Softening Methods Of Waters With High Total Dissolved Solids

Softening Methods Of Waters With High Total Dissolved Solids

Soft water is required to generate steam without formation of harmful scale on the tubes of the boiler. Softening of low TDS (Total Dissolved Solids) water is done commonly with a Strong Acid Cation (SAC) exchange resin operating in the sodium form and regenerated with brine (Sodium Chloride). As the TDS of the water to be softened increases, removal of hardness to low levels becomes more difficult and processes such as counter-flow regeneration need to be used. As the TDS increases even higher, a weak acid cation (WAC) exchange resin has to be used.

WAC resin is much less affected by salt concentration in the water to be softened, which also means that brine cannot be used efficiently for regeneration. Regeneration in this case, is done by regenerating the resin first to the hydrogen form with acid, followed by conversion to the sodium form with a sodium alkaline solution such as caustic (NaOH).

Weak acid cation exchange resins are also used very efficiently to reduce the alkalinity of water and this property can be used to soften and dealkalize waters that contain a higher level of alkalinity than hardness.

Low TDS water (< 5000 ppm) can be softened using SAC exchange resin in the sodium form (effluent hardness < 1ppm). This type of resin is similar to those used for home water softeners. As water, containing hardness (calcium & magnesium), flows over the resin in the sodium form, the calcium and magnesium are exchanged for the sodium from the resin. As a result, the effluent water no longer contains calcium or magnesium, which is soft water. This process is made possible because SAC resin has a higher selectivity for calcium and magnesium than for sodium. After treating a certain volume of water, the SAC resin will become saturated with calcium and magnesium and therefore, will no longer soften the water. The resin needs to be regenerated by replacing the calcium and magnesium with sodium.

Since the resin has a higher selectivity for calcium and magnesium than for sodium, the only way to displace the calcium and magnesium off the resin, with sodium, is by feeding the sodium at a sufficiently high concentration. Normally 10 to 12% brine is used for regeneration.

Sodium Chloride is commonly used for regeneration because it is an inexpensive source of sodium. In some cases KCI is use as regenerant. This converts the resin to the potassium form, which is similar to the sodium form.

Most softeners are operating down-flow for service, which means that the water to be softened enters at the top of the ion exchange vessel and leaves from the bottom. Two methods are available for regeneration: Co-Current Regeneration, which consists of injecting the brine in the same direction as the service flow (from top of the bed down) and Counter-Current Regeneration (CCR), where the regenerant is injected in the opposite direction of the service flow (from the bottom of the bed up).