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 Influent 
controls. 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).