Chapter 6.0 How Reverse Osmosis Works

6.0  How Reverse Osmosis Works

In the reverse osmosis process, the tap water is allowed to pass through a semi permeable membrane.  Ever since its inception, this system has become very popular in domestic water purification.

Basically, part of the water actually allows the rejection of impurities that are selected initially by the membrane.  This membrane is usually made of polyamide TFC.  Although this membrane is highly effective, it is also very sensitive to chlorine.  Thus, it must be protected with active carbon filters.  In normal operating conditions, a reverse osmosis system lasts 5 to 8 years.

6.1 Process

Water with low concentration of dissolved particles has the tendency to move across a semi permeable membrane to an area with a high concentration of dissolved particles. In effect, water will try to achieve equilibrium on both sides.

In reverse osmosis, water is forced through a semi-permeable membrane in the opposite direction of the natural osmotic flow.  As such, the dissolved particles are left behind in the more highly concentrated solution.  Thus, for reverse osmosis to occur, the pressure applied must be greater than the osmotic pressure of the water.

Fig.12. Reverse Osmosis Diagram

Two types of membranes:

This membrane is made of cellulose triacetate

This is composed of thin film composite.

Fig.13. TFC Membrane

6.2 Selecting a Reverse Osmosis System

In the selection of a reverse osmosis system, there are a number of important factor to be considered.

Factors to be considered:

The water supply should be potable and one that is deemed safe for human consumption.  It must be sufficiently disinfected or sterilized on a continuous manner.

It is essential to determine if the water is chlorinated or not.  This will determine the type of membrane to be used in the RO process.  For chlorinated water, a CTA membrane is most advisable to use.  For unchlorinated wter, a TFC membrane can be used.  Most systems however use TFC membranes.

The RO system must be able to provide and sustain the daily requirements.  A residential system should more or less be capable of producing an excess of at least ½ gallon of water per person in one day.  Commercial units are expected to be capable of producing more.

Contaminants such as manganese, iron, and hydrogen sulfide must be significantly reduced or removed by pre-treatment processes in accordance with membrane tolerances.

The tolerable level of Total Dissolved Solids for various types of water should be known prior to reverse osmosis.

The pH of the feedwater should also be known in the selection of an RO system.  Most city water supplies have a pH of 6.9 to 7.5.

If the water pressure is less than 50 psi, a booster pump may be needed. Moreover, if the inlet water temperature is very low, a booster pump will also be necessary.

6.3 Factors affecting performance of RO units

There are basically three factors that have the greatest potential impact on the quality of reverse osmosis water that an individual system will produce.

Fig.12. RO Filters

Factors that affect performance of RO units:

In the determination of the amount of water produced, the net pressure across the membrane is a major factor. It should be noted that as the pressure increases, the rate of water production also increases. The minimum pressure of the water required for residential units is 50 psi.  If the water pressure is lower than this, a booster pump may be required.

The molecular forces increase as the TDS increase. Before water molecules can start to separate, these molecular forces must be broken.  This can be done with the application of pressure.  Every 100 mg/l of TDS requires 1 psi (pounds per square inch) just to overcome osmotic pressure.

The actual rate of production is greatly affected by water temperature.  The production decreases as the temperature of the water decreases.  This means that the cooler the water is, the lower of the rate of production becomes.

Reverse Osmosis System Diagram

Typical water problems and some common treatments are listed below (1).