POWER March 2016 - 24

WATER & WASTEWATER
Desalination Technologies
O is a well-understood process
Separating salts and other impurities
from H2
with a long history. Desalination of seawater,
brackish water, and recycled water
is widely practiced around the world in a
variety of ways.
Generally, two types of separation technologies-thermal
and membrane-dominate,
each with about half of the global
market (for a more detailed discussion,
see " Adding Desalination to Solar Hybrid
and Fossil Plants " in the May 2010 issue
online at powermag.com).
Thermal Desalination
Thermal technologies use heat to vaporize
seawater, condensing the steam as pure
water. The three approaches used with
thermal desalination are multi-stage flash
distillation (MFD), multi-effect distillation
(MED), and vapor compression distillation
(VCD).
In MFD, feedwater is heated under high
pressure and then flows as a liquid into
a successive series of chambers with progressively
lower pressures. Because each
stage is lower in pressure than the one
before, the liquid water continues to flash
to steam, which is collected by heat exchange
tubing running through each
stage. MFD technology dates to the 1950s.
MFD plants dominate the thermal sector,
and many have been built in the Middle
East, where water is scarce but energy resources
are cheap and plentiful.
MED was first used in the late 1950s
and early 1960s. In MED plants, a series
of evaporator vessels are held at progressively
lower temperatures and pressures.
Because the boiling point of water decreases
as pressure decreases, the vapor
boiled off in one vessel can be used to
heat the next, and only the first vessel
requires an external source of heat. Three
MED plants with combined capacity of 3.5
tion plants operating around the world, with
the largest in Saudi Arabia, the UAE, and
Israel. The Saudi Shoaiba complex produces
over 232 million gallons (880,000 m3) daily,
while the Al Jubail complex produces over
211 million gallons (800,000 m3) per day.
The big Saudi plants use a variety of desalination
technologies.
Israel's Sorek plant (Figure 2) is the largest
RO plant in the world, with a daily output of
24
million gallons (13,250 m3) per day operate
in the U.S. Virgin Islands, serving as
the principal water supply.
VCD uses heat from compression of vapor,
rather than an external heat source.
Typically, a mechanical compressor is
used, often powered by a diesel engine.
These desalination units are generally
small and can be used at hotels, resorts,
and in industrial applications.
Membrane Desalination
Membrane technologies separate salts from
water using exceptionally fine screens or
membranes. The two categories are electrodialysis
(ED) and reverse osmosis (RO).
ED, introduced in the 1960s, is voltagedriven
and generally used for treating
brackish water. Most salts dissolved in
water are negatively or positively charged
ions. The technology uses electrodes of
opposite charge to attract the ions, with
membranes to permit selective passage of
either positively charged cations or negatively
charged anions. An ED stack consists
of several hundred such cells that the
feedwater is pumped through.
RO is the latest technology, commercialized
in the 1970s and based on Israeli
research and development. It is the most
widely used desalination technology in the
U.S. This process reverses normal osmosis-in
which a solvent moves from zones
of low solute concentration to zones of
high concentration-by applying pressure
to the zone of high concentration. This
causes the pure solvent-in this case, purified
water-to flow continuously to the
low-concentration side of the membrane.
RO works for both seawater and brackish
water, and removes all impurities, not just
salt.
ED and RO can be used together, with
the ED stack treating both the RO feedwater
and its brine stream.
165 million gallons (624,000 m3). The developer,
IDE Technologies, is a key player in the
Carlsbad, Calif., project and also has projects
under way or under development in Israel, China,
India, Australia, Chile, Italy, and Germany,
using a variety of desalination technologies.
Desalination's Long History
Turning seawater into drinking water is nothing
new. Greek sailors boiled seawater to get
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potable water. Romans used clay to filter
brackish water to drink and boiled seawater
in ceramic urns, although they may have
been as interested in retrieving the salt as the
water.
In 1791, then-U.S. Secretary of State
Thomas Jefferson offered a technical paper
describing a simple way to distill seawater
for sailors to drink. It was printed on the back
of shipboard papers to ensure a source of
freshwater in an emergency.
Australia's Victoria state government recounts
" wood-fired stills at the Coolgardie
goldfields 100 years ago, solar ponds at Coober
Pedy and electrodialysis for the first plant
at Yulara. "
When steam ships began to ply the high
seas in the 19th and 20th centuries, seawater
conversion technologies allowed sailors
to create freshwater as needed, freeing holds
to store more lucrative cargos. According to
a paper by Rensselaer Polytechnic Institute,
" By the Second World War, hundreds of mobile
desalination units were in use and all
major vessels had them. " The technology was
known in those days as " desalting. "
A feared " water crisis " after World War
II pushed the U.S. Congress in 1952 to pass
the " Saline Water Act, " establishing an Office
of Saline Water in the Interior Department.
A 1958 article in the New York Times proclaimed
hyperbolically, " In the future-not
so long as time is measured-Americans will
be taking the 'water cure' on a mass scale.
That does not, however, presage a large drop
in the intake of hard liquor. Rather, it means
that many communities, industrial plants and
other organizations probably will be using
fresh water distilled from the sea. "
The Interior Department's desalination
office went through several reorganizations
over the years. It funded research and development
(R&D) of desalination technologies
and left behind an archive of articles and papers.
It no longer exists as a separate government
bureaucracy.
One of the first demonstration plants built
with federal funds was in Freeport, Texas, in
1961. Dow Chemical erected a million-gallon-per-day
plant using thermal technology
at a cost of $1.2 million. It produced water
for Dow operations and the city of Freeport.
President John Kennedy officially started the
plant by pressing a button from his office
in the White House, an event that mirrored
President Dwight Eisenhower kicking off the
Shippingport nuclear plant in Pennsylvania
in 1957 from the Oval Office. Then-Vice
President Lyndon Johnson attended the Freeport
event in Texas, his home state.
Dedicating the plant, Kennedy said, " No
water resources program is of greater longrange
importance than our efforts to convert
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POWER March 2016

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