer

Reverse osmosis water purification is a method for removing salt, minerals and other impurities from water. This is important for achieving pure, healthy drinking water that is free from salt, minerals and other contaminants.



To understand how reverse osmosis water purification works, you must first know the definition of osmosis. And just in case you don’t remember this from high school science class, here it is! Osmosis describes water moving or passing from an area of high water concentration to an area of low water concentration through a semipermeable membrane. Clear as mud, right?

We usually think of osmosis in terms of cells. Water molecules are small enough to freely pass through cell walls. If your body is retaining water, and you feel bloated and puffy, the cells are holding a high concentration of water. Osmosis occurs when the water moves out of those cells into other areas of the body, and the puffiness goes away. Osmosis basically equalizes the water on the two sides of the membrane. The cells and fluids want to be in a state of equality. In our bodies, water is drawn to the areas that have higher concentrations of salt to equalize the concentrations.

In reverse osmosis water purification, we want to remove impurities like sediment, nitrates and other substances. A pump forces water through a filter drawing the pure water molecules away from the contaminants. The pure water passes through the filter for use by the consumer, while the contaminated water is discarded. This process for water purification produces very high quality water both for drinking or for routine laboratory uses.

Reverse osmosis
Encyclopedia definition.Reverse osmosis is a technological term developed from osmosis. Osmosis is a natural phenomena occuring in live cells where solvent molecules (usually water) will flow from a low solute region to a high solute region through a semipermeable membrane. The semipermeable membrane is referring to cell membrane or any membrane that has a similar structure or contituents to cell membrane. The movement of the solvent continues until an equilibrium of concentration is achieved on both sides of the membrane.

Reverse Osmosis is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. In simpler terms, reverse osmosis is pushing a solution through a filter that traps the solute from one side and allows the obtainment of the pure solvent from the other side.

Method

When two solutions with different concentrations of a solute are mixed together, the total amount of solutes in the two solutions will be equally distributed in the total amount of solvent from the two solutions. This is achieved by diffusion, in which solutes will move from areas of higher concentration to areas of lower concentrations until the concentration in all the different areas of the resulting mixture are the same, a state called equilibrium.

Instead of mixing the two solutions together, they can be put in two compartments where they are separated from each other by a semipermeable membrane. The semipermeable membrane does not allow the solutes to move from one compartment to the other, but allows the solvent to move. Equilibrium cannot be achieved by the movement of solutes from the compartment with high solute concentration to the one with low solute concentration. Equilibrium is, instead, achieved by the movement of the solvent from areas of low solute concentration to areas of high solute concentration. When the solvent moves away from low concentration areas, it causes these areas to become more concentrated. On the other side, when the solvent moves into areas of high concentration, solute concentration will decrease. This process is termed osmosis. The tendency for solvent to flow through the membrane can be expressed as “osmotic pressure”, since it is analogous to flow caused by a pressure differential.

In reverse osmosis, in a similar setup as that in osmosis, pressure is applied to the compartment with high concentration. In this case, there are two forces to consider regarding the movement of water: the force of solute concentration difference between the two compartments (the osmotic pressure) and the force caused by the externally applied pressure. In the same manner, the solute cannot move from areas of high pressure to areas of low pressure, because the membrane is not permeable to it. Only the solvent can move in this way. When the effect of the externally applied pressure is greater than that of the concentration difference, net solvent movement will be from areas of high solute concentration to low solute concentration, and reverse osmosis occurs.
 Drinking water purification

In the United States, household drinking water filtration systems, including a reverse osmosis step, are commonly used for improving water for drinking and cooking. Such systems typically include four or five stages:

* a sediment filter to trap particles including rust and calcium carbonate
* optionally a second sediment filter with smaller pores
* a carbon filter to trap organic chemicals and chlorination
* a reverse osmosis filter with a thin film composite membrane (TFM or TFC)
* optionally a second carbon filter

In some systems, the carbon pre-filter is omitted and cellulose triacetate membrane (CTA) is used. The CTA membrane is prone to rotting unless protected by the chlorinated water, while the TFC membrane is prone to breaking down under the influence of chlorine. In CTA systems, a carbon post-filter is needed to eliminate the chlorine.

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