Reverse osmosis was developed in the 70s by scientists who were working on water research in the University Of California Los Angeles. It was traditionally used to remove salt from sea water. Its uses have become wider and it is also applied to purify water for drinking. The water is demineralized or deionised by pushing it through a reverse membrane under pressure.
To understand reverse osmosis, it is good to understand the osmosis process first. This is one of the most common processes in nature. It refers to a situation where a weaker solution tends to move to a stronger solution. It is applicable in human bodies when kidneys absorb water from the blood or when the roots of plants absorb water from the soil. There has to be a semi-permeable membrane in between the two saline solutions.
Reverse osmosis membrane
A reverse osmosis membrane works with the same concept of a semi-permeable membrane. What a semi permeable membrane does is allow some molecules to pass through while retaining others. With a reverse osmosis membrane, the tap water is pushed through a semi-permeable membrane by using household water pressure.
The pressure that has to be applied must be greater than the osmotic pressure that is naturally occurring in order to push the water through the membrane. The amount of pressure will also be determined by the amount of salt or contaminants in the feed water. The higher the amount of dissolved salts, the higher the amount of salt required to remove them from water.
Through the membrane which is as thick as cellophane, only the water molecules pass through. The other dissolved substances such as salts, pyrogens, organic bacteria, and organics are washed down the drain. The performance of a reverse osmosis system is determined by incoming water pressure, water temperatures, nature, and amount of dissolved solids in the tap water and the quality of the membranes and filters used.
Components of a reverse osmosis system
– Cold water line valve – this is the valve that is fitted onto the cold water supply line.
– Pre-filters – this is the first entry where the cold water supply enters the reverse osmosis chamber. Sediment filters are the most commonly used pre-filters in most reverse osmosis systems. Sediment filters remove dirt, sand, silt, and sediments. Carbon filters are used to remove chlorine. Build up of chlorine can damage the thin film material or membrane used. If the reverse osmosis system uses cellulose tri-acetate membrane, then the carbon filters are not used.
– Reverse osmosis membrane: this is the main component of the system and is most commonly used membrane is a spiral wound which comes in two different options. One is the thin film composite material which is not tolerant to chlorine and the other is cellulose tri-acetate which is chlorine tolerant.
– Post filters – this is where the product water goes through after it has left the reverse osmosis storage tank and before entering the reverse osmosis faucet. The post filter is made from carbon material, either in block or granular form. This is the process where remaining tastes and odors are removed from the water.
– Automatic shut off valve – this is a valve that is designed to conserve water in the reverse osmosis system. This valve blocks any more water from entering the membrane when the storage tank is full. As such, there will be no more production of water. This valve shuts the flow of water and prevents water from entering the drain. Consumption of water from the drinking water faucet will lead to a drop in pressure in the storage tank and the valve opens to allow more water.
– Check valve – this is located at the end of the outlet of the housing of the reverse osmosis membrane. The function of this valve is to prevent the water from flowing backwards. It also prevents the product water from flowing back to the reverse osmosis storage tank. Backward flow of water can lead to the rupture of the reverse osmosis membranes.
– Flow restriction – this is designed to regulate the flow of water through the reverse osmosis membrane. A flow restrictor maintains the flow rate in such a way that only the highest quality of drinking water is obtained.
– Storage tank – a standard reverse osmosis storage tank can hold up to 2.5 gallons of water. It is furnished with a bladder inside that is meant to maintain water in a pressurized state when the tank is full.
Reverse osmosis is one of the best ways to clean water. Although the systems may look similar in terms of components and design, they come with several differences. The difference in components can have an impact on the quality of product water.