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8/13/2019 8_WaterDesalinationProcesses
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America’s Authority in Membrane Treatment
Improving America’s Waters Through Membrane Treatment and Desalting
Water desalting, or desalination, has long
een utilized by water-short nations world-wide to produce or augment drinking water
upplies. The process dates back to the 4thentury BC when Greek sailors used an
vaporative process to desalinate seawater.Today, desalination plants worldwide havehe capacity to produce over 11 billion
allons a day – enough water to providever 36 gallons a day for every person in the
United States. About 1,200 desalting plantsre in operation in USA.
n the United States, water is relativelynexpensive compared to many other parts
f the world. However, the vagaries of weather, population growth and subsequent
ncreases in demand for water in arid, semi-rid and coastal areas are contributing to aeightened interest in water desalination as
means to augment existing supplies. Inddition, many communities are turning to
esalting technologies as a cost-effective
method of meeting increasingly stringentwater quality regulations. Most potable wateresalination plants in the United States utilize
membrane processes for desalting brackish
moderately saline) water and for softening nd organics removal in ground water (low
aline) supplies. However there are severalarge seawater plants in the planning phase.
Desalination is generally divided into tworimary categories: Distillation Processes and
Membrane Processes.
Thermal (Distillation) Processes
Nature, through the hydrologic cycle,rovides our planet with a continuousupply of fresh, distilled water. Water
vaporates from the ocean (seawater) andther water bodies, accumulates in clouds as
apor, and then condenses and falls to theEarth’s surface as rain or snow (fresh water).
Distillation desalting processes work in theame way. Over 60 percent of the world’sesalted ocean water is produced by boiling
eawater to produce water vapor that is thenondensed to form fresh water.
ince thermal energy represents a large portion
f the overall desalting costs, distillation
Water Desalination Processes processes often recover and reuse waste heat
from electrical power generating plants todecrease overall energy requirements. Boiling
in successive stages each operated at a lowertemperature and pressure can also significantly
reduce the amount of energy needed.
Vapor phase, or evaporative processes are used
primarily for seawater conversion, and consistof the following well established methods:
•
Multistage flash evaporation (MSF)• Multieffect distillation (MED)
• Vapor compression (VC)
MSF and MED require thermal input in
addition to electric power, and because theyhandle hot seawater, materials selectionbecomes a critical factor in design. VC uses
only electric power, with the thermal inputcoming from heat of compression. VC is
generally the most economical evaporative process, but the fan compressors that are used
limit the output capacity of the equipment.
Depending on the plant design, distilledwater produced from a thermal desalination plant typically has salt concentrations of between 5 to 50 parts per million (ppm) of
Total Dissolved Solids (TDS). Between 25and 50 percent of the source water is
recovered by most distillation methods.
Membrane Processes
In the late 1940s, researchers began examin-ing ways in which pure water could be
extracted from salt water. Significant researchwas done in the 1950’s at the University of
Florida to demonstrate semi-permeable(desalination properties) of cellulose acetate(CA) membranes. During the John F.
Kennedy administration, saline waterconversion to fresh water was a high priority
technology goal, “go to the moon and makethe desert bloom”, was the slogan. Sup-
ported by federal and state funding, a number of researchers advanced the scienceand technology of saline water conversion,
but UCLA made a significant breakthroughin 1959 and became the first to demonstrate
to be practical a process known as reverse
osmosis (RO).
Professor Sidney Loeb and engineer Ed Selover remov
manufactured reverse osmosis membrane from plate-
frame product ion unit circa 1960.
About the same time, some researchers investigating a non-permeable membran
technology known as electrodialysis. Bothe electrodialysis (ED) and Reverse
Osmosis (RO) processes use membraneseparate dissolved salts from water.
Electrodialysis
Electrodialysis is an electrochemical procewhich the salts pass through the cation aanion membranes, leaving the water behIt is a process typically used for brackish w
Because most dissolved salts are ionic (eit positively or negatively charged) and the i
are attracted to electrodes with an opposielectric charge, membranes that allow sele
passage of either positively or negativelycharged ions accomplish the desalting.Freshwater recovery rates for this type of
process range from 75 to 95 percent of thsource water.
Reverse Osmosis
When two solutions with differentconcentrations of a solute are mixed, th
total amount of solutes (i.e. salts) in the
ED / EDR Schematic
Saline Feed
Negative Polarity
caca
c
ac
Cation
Membr
Anion
Memb
Positive Polarity +
Na+ Cl-
Na+
Na+
Na+
Cl-
Cl-
Cl-
-
Product
8/13/2019 8_WaterDesalinationProcesses
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(FS-8) Feb. 2
wo solutions will be equally distributed inhe total amount of solvent (i.e. water) from
he two solutions. In the natural occurring henomenon of osmosis this is achieved by
iffusion, in which solutes will move from
reas of higher concentration to areas of
ower concentrations until the concentrationn both sides of a membrane and theesulting mixture are the same, a state called
quilibrium . Equilibrium occurs when theydrostatic pressure sifferential resulting rom the concentration changes on both
ides of the semi-permeable membrane isqual to the osmotic pressure of the solute.
n Reverse Osmosis, salt water on one side
f a semi-permeable plastic membrane isubjected to pressure, causing fresh water toiffuse through the membrane and leaving
ehind a more concentrated solution thanhe source supply containing the majority of
he dissolved minerals and other contami-ants. The major energy requirement for
everse osmosis is for pressurizing theource, or “feed” water.
Depending on the characteristics of the feedwater, different types of membranes may be
sed. Because the feed water must passhrough very narrow passages as a result of he way the membrane packaged, fine
articulates or suspended solids must beemoved during an initial treatment phase
pretreatment). Brackish water RO plantsypically recover 50 to 80 percent of the
ource water and seawater RO recovery rates
ange from 30 to 60 percent.
A “loose” version of RO nanofiltrationNF) plants typically operate at 85 to 95
ercent recovery, which is typically used forrganic removal and softening (reducing alcium and magnesium hardness).
Applying the TechnologyNo one desalting process is necessarily “the
est.” A variety of factors come into play inhoosing the appropriate process for a
articular situation. These factors include theuality of the source water, the desired
uantity and quality of the water produced,
pretreatment, energy and chemical require- ments, and methods of concentrate disposal.
Uses of DesaltingThe conversion of seawater to drinking
water is the most publicly recognized use of desalination. Desalination is also used for
improving the quality of drinking waterfrom marginal or brackish sources. Mem-brane desalting technologies are also used in
home or tap water treatment systems, inindustrial wastewater treatment to reclaim
and recycle, to produce high-quality water forthe semi-conductor and pharmaceutical
industries and for the treatment andrecycling of domestic wastewater.
Membrane desalting technologies are not onlyused to remove salt and other dissolved minerals from water but in addition contami-
nants, such as dissolved heavy metals,radionuclides, pathogens, arsenic, bacterial
and dissolved organic matter may also beremoved in a variety of methods.
In the last twenty years, there has been asignificant reduction in power requirements
of membrane desalination technologies,with improvements in membrane salt
rejection and flux properties. As an exampleIsland Water Association’s 5 MGD BrackishRO WTP originally installed in 1980 with
CA membrane elements operating at 550 psi
and 75% recovery with 10% salt passagecurrently operates non-CA membrane thin-film composite polyamide (TFCPA) at 170
psi with only 5% passage. In fact this plantwas a pioneer initially converting to TFCPA membrane elements in 1984. In charted
performance from December 1986 to October1998, this plant experienced a decrease from
3.9 KWH / Kgal to 2.7 KWH / Kgal.
Desalination FutureWater from desalination is not bound by
many of the conditions that plague traditionalfresh water development. The increase in the
public awareness of the environmental
problems associated with fresh water sourcoupled with the new, more stringent drin
water quality regulations make developme new traditional water resources more difficand costly. Unlike traditional water supplie
alternative desalted water supplies are not
vulnerable to weather (droughts).Membrane desalting technologies also a plants to be built in stages to meet dem
unlike traditional water development wits high initial capital outlay. Finally, memb
desalted water is in many cases comparain cost to water from traditional water
supplies, especially if utilized to augmencurrent supplies.
From initial experiments conducted in t1950’s which produced a few drops perhour, the reverse osmosis membrane
industry has today resulted in combinedworldwide production in excess of 6.8
billion gallons per day. With demand fo pure water ever-increasing, and water
shortages world-wide, the growth of threverse osmosis industry is poised well
the next century.
In the early 1980’s, research in US Gover
ment Labs resulted in the first ComposPolyAmide membrane. This membranesignificantly higher permeate flow and b
salt rejection than CA membranes. Tod
with the advancement of non-CA thin-composite polyamide membrane elemethe industry has attained a 20-times incr
in energy efficiency over the original CA membrane elements, with a similar ord magnitude decrease in salt passage.
Experts around the world continue to
research better membranes for desalinatas well as membranes for contaminantremoval, water reclamation, re-use, and
industrial applications. The primary focthe research is on reduction in energy
requirements and making elements withhigher and selective rejection properties,
while minimizing fouling tendencies.
This material has been prepared as an educational tool by
American Membrane Technology Association (AMTA). I
designed for dissemination to the public to further the
understanding of the contribution that membrane water
treatment technologies can make toward improving the qu
water supplies in the US and throughout the world.
For more information, please contact:
American Membrane Technology Associ
2409 SE Dixie Hwy., Stuart, Florida 3499
Phone: (772) 463-0820 Fax: (772) 463-0860
Email : admin@amtaorg.com
or visit our website at: ww w.am taorg.co
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