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Ion exchange vs. reverse osmosis

WatMan is able to deliver both IX and RO technology, so we have no favourites among these technologies. Enclosed are some parameters that affect the decision of which technology to choose.

The total investment in a DI water system depends on the nature of the total plant, especially on the pretreatment and the possible post treatment. The layout of the system should be considered carefully. For waters of low TDS, packed-bed ion exchange is more economical to operate than reverse osmosis (RO). Depending on the amount of bicarbonate and silica in medium TDS water, packed-bed may still be more economical than RO. There is no point in using IX with high TDS waters because of the increasing consumption of chemicals.

IX needs strong chemicals, however chemicals such as NaOH and H2SO4 or HCl are inexpensive. Strong acids and caustics need neutralization equipment. RO may require special antiscalants and/or membrane cleaning chemicals. RO systems normally use only minute amounts of corrosive chemicals. Neutralization may be needed depending on the nature of the membrane cleaning chemicals. Chemical usage and investment in the neutralization equipment should be considered when making cost calculations. Possible usage of brine in the softeners should also be considered.

RO creates more waste water because the recovery of RO is typically 50-80%: for each cubic metre produced by an RO system, 0.25-1 cubic metres are fed into the sewer. By comparison, the recovery of a modern ion exchange system is usually 96%. It should be noted, however, that the RO reject may still be suitable for a secondary use such as cooling. In a situation like this, the total yield of RO may be close to 100%.

IX produces a higher quality of water compared to RO, which normally rejects 95-98% of the minerals. Modern IX technology retains 99% of the salts, or more. It is possible to add a polisher both after the IX and after the RO. As regards feed water quality, ion exchange is more forgiving than RO. Slow fouling of the RO membranes can be noticed from decreasing flux or from increasing feed pressure. A ruptured membrane always requires an immediate plant shutdown. There is no sudden failure with ion exchange, processes such as slow fouling of the resin are time consuming.

RO removes all the particles very effectively, but suitable pretreatment must be involved to remove the suspended solids, iron and hardness from the feed water. IX effluent quality is almost independent of the temperature; RO permeate conductivity will be made higher by the rising temperature and lowering pressure correspondingly. RO loses up to 20% of the flux (capacity) with a 10 °C temperature decrease.

Low pressure RO membranes working on very low pressure - as low as 5-8 bars - remarkably save energy compared to high to medium pressure membranes. Conventional RO requires up to 10 times more kWh to operate than ion exchange. In large-scale production, energy consumption creates a noticeable expense. Comparing different methods, the type of IX plant and the type of RO unit - especially the type of membranes - must be observed and considered.

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