13.2 Ion Exchange & Inorganics Removal (Subclass 9)

Ion Exchange as a Unit Process

Ion exchange passes water through a bed of resin beads that hold loosely bound ions and swap them for ions in the water. The DEP subclassification pairs ion exchange with greensand because both are regenerable media beds operated on a service and regeneration cycle, and both are commonly the only treatment at small Pennsylvania groundwater systems.

The selectivity order matters. A standard strong base anion resin prefers sulfate over nitrate, which creates the peaking behavior described below.

Capacity and Run Length

Resin capacity is rated in kilograins of hardness as calcium carbonate removed per cubic foot of resin, commonly 20 to 30 kilograins per cubic foot at typical salt doses. The operator-level calculation is:

• Grains per gallon of hardness equals hardness in mg/L as calcium carbonate divided by 17.1.
• Total capacity in grains equals resin volume in cubic feet times rated capacity in kilograins per cubic foot times 1,000.
• Gallons to exhaustion equals total capacity in grains divided by grains per gallon.

Higher salt dosages during regeneration buy more capacity per cubic foot, but with diminishing returns and rising brine strength and volume. Selecting the salt dose is therefore a direct trade between run length and waste load.

The Regeneration Cycle

1. Backwash. Upflow at a rate that expands the bed, removing accumulated particulates and reclassifying the resin. Skipping backwash lets the bed channel.
2. Brine draw and slow rinse. Concentrated sodium chloride solution, typically 10 to 26 percent, is drawn slowly through the bed. Contact time is what actually regenerates the resin, so this step is deliberately slow. The slow rinse that follows displaces brine at the same low rate.
3. Fast rinse. Full-rate service water rinses residual brine to waste until conductivity or chloride falls to the service standard.
4. Return to service.

The two classic failures are short-cycling the slow rinse, which sends salty water to the distribution system and generates customer complaints and possible sodium concerns, and regenerating on time rather than on volume, which either wastes salt or breaks through on hardness.

Nitrate and Chromatographic Peaking

Nitrate is a health-based MCL at 10 mg/L as nitrogen and is acutely dangerous to infants. On a standard chloride-form anion resin the bed captures both sulfate and nitrate, but because the resin prefers sulfate, continued operation past exhaustion causes sulfate to displace previously captured nitrate, releasing it in a slug at a concentration higher than the raw water. An operator who runs a standard resin past breakthrough can therefore deliver water worse than untreated.

Controls:

• Use a nitrate-selective resin, which prefers nitrate over sulfate.
• Regenerate on treated volume, monitored against the calculated capacity, not on a clock.
• Monitor nitrate at the effluent and blend with a controlled fraction of raw water only where the blend is continuously verified.

Other Inorganic Contaminants

Resin Fouling and Loss

• Iron fouling coats beads and reduces capacity; symptoms are shortened runs and rusty backwash. Treated with an acid or a reducing cleaner.
• Organic fouling of anion resin by natural organic matter produces gradual capacity loss and color throw; treated with a warm brine and caustic soak.
• Oxidant damage. Free chlorine attacks resin structure, so chlorinated water must not be applied to a resin bed unless the resin is rated for it. Dechlorinate ahead of the vessel.
• Physical loss. Excessive backwash rate carries beads out the top; a broken underdrain lets them out the bottom, and the giveaway is resin appearing in the distribution system or in a downstream strainer.

The Brine Problem

Every regeneration produces a short, high-strength waste containing the salt used plus everything removed. Typical volumes are 2 to 5 percent of the water treated but the strength is extreme. Discharge options are a permitted sanitary sewer discharge, which may be limited by the receiving wastewater plant's ability to tolerate chloride and by its own permit limits, or a surface water NPDES discharge, which is difficult to obtain for total dissolved solids. Where radium or uranium is involved, the waste may be subject to radioactive material handling requirements. Securing the disposal route is part of selecting the technology, not an afterthought.