How Does Reverse Osmosis Work?

Reverse osmosis is a process of removing contaminants from water. This is accomplished by pushing the water under pressure through a semi-permeable membrane. The main goal is to make the water safe and clean for drinking and other household usage. In this article we are going to look at how a reverse osmosis system works to achieve this goal.

how does reverse osmosis work

To understand this process, we first examine osmosis process that occurs naturally. In the naturally occurring osmosis process, molecules move across a semipermeable membrane from less concentrated solution to a high one. A semi-permeable membrane allows some molecules to pass and blocks others.

A reverse osmosis system is the opposite of the naturally occurring osmosis. As a results it requires energy to push molecules across the membranes. The reverse osmosis membranes allows water molecules to pass through and blocks some dissolved salts, bacteria and other contaminants. The water, however, has to be pushed under high pressure to allow pure water to pass through while holding back the contaminants.

Feed/tap water is introduced to the reverse osmosis system via a pipe. The feed water enters the membranes under high pressure. These membranes are made from a flexible membrane sheet that has three layers; a 0.2 microns polyamide layer, polystyrene layer and a polyester fabric support base.

The polyamide barrier layer removes bacteria, viruses and chemicals from the water. A high pressure pump is used to force feed water across the semi-permeable membranes leaving behind close to 99% of the dissolved impurities. The amount pressure required to eliminate the impurities depends on the impurities concentration in the feed water. The more the concentration, the higher the pressure is needed.

How does reverse osmosis work


Water first enters the pre-filters. The pre filters may be more than one in the RO system. Sediment pre-filters are the most commonly used.  They are used to eradicate sediments like sand, silt, dirt etc. in this stage, carbon filters may also be used for removing chlorine. Chlorine is removed since it has effects on thin film material (TFM) and thin film composite (TFC) membranes. The TFC rejects up to 95% of the dissolved impurities. However, the carbon pre filters are not used if the system has cellulose triacetate (CTA) membrane.

The activated carbon filters have about 10 micron filtration ability. This helps in removing odor from water, the insoluble lead contaminants and removing bad taste from water. Most of the RO systems have Coconut shell Carbon Block cartridges that remove chemicals and chlorine from water.

After passing through the pre-filters, water enters the reverse osmosis membranes which is the core of the system.  The commonly used membranes are CTA which chlorine tolerant and TFC/TFM which are not chlorine tolerant.

Before the water enters the RO faucet after leaving the storage tank, it passes through the post filters. These post filters are generally granular or carbon blocks.  They have a major role of removing any remaining bad tastes and odors from the water. The water that comes out of the faucet is therefore pure and safe for consumption.

Modern RO system have alkaline filters that aid in retaining useful mineral in the water. Most of the RO systems have filters that eliminate all the inorganic minerals from water including the useful ones. A good example is magnesium and calcium minerals. These two minerals are important in leading a healthy life. It is the work of the alkaline filers to return them back to water.

How Does Reverse Osmosis Work

The entire purification system is regulated by a flow restrictor. There are various types of flow controls. The flow restrictor maintains the required rate to get the highest quality of drinking water. This is mainly dictated by the gallon capacity of the membranes. The restrictor also aids in maintaining the inlet side pressure. Without this flow control, only a small percentage of drinking water would be collected since water will take the least resistance path. A lot of water will therefore flow down the drain line.

An RO system is incorporated with an automatic shut off valve used to conserve water. When the storage tank gets full, the valve stops water from entering the membranes hence stopping water dispensation. As a result of stopping water from entering the membranes, the valve also stops water from entering the drain. The valve opens again after water is drawn through the faucet. The pressure in the storage tank drops making the valve to open.

The drain line is fitted in the outlet end of the RO system. The line is used to dispose impurities from the tap/feed water.

Reverse osmosis water purification process has been tested and verified to deliver safe drinking water. There is need to understand how the entire process works. Besides getting the knowledge, this can help you in deciding the type of reverse osmosis system to purchase. If you understand how the system operates it becomes easy to do minor repairs to your system. This will assist in saving the cost of hiring professionals in the long run.

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