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NATIONAL DRINKING WATER CLEARINGHOUSE

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A NATIONAL DRINKING WATER CLEARINGHOUSE FACT SHEET

Ion Exchange and

Demineralization

Summary

Ion exchange and membrane processes are becoming used extensively in water and wastewater

treatment. Ion exchange is primarily used for the removal of hardness ions, such as magnesium and

calcium, and for water demineralization. Reverse osmosis (RO) and electrodialysis, both membrane

processes, remove dissolved solids from water using membranes.

Figure 1. Ion exchange treatment system

systems where there is a high degree of seasonal fluctuation in water demand.

Electrodialysis is a process that also uses

membranes. However, in electrodialysis, direct

electrical current is used to attract ions to one

side of the treatment chamber. Electrodialysis

systems include a source of pressurized water,

a direct current power supply, and a pair of

selective membranes.

Ion exchange units can be used to remove any

charged (ionic) substance from water, but are

usually used to remove hardness and nitrate

from groundwater.

Water is pretreated to reduce the suspended

solids and total dissolved solids (TDS) load to

the ion-exchange unit. Methods of pretreatment

include:

• filtration,

• coagulation and filtration,

• cold lime with or without

soda ash,

• hot lime with or

without soda ash,

• evaporation or

distillation,

• electrodialysis,

• RO,

• continuous deionization,

• ultrafiltration,

• degasification, or

• combinations of

the above.

(Source: Owens,

1995)

RO systems are

compact, simple to

operate, and require

minimal labor, making them suitable for

small systems. They

are also suitable for

Source: U.S. Environmental Protection Agency, 1989

Ion Exchange and Demineralization Are Becoming Widely Used

Salt Loading

Brine

Tank Overflow

Potable

Water

to Brine

System

Pressure Tank

Distribution

System

Well Supply

Alarm and

Shutdown

Conductivity

Monitor

Nitrate

Analysis

Backwash

Waste to

Disposal

Vessel 1

Blending Valves

Treated Water

Raw Water

Brine

Vessel 2 Vessel 3
PAGE OF FOUR

 FOUR • May 1997

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Ion exchange

Ion exchange effectively removes more than 90 percent

of barium, cadmium, chromium (III), silver, radium,

nitrites, selenium, arsenic (V), chromium (VI), and nitrate.

Ion exchange is usually the best choice for small systems

that need to remove radionuclides.

ADVANTAGES

• Ion exchange process, like reverse osmosis, can

be used with fluctuating flow rates.

• Effluent contamination is virtually impossible.

• Large variety of specific resins are available from

suppliers. Each resin is effective in removing

specific contaminants.

LIMITATIONS

• Ion exchange waste is highly

concentrated and requires careful

disposal.

• Potential for unacceptable levels

(peaks) of contamination in effluent.

• Usually not feasible with high levels

of TDS.

• Pretreatment required for most

surface waters.

• Ion exchange units also are

sensitive to the presence of

competing ions. For example,

influent with high levels of hardness

will compete with other cations

(positive ions) for space on the

exchange medium, and the

exchange medium must be

regenerated more frequently.

PROCESS

Inorganics removal is accomplished

through adsorption of contaminant

ions onto a resin exchange medium.

As the name implies, one ion is

substituted for another on the charged surface of the

medium, which is usually a synthetic plastic resin. This

resin surface is designed as either cationic or anionic

(negatively charged). The exchange medium is saturated

with the exchangeable ion before treatment operations.

During ion exchange, the contaminant ions replace the

regenerant ions because they are preferred by the

exchange medium. When there are no ions left to replace

the contaminant ions, the medium is regenerated with a

suitable solution, which resaturates the medium with the

appropriate ions. Because of the required “down time,”

the shortest economical regeneration cycles are once

per day.

The resin exchange capacity is expressed in terms of

weight per unit volume of the resin. The calculation of the

breakthrough time for an ion exchange unit requires

knowledge of the resin exchange capacity, the influent

contaminant concentration, and the desired effluent

quality.

EQUIPMENT

Typical ion exchange units consist of prefiltration, ion

exchange, disinfection, storage, and distribution elements. (See figure 1 on page 1.)

CHEMICALS

Sodium chloride is often used to regenerate the exchange

medium in ion exchangers because of the low cost of the

chemical. However, this can result in a high sodium

residual in the finished water, which may be unacceptable

for individuals with salt restricted diets. This problem can

be avoided by using other regenerant materials, such as

potassium chloride.

Reverse osmosis (RO)

RO can effectively remove nearly all inorganic contaminants from water. It removes more than 70 percent of

arsenic (III), arsenic (IV), barium, cadmium, chromium

(III), chromium (VI), fluoride, lead, mercury, nitrite,

selenium (IV), selenium (VI), and silver. Properly

operated units will attain 96 percent removal rates. RO

can also effectively remove radium, natural organic

substances, pesticides, and microbiological contaminants. RO is particularly effective when used in series.

Water passing through multiple units can achieve near

zero effluent contaminant concentrations.

ADVANTAGES

• Removes nearly all contaminant ions and most

dissolved non-ions.

• Relatively insensitive to flow and TDS level, and thus

suitable for small systems with a high degree of

seasonal fluctuation in water demand.

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Ion Exchange/Demineralization Systems:

Figure 2. Schematic of a reverse osmosis system aaaaaaaaaaaaaaa aaaaaaaa

Source: U.S. Environmental Protection Agency, 1989

Generator Set

High Service Pumps

Brine to

Percolation

Pond

Permeators

Control Panel

and Pumps

Product to Degasifier

and Reservoir

PO4 Feed

Feed

Acid Feed Filter

Mixing

Tank Cleaning

Solution

Sodium

Silicate Feed
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NATIONAL DRINKING WATER CLEARINGHOUSE

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• RO operates immediately, without any minimum

break-in period.

• Low effluent concentration possible.

• Bacteria and particles are also removed.

• Operational simplicity and automation allow for less

operator attention and make RO suitable for small

system applications.

LIMITATIONS

• High capital and operating costs.

• Managing the wastewater (brine solution) is a

potential problem.

• High level of pretreatment is required in some cases.

• Membranes are prone to fouling.

PROCESS

RO removes contaminants from water using a semipermeable membrane that permits only water, and not

dissolved ions (such as sodium and chloride), to pass

through its pores. Contaminated water is subject to a

high pressure that forces pure water through the

membrane, leaving contaminants behind in a brine

solution. Membranes are available with a variety of pore

sizes and characteristics.

EQUIPMENT

Typical RO units include raw water pumps, pretreatment,

membranes, disinfection, storage, and distribution

elements. (See figure 2 on page 2.) These units are able

to process virtually any desired quantity or quality of

water by configuring units sequentially to reprocess

waste brine from the earlier stages of the process. The

principal design considerations for reverse osmosis

units are:

• operating pressure,

• membrane type and pore size,

• pretreatment requirements, and

• product conversion rate (the ratio of the influent

recovered as waste brine water tothe finished water).

Electrodialysis

Electodialysis is very effective in removing fluoride and

nitrate, and can also remove barium, cadmium, and

selenium. (See figure 3 below.)

ADVANTAGES

• All contaminant ions and most dissolved non-ions

are removed.

• Relatively insensitive to flow and TDS level.

• Low effluent concentration possible.

LIMITATIONS

• High capital and operating costs.

• High level of pretreatment required.

• Reject stream is 20–90 percent of feed flow.

• Electrodes require replacement.

PROCESS

The membranes adjacent to the influent stream are

charged either positively or negatively, and this charge

attracts counter-ions toward the membrane. The membranes are designed to allow either positively or negatively charged ions to pass through the membrane, thus

ions move from the product water stream through a

membrane to the two reject water streams. (See figure 4

on page 4.)

EQUIPMENT

The three essential elements of the system are (1) a

source of pressurized water, (2) a direct current power

supply, and (3) a pair of selective membranes. The

average ion removal varies from 25 to 60 percent per

stage. Multistage units can increase the efficiency of

removal. Many membrane pairs are “stacked” in the

treatment vessel.

CHEMICALS

Fouling of membranes may limit the amount of water

treated. Fouling is caused when membrane pores are

clogged by salt precipitation or by physical obstruction of

suspended particulates.

Particulates, suspended in

water, can be removed in

pretreatment but salts that

exceed their solubility

product at the membrane

surface must be controlled

chemically by pH reduction

(to reduce carbonate

concentration) or chelation

of metal ions (by use of

phosphate, for example). A

reversal of the charge on the

membranes, a process

called electrodialysis

reversal (EDR), helps to

flush the attached ions from

the membrane surface, thus

extending the time between

cleanings.

Figure 3. Basic components of an

electrodialysis unit

Source: U.S. Agency for International Development, 1980

Raw

Feedwater

Special Treatment

(If Required)

Holding

Tank

LowPressure

Circulation

Pump

Cartridge

Filters

Electrode

Electrode

AC

Power

Source

DC Power

Supply

(Rectifier)

Membrane

Stack Product

Water

Concentrate

Discharge
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 FOUR • May 1997

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Information on ion exchange/demineralization

was primarily obtained from two sources:

Environmental Pollution Control Alternatives:

Drinking Water Treatment for Small Communities, EPA/625/5-90/025; and Technologies for

Upgrading Existing or Designing New Drinking

Water Treatment Facilities, EPA/625/4-89/023.

Both may be ordered free from the U.S. Environmental Protection Agency Office of Research and

Development at (513) 569-7562.

These publications also may be ordered from

the National Drinking Water Clearinghouse

(NDWC); however, copying costs apply. The first

Where can I find more information? book, item #DWBKGN09, an 82-page publication, costs $11.82; and the second, item

#DWBKDM04, a 209-page book, costs $30.05.

Shipping and handling charges also apply.

For further information or to order copies of

“Tech Brief: Ion Exchange and Demineralization,” item #DWBLPE56, “Tech Brief: Filtration,”

item #DWBLPE50, “Tech Brief: Disinfection,”

item #DWBLPE47, or “Tech Brief: Corrosion

Control,” item #DWBLPE52 call the NDWC at

(800) 624-8301 or (304) 293-4191. These fact

sheets are available at no cost, except for

shipping and handling charges.

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Many of the substances which make up the total dissolved solids

(TDS) in brackish water are strong electrolytes. When dissolved in

water they ionize; that is, the compounds dissociate into ions which

carry an electric charge. Typical of the ions in brackish water are

Cl-1, Na+1, HCO3-1, Mg+2, SO4-2, and Ca+2. These ions tend to

attract the dipolar water molecules and to be diffused in times, fairly

evenly throughout a solution.

If two electrodes are placed in a solution of ions and energized by a

battery or other direct current source, the current is carried through

the solution by the charged particles and the ions tend to migrate to

the electrode of the opposite charge.

If alternately fixed charged membranes (which are selectively

permeable to ions of the opposite charge) are placed in the path

of the migrating ions, the ions will be trapped between the alternate

cells formed.

Note 1: A positively fixed charge (anionic) membrane will allow

negative ions to pass, but will repel positive ions.

Note 2: A negatively fixed charge (cationic) membrane will allow

positive ions to pass, but will repel negative ions.

If this continued, almost all the ions would become trapped in the

alternate cells (concentrate cells). The other cells, which lack ions,

would have a lower level of dissolved constituents and would have

a high resistance to current flows.

The phenomenon illustrated above is used in electrodialysis to

remove ions from incoming saline water on a continuous basis.

Feedwater enters both the concentrate and product cells. Up to

about half of the ions in the product cells migrate and are trapped in

the concentrate cells. Two streams emerge from the device: One of

concentrated brine and the other with a much lower concentration of

TDS (product water).

Figure 4. Movement of ions in the electrodialysis process

Source: U.S. Agency for International Development, 1980

Battery

Positive

Electrode

Negative

Electrode

Switch

Note 1 Note 2

Anionic

Membranes

Cationic

Membranes

Concentrate Cells

Feedwater

Product Water

Brine