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Reverse Osmosis is a
process in which a semipermeable membrane is used to separate fluids of
various qualities into a highly saturated concentrate (brine) and a high
quality permeated fluid low in dissolved solids. The separation is
accomplished by passing the fluid across the membrane at a specified
pressure and velocity. The membranes contain pores which approximate two
hundred molecular weight in diameter. This allows the fluid and
approximately one to two percent of the solids to pass through and be
collected for storage. The concentrated solution (brine) unable to pass
through the membrane is then processed out of the system. Unlike standard
filtration where contaminates continually build up on the filter surface
area and gradually decrease the filtering capabilities of the system,
reverse osmosis filtration systems provide a self cleaning system by
allowing the unfiltered fluid to continuously pass across the membrane
surface removing the contaminants as a concentrated solution and
preventing surface fouling.
Typical applications for Reverse Osmosis are the purification of seawater,
waste water treatment, brackish well water, and city water. However, as
the industry grows many more applications are being both implemented and
researched at this time, many of which have nothing to do with water
purification. Presently this technology is being expanded to include gas
separation in the oil industry, protein concentration, breweries, wine and
food processing, maple syrup, dairies, removal of cholesterol from butter,
undesirable bacteria removal, undesirable pyrogen removal from I.V.’s,
etc. As the market expands I believe the industries future prospects are
excellent and should continue to expand well into the next century.
Semipermeable Membranes
Reverse Osmosis Membranes are a spiral wound
filtration system using alternating semipermeable and permeable
materials to process and separate the product fluid from the concentrate
solution. Their filtration capabilities and application are dependent on
several factors; chemical composition of the fluid to be filtered and
semipermeable material required due to this composition, fluid
temperature, operating pressure, total dissolved solids to be removed,
as well as several other small factors to be taken into account. The
filter sizing or permeate flow rates are determined as a function of the
sq.ft. of semipermeable material used in the membrane as well as the
factors described above. The typical rejection rate of Reverse Osmosis
Filters is 90 to 99.9 percent of Sodium Chloride as well as many other
impurities. This constitutes an excellent utilization of available water
resources for large scale water purification requirements. Filter sizing
for both commercial and residential applications generally range from
2” X 12” to 8” X 40”. These sizes will meet almost all
applications in today’s water purification industry.
Typical Membrane Elements
Semipermeable Materials
Only three major membrane groups are being used in
water desalinization at this time. A brief description of each is given
below as well as there usage’s.
Cellulose Acetate (CA)
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Chlorine
tolerant
-
Non-bacteria
resistant
-
pH
range is 6.0 to 8.0
-
Good
water production rate
-
Must
be used with a chlorinated water supply
-
Most
widely used membrane on the market
Cellulose Tri-Acetate (CTA)
-
Chlorine
tolerant
-
Resistant
to most bacteria
-
pH
range is 4.0 to 8.0
-
Excellent
water production rate
-
Chlorinated
water supply will extend membrane life
Thin Film Composite (TFC)
-
Chlorine sensitive, chlorine must be removed up stream of the membrane
-
Bacteria resistant
-
pH range is 3.0 to 11.0
-
Highest water production rate of all reverse osmosis membranes
-
Highest salt rejection characteristics
-
Longest membrane life
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