### ION EXCHANGE/ADSORPTION

Ion exchange (IX) and adsorption processes

are used to remove dissolved ions and other charged species from water. IX processes are

reversible chemical reactions that remove dissolved ions from solution and replace them with other similarly charged ions. Adsorption processes rely on surface charges to adsorb charged ionic species.

Most IX and adsorption processes in water treatment operate in a continuous mode. Ion exchange or adsorption occur as water flow (typically in a down-flow mode) through a packed-bed of IX resin or adsorption media.

In water treatment, the most common IX process is cation exchange softening in which calcium and magnesium are removed. Radium can also be removed from drinking water by cation exchange. Anion exchange processes can be used for the removal of contaminants such as nitrate, fluoride, perchlorate, uranium, selenium, arsenic, sulfate, and natural organic matter (NOM), as well as others. Adsorption processes, such as activated alumina and granular ferric hydroxide, are used to remove arsenic and similar species.

Competition for ion exchange or adsorption sites can greatly impact a given system’s efficiency in removing contaminants. Generally, ions with higher valence, greater atomic weights and smaller radii are preferred by IX resins and adsorption media. Competing ions lead to a reduction in capacity for the target contaminant. When the capacity of the IX resin is exhausted, it is necessary to regenerate the resin using a saturated solution of the exchange ion (e.g.,

Na + or Cl

- )

Anion exchange processes will generally preferentially remove sulfate over other target contaminants. Removal of sulfate and increased chloride concentrations (as a result of the exchange) can cause an increase in the chloride-to-sulfate ratio, which has been

demonstrated to causes increases in lead corrosion in some distribution systems.

Strong acid anion exchange resins are generally available in two types: Type I and Type II. Type I resins contain trialkyl ammonium chloride or hydroxide, and Type II contain dialkyl 2-hydroxyethyl ammonium chloride or hydroxide. Type II resins have

been demonstrated to release nitrosamines when preceded by chlorination in the treatment process or by an initial rinse following installation or regeneration.

The cation exchange functional group includes sodium, which can result in an

increase in finished water sodium concentrations. Although there is no maximum contaminant level (MCL) for sodium, USEPA has established a Drinking Water Equivalence Level (i.e., guidance level) for sodium of 20 mg/L.

Depending on the TDS and other contaminant concentrations in the spent regenerant, it may be necessary to evaluate the impacts on wastewater treatment plant discharges and National Pollutant Discharge Elimination System (NPDES) requirements. Similarly, disposal of spent media with high

concentrations of removed contaminants may require disposal as a hazardous waste.

Approximate capital and operations and maintenance (O&M) costs for IX and adsorption are provided in Table 1 . Capital

costs include the addition of IX or adsorption beds, chemical storage, associated piping and valves, and instrumentation and Contaminated Influent

Resin or Adsorption

Media Bed Upper Distributor

Underdrain

Treated Influentcontrols. O&M costs include regenerant/chemicals, power, replacement parts, and maintenance labor. Costs do not include pH adjustment (which can enhance the treatment process and can represent a significant O&M expenditure).

Table 1. Approximate Costs of Ion Exchange and Adsorption

Design Flow (mgd) 0.01 0.1 1.0 10 100

Average Flow (mgd) 0.005 0.03 0.35 4.4 50

Capital Cost ($/gal) 1 $2.50 $0.75 $0.50 $0.50 $0.50

Annual O&M Cost ($/kgal) 2 $3.00 $1.00 $0.75 $0.50 $0.50

1. Capital costs are based on $ per gallon of treatment plant capacity. For example, addition of IX at a treatment facility with a capacity of 10,000 gpd would be expected to cost approximately $25,000 ($2.50/gal  10,000 gal = $25,000). 2. Annual O&M costs are based on $ per thousand gallons treated. For example, annual O&M costs for a system with an average daily flow of 5,000 gallons (5 kgal) would be approximately $5,475 ($3.00/kgal  5 kgal/day  365 days/year = $5,475).