Ion exchange (IX) and reverse osmosis (RO) can be used to solve some of the same separation needs, and are sometimes even used in combination with one another. Still, IX and RO differ in a number of ways that affect their overall suitability and cost-effectiveness for a specific process or application.
If you’re wondering how IX and/or RO might fit into your process, let us get to know first “What ion exchange and reverse osmosis are?”
What is Ion Exchange?
In Ion Exchange 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 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.
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.
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.
Which one to choose; RO or IEX?
IX resins are best used to treat streams with low to moderate TDS levels. This is because high TDS levels will quickly exhaust IX resins, resulting in a need for more frequent regeneration cycles. As a result, the facility will see higher costs due to greater consumption of regenerant chemicals, more downtime for regeneration, and more frequent replacement/disposal of IX resins due to accelerated degradation.
IWhile IX can theoretically be used to treat streams with any level of TDS, excessive maintenance requirements and short service cycles become limiting factors for most facilities. In short, the higher the TDS, the less practical IX becomes.
For most facilities, the cost of maintaining an IX system is simply not economical for treating streams with TDS levels of 500-1000 parts per million (ppm). While RO systems become more economical for demineralisation of water with higher TDS levels.
TDS data gives us an easy lead regarding to decide for technology choice, there is also other parameters where TDS level of treated water might be seen suitable for both. From a point of economical view here steps Return of the Investment, APEX and OPEX calculations in.