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04-10

Desalination Methods
for Producing Drinking Water
*Justin K. Mechell and Bruce Lesikar
s populations increase and sources of high- Source waters

A quality fresh drinking water decrease, many

communities have considered using desalina-
tion processes to provide fresh water when other
Several factors influence the selection of source
waters to feed desalination plants: the location of the
plant in relation to water sources available, the deliv-
sources and treatment procedures are uneconomical ery destination of the treated water, the quality of
or not environmentally responsible. the source water, the pretreatment options available,
Desalination is any process that removes excess and the ecological impacts of the concentrate dis-
salts and other minerals from water. In most desali- charge.
nation processes, feed water is treated and two
streams of water are produced: Seawater
• Treated fresh water that has low concentra- Seawater is taken into a desalination plant ei-
tions of salts and minerals ther from the water’s surface or from below the sea
• Concentrate or brine, which has salt and floor. In the past, large-capacity seawater desalina-
mineral concentrations higher than that of tion plants have used surface intakes on the open
the feed water sea.
The feed water for desalination processes can Although surface water intake can affect and be
be seawater or brackish water. Brackish water con- affected by organisms in the ocean, the issues related
tains more salt than does fresh water and less than to this method can be minimized or resolved by
salt water. It is commonly found in estuaries, which proper intake design, operation, and maintenance of
are the lower courses of rivers where they meet the technologies. The technologies include passive
sea, and aquifers, which are stores of water under- screens, fine mesh screens, filter net barriers, and
ground. behavioral systems. They are designed to prevent or
An early U.S. desalination plant was built in minimize the environmental impact to the sur-
1961 in Freeport, Texas. It produced 1 million gal- rounding intake area and to minimize the amount of
lons per day (mgd) using a long vertical tube distil- pretreatment needed before the feed water reaches
lation (LVT) process to produce water for the City of the primary treatment systems.
Freeport, Texas. As technology rapidly improves, Subsurface intakes are sometimes feasible if the
two other processes—thermal and membrane—are geology of the intake site permits. When the water is
becoming viable options to convert saline water to taken in from below the surface, the process causes
drinkable fresh water. less damage to marine life. However, if the geology
* Extension Program Specialist, and Professor and Extension
Agricultural Engineer, The Texas A&M University System

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Figure 1. Example of a multi-stage flash distillation (MSF) process (Source: Buros, 1990).

of the site is unfavorable, a subsurface intake can tion (MED), and vapor compression distillation
harm nearby freshwater aquifers. Methods of sub- (VCD). Another thermal method, solar distillation,
surface intake include vertical beach wells, radial is typically used for very small production rates.
wells, and infiltration galleries. Membrane distillation technologies are prima-
A major advantage to using a subsurface intake rily used in the United States. These systems treat
is that the water is filtered naturally as it passes the feed water by using a pressure gradient to force-
through the soil profile to the intake. This filtration feed the water through membranes. The three major
improves the quality of feed water, decreasing the membrane processes are electrodialysis (ED), elec-
need for pretreatment. trodialysis reversal (EDR), and reverse osmosis
(RO).
Brackish water
Brackish water is commonly used as a source Thermal technologies
for desalination facilities. It is usually pulled from Multi-stage flash distillation
local estuaries or brackish inland water wells. Be- Multi-stage flash distillation is a process that
cause it typically has less salt and a lower concentra- sends the saline feed water through multiple cham-
tion of suspended solids than does seawater, bers (Fig. 1). In these chambers, the water is heated
brackish water needs less pretreatment, which de- and compressed to a high temperature and high
creases overall production costs. However, a disad- pressure. As the water progressively passes through
vantage is that disposing of the brine from an inland the chambers, the pressure is reduced, causing the
desalination location increases the cost and can raise water to rapidly boil. The vapor, which is fresh
environmental concerns. water, is produced in each chamber from boiling
and then is condensed and collected.
Desalination technologies
Two distillation technologies are used primarily Multi-effect distillation
around the world for desalination: thermal distilla- Multi-effect distillation employs the same prin-
tion and membrane distillation. cipals as the multi-stage flash distillation process ex-
Thermal distillation technologies are widely cept that instead of using multiple chambers of a
used in the Middle East, primarily because the re- single vessel, MED uses successive vessels (Fig. 2).
gion’s petroleum reserves keep energy costs low. The The water vapor that is formed when the water boils
three major, large-scale thermal processes are multi- is condensed and collected. The multiple vessels
stage flash distillation (MSF), multi-effect distilla- make the MED process more efficient.

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Figure 2. Example of a multi-effect distillation (MED) process (Source: Buros, 1990).

Vapor compression distillation solar distillation units vary greatly, the basic princi-
Vapor compression distillation can function in- pals are the same. The sun provides the energy to
dependently or be used in combination with an- evaporate the saline water. The water vapor formed
other thermal distillation process. VCD uses heat from the evaporation process then condenses on
from the compression of vapor to evaporate the feed the clear glass or plastic covering and is collected as
water (Fig. 3). VCD units are commonly used to fresh water in the condensate trough. The covering
produce fresh water for small- to medium-scale pur- is used to both transmit radiant energy and allow
poses such as resorts, industries, and petroleum water vapor to condense on its interior surface. The
drilling sites. salt and un-evaporated water left behind in the still
basin forms the brine solution that must be dis-
Solar distillation carded appropriately.
Solar desalination is generally used for small- This practice is often used in arid regions
scale operations (Fig. 4). Although the designs of where safe fresh water is not available. Solar distil-

Figure 3. Example of a vapor compression distillation (VCD) process (Source: Buros, 1990).
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 built to allow passage of ei-
ther positively or negatively
charged ions, but not both.
Ions are atoms or molecules
that have a net positive or
net negative charge. Four
common ionic molecules in
saline water are sodium,
chloride, calcium, and car-
bonate.
Electrodialysis and
electrodialysis reversal use
the driving force of an elec-
Figure 4. Example of a solar still desalination process (Source: Buros, 1990).
trical potential to attract
and move different cations
(positively charged ions) or
lation units produce differing amounts of fresh anions (negatively charged ions) through a perme-
water, according to their design and geographic lo- able membrane, producing fresh water on the other
cation. Recent tests on four solar still designs by the side (Fig. 5).
Texas AgriLife Extension Service in College Station, The cations are attracted to the negative elec-
Texas, have shown that a solar still with as little as trode, and the anions are attracted to the positive
7.5 square feet of surface area can produce enough electrode. When the membranes are placed so that
water for a person to survive.

Membrane technologies
A membrane desalination process
uses a physical barrier—the mem-
brane—and a driving force. The driving
force can be an electrical potential,
which is used in electrodialysis or elec-
trodialysis reversal, or a pressure gradi-
ent, which is used in reverse osmosis.
Membrane technologies often re-
quire that the water undergo chemical
and physical pretreatment to limit
blockage by debris and scale formation
on the membrane surfaces. Table 1
(page 5) details the basic characteristics
of membrane processes.

Electrodialysis
and electrodialysis reversal
The membranes used in electro-
Figure 5. Example of an electrodialysis process showing the basic
dialysis and electrodialysis reversal are movements of ions in the treatment process (Source: Buros, 1990).

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Table 1. General characteristics of membrane processes (Source: Metcalf and Eddy, 2003).

Membrane Membrane Typical Operating Typical Permeate Typical

process driving separation structure operating description constituents
force mechanism (pore size) range, (µm) removed

Microfiltration Hydrostatic Sieve Macropores 0.08–2.0 Water + TSS, turbidity,

pressure (> 50 nm) dissolved protozoan
difference or solutes oocysts and
vacuum in open cysts, some
vessels bacteria and
viruses

Ultrafiltration Hydrostatic Sieve Mesopores 0.005–0.2 Water + Macromolecules,

pressure (2–50 nm) small colloids,
difference molecules most bacteria,
some viruses,
proteins

Nanofiltration Hydrostatic Sieve + Micropores 0.001–0.01 Water + Small

pressure solution/ (< 2 nm) very small molecules,
difference diffusion + molecules, some
exclusion ionic solutes hardness,
viruses

Reverse Hydrostatic Solution/ Dense 0.0001– Water + Very small

osmosis pressure diffusion + (< 2 nm) 0.001 very small molecules,
difference exclusion molecules, color,
ionic solutes hardness,
sulfates,
nitrate, sodium,
other ions
Dialysis Concentration Diffusion Mesopores -- Water + Macromolecules,
difference (2–50 nm) small colloids,
molecules most bacteria,
some viruses,
proteins

Electrodialysis Electromotive Ion exchange Micropores -- Water + Ionized

force with selective (< 2 nm) ionic solutes salt ions
membranes

some allow only cations to pass and others allow Reverse osmosis
only anions to pass, the process can effectively re- Reverse osmosis uses a pressure gradient as the
move the constituents from the feed water that make driving force to move high-pressure saline feed
it a saline solution. water through a membrane that prevents the salt
The electrodialysis reversal process functions as ions from passing (Fig. 6).
does the electrodialysis process; the only difference There are several membrane treatment
is that in the reverse process, the polarity, or charge, processes, including reverse osmosis, nanofiltration,
of the electrodes is switched periodically. This rever- ultrafiltration, and microfiltration. The pore sizes of
sal in flow of ions helps remove scaling and other the membranes differ according to the type of
debris from the membranes, which extends the sys- process (Fig. 7).
tem’s operating life.

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 Because the RO
membrane has such
small pores, the feed
water must be pretreated
adequately before being
passed through it. The
water can be pretreated
chemically, to prevent
biological growth and
scaling, and physically,
to remove any sus- Figure 6. Basic components of a membrane treatment process.
pended solids.
The high-pressure
feed water flows through
the individual mem-
brane elements. The
spiral RO membrane ele-
ment is constructed in a
concentric spiral pattern
that allows alternating
layers of feed water and
brine spacing, RO mem-
brane, and a porous
product water carrier
(Fig. 8). The porous
product water carrier
allows the fresh water to
flow into the center of
the membrane element
to be collected in the
product water tube.
To enable each
pressure vessel to treat Figure 7. Range of nominal membrane pore sizes for reverse osmosis (RO),
nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF) (Source: Metcalf
more water, the individ- and Eddy, 2003).
ual membrane elements
are connected in series (Fig. 9). After the water chemicals that were either removed during the de-
passes through the membrane elements within the salination process or added to help pretreat the feed
pressure vessels, it goes through post treatment. Post water. For all of the processes, the brine must be dis-
treatment prepares the water for distribution to the posed of in an economical and environmentally
public. friendly way.
Options for discharging the brine include dis-
Concentrate management options charge into the ocean, injection through a well into a
Both thermal and membrane desalination saline aquifer, and evaporation. Each option has ad-
processes produce a stream of brine water that has a vantages and disadvantages. In all cases, the brine
high concentration of salt and other minerals or water should have a minimal impact on the sur-

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Figure 8. Cutaway view of a spiral reverse osmosis membrane element (Source: Buros, 1990).

rounding water bodies or aquifers. Specific consid- implementation, desalination processes need tech-
erations for the water quality include saline concen- nological improvements and increased energy
tration, water temperature, dissolved oxygen efficiency.
concentrations, and any constituents added as pre-
treatment.
References
Buros, O. K., The ABC’s of Desalting,
Summary International Desalination Association.
As high-quality freshwater resources decrease, Topsfield, Massachusetts. 1990.
more communities will consider desalination of Krishna, Hari J., Introduction to Desalination
brackish and salt water to produce drinking water. Technologies, Texas Water Development
All desalination technologies have advantages and Board. 2004.
disadvantages based on site-specific limitations and Metcalf and Eddy, Wastewater Engineering:
requirements. Small-scale desalination of brackish Treatment and Reuse. McGraw-Hill, Inc.,
water using solar stills is a promising method in re- New York. 2003.
mote locations where good-quality water for drink- Pankratz, Tom, Desalination Technology
ing and cooking is unavailable. For more widespread Trends, CH2M Hill, Inc. 2004.

Figure 9. Cross section of a pressure vessel with three membrane elements (Source: Buros, 1990).

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 This material is based on work supported by the
National Institute of Food and Agriculture, U.S. Department of Agriculture,
under Agreement No. 2009-34461-19772 and Agreement No. 2009-45049-05492.