The Process and Water Quality Specialists

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Ion Exchange

Ion exchange is a technology that allows, as the name implies, for the exchange of ions from the aqueous phase to the ion exchange resin phase.  The ion exchange resin is “presaturated” with an ion that exchanges with the ions in solution.  There are two general types of ion exchange resins, cation exchange resins and anion exchange resins, with the cation exchange resins removing positively-charged ions and anion exchange resins removing negatively charged ions.  The presaturant ion in the case of cation exchange resins is typically sodium though hydrogen is sometimes used.  The presaturant ion in the case of anion exchange resins is typically chloride, though hydroxide is sometimes used.  The cation exchange resins are further divided into two types: strong acid cation (SAC) and weak acid cation (WAC) ion exchange resins.  The anion exchange resins are also classified in two types:  strong base anion (SBA) and weak base anion (WBA) exchange resins.  After the ion exchange process runs for some time, the resin bed is exhausted and the ions breakthrough similar to the adsorption process.  At that point it is necessary to “regenerate” the resin, which is typically accomplished by running a highly concentrated salt solution through the bed to remove the ions and return the presaturant to the resin.  WAC and WBA resins are sometimes used because the ions are less strongly bonded to the resin phase compared to SAC and SBA resins, making it easier to remove the ions during the regeneration step.

Ion exchange technology has a rich history.  When the Colorado River Aqueduct was first built to deliver Colorado River Water from the Parker Dam to Southern California, it was thought to be too hard for potable use.  As a consequence, when the first large water treatment plants were built to treat Colorado River Water, they were designed to soften the water.

The first was the Weymouth Water Treatment plant in La Verne, CA.  Weymouth was built to include the capability for softening with both lime precipitation, as well as ion exchange.  At it peak, between 1946 and 1970, the plant continuously operated a 200 mgd ion exchange softening facility, the largest the world has ever seen.  The ion exchange was accomplished using polystyrene strong acid cation gel resin of the same type that is still used today.  The resin was placed in concrete gravity flow filters, each containing approximately 4,000 cf of resin.  A brine line 22 miles in length was constructed in order to connect the plant to ocean-bound sewers, which were used for brine disposal.

A similar plant of half the capacity was built in San Diego, but the ion exchange process was never operated because the brine line was never completed.  Today the City of Santa Monica uses concrete, gravity softeners of similar design in a smaller 10 mgd softening facility used on local groundwater.  The softening at the Weymouth plant was abandoned in the early 1970s, following the introduction of the much softer water supply from Northern California.  More recent applications of ion exchange technology include removal of perchlorate and nitrate from contaminated groundwater