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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

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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