Difference Between Ion Exchange and Reverse Osmosis

The key difference between ion exchange and reverse osmosis is that ion exchange is a physio-chemical method that selectively removes contaminants by effectively exchanging ions of similar electrical charges while reverse osmosis is a physical method in which water is passed across a semi-permeable membrane against a concentration gradient, applying a pressure.

Water purification is an important process in providing clean water to the community. There are many steps involved in the water purification process, which include biological, chemical and physical procedures. Ion exchange and reverse osmosis are two processes used in water purification. Both remove dissolved solids from water. Some water purification processes use a combination of both methods. Moreover, both are commonly used across a variety of industries.

CONTENTS

1. Overview and Key Difference
2. What is Ion Exchange
3. What is Reverse Osmosis
4. Similarities Between Ion Exchange and Reverse Osmosis
5. Side by Side Comparison – Ion Exchange vs Reverse Osmosis in Tabular Form
6. Summary

What is Ion Exchange?

Ion exchange is a technique used in demineralization of wastewater and domestic water softening. In this technique, ions are removed from the aqueous solution by exchanging them with another ionic species. In this way, weakly bound ions can be displaced by a strongly binding ionic species. We call this effect principle of selectivity. Using this principle, unwanted ions in the water are replaced by other ions during the ion exchange technique.

Ion exchange technique can be executed in batch or continuous mode. It is applied in wastewater purification for the removal of nitrogen, phosphorus and heavy metals. Moreover, it is used to remove specific impurities selectively and to recover valuable trace metals like chromium, nickel, copper, lead and cadmium from industrial waste discharges.

Ion exchange resins are made from small porous beads that are insoluble in water and organic solvents. The most widely used base-materials are polystyrene and polyacrylate. Moreover, many naturally occurring minerals, especially aluminium silicate minerals, have this property of ion exchange.

What is Reverse Osmosis?

Reverse osmosis is the process where a pressure greater than the hydraulic pressure is applied to the system in order to allow movement of water through a semi-permeable membrane. The movement takes place against a concentration gradient. The membranes that are used in reverse osmosis are termed as reverse osmosis (RO) membranes. The materials which are commonly used to prepare commercial RO membranes are polyamide thin-film composites (TFC), cellulose acetate (CA) and cellulose triacetate (CTA). Depending on the type of membrane material, the efficiency and the speed of the technique differ.

The reverse osmosis setup is composed of a hollow fibre with the membrane material being spirally wound around the fibre. These fibres are bound together to increase the surface area for reverse osmosis. Once the flowing water is subjected to high pressure, the water and small molecules pass through the semi-permeable membrane. This retains the large particles and the rest of the unwanted particles. The filtered water is then passed for downstream processing.

RO membranes can filter out virtually all particles including germs, organic matter, ions and other particulate matter. Filtration of large molecules up to a molecular weight of >300 Da is possible with reverse osmosis technique.

Advantages of Reverse Osmosis in Water Purification

• Cost effectiveness
• Can filter out almost all the particles including ions and heavy metals
• Can be used to eliminate radioactive particles from water samples
• Chemical usage is minimized

What are the Similarities Between Ion Exchange and Reverse Osmosis?

• Ion exchange and reverse osmosis are two processes commonly used in water purification processes.
• Combination of both methods provides a high quality purification.
• Ion exchange resins can be installed in front of a reverse osmosis unit.
• The use of both methods depends on the specific substances present as well as the target purity of the treated stream.

What is the Difference Between Ion Exchange and Reverse Osmosis?

Ion exchange is a physio-chemical method which exchanges ions between a liquid phase and an ion-exchange resin. On the other hand, reverse osmosis is the process where water is passed across a semi-permeable membrane against the concentration gradient, which is facilitated by high pressure. So, this is the key difference between ion exchange and reverse osmosis. Ion exchange is a physio-chemical method, while reverse osmosis is a physical method. Moreover, ion exchange process utilizes ion exchange resins while reverse osmosis utilizes reverse osmosis membranes.

The below infographic tabulates the difference between ion exchange and reverse osmosis.

Summary – Ion Exchange vs Reverse Osmosis

Ion exchange and reverse osmosis are two techniques used in the water purification process. Ion exchange method is a physio-chemical process which exchanges ions (contaminants) in the water with an ion-exchange resin. In contrast, reverse osmosis is a physical method which filters all most all contaminants based on the size. In reverse osmosis, water is passed through a semi-permeable membrane. Reverse osmosis utilizes pressure to force water across the membrane. Ion exchange removes specific substances based on the ionic charges while reverse osmosis utilizes an ionic exclusion process.

Reference:

1. “Different Water Filtration Methods – Reverse Osmosis/ Distillation/ Ion Exchange/ Carbon | APEC Water”. Freedrinkingwater.Com, 2020, Available here.

Image Courtesy:

1. “Ion exchange” By Jspiteri at English Wikibooks – Transferred from en.wikibooks to Commons (Public Domain) via Commons Wikimedia
2. “Simple RO schematic”  By Colby Fisher – Own work (CC BY-SA 3.0) via Commons Wikimedia