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PRINCIPLES OF REVERSE OSMOSIS
Reverse osmosis is a process for
removing dissolved mineral salts, organic molecules, and
certain other impurities from water by permitting water under
increased pressure to pass through a semi-permeable membrane.
It is called reverse osmosis as it is the reverse of the
natural osmotic process in which fluids with a low
concentration of suspended and dissolved solids pass through a
membrane into an area of higher concentration. With reverse
osmosis water treatment, water is make to pass from a state of
high dissolved solids concentration to a state of low
concentration.
Since reverse osmosis does not occur
naturally, it must be created by applying higher pressure to
the high dissolved solids water in order to force it through
the membranes. Membranes must be strong and resistant enough to
withstand the high pressures of the RO operation… from
200 to 400 psi in most applications: 1000, or even 1200psi for
sea water desalination. The pressure applied to the feed side
of the RP membrane must be higher than the natural osmotic
pressure of the water in order for the osmotic process to be
reversed. High pressure pumps are used to created the needed
pressure.
Reverse Osmosis Membranes – Several types of
membranes have been developed for RO applications. Three types are
widely used.
Thin
Film Composite
The first
type of commonly available membrane capable of high salt
rejection is the composite membrane, usually called a thin film
composite (TFC) membrane. TFC membranes are three layers of
material; a thin (0.25um) barrier coating on the surface of a
micro-porous layer of polymers, such as polyamines, polyimines,
or polyethers.
TFC membranes have high salt rejection
rates, usually operate at lower pressures that CA or HF, and
have exhibited good performance under wide ranging pH and
temperature conditions. They are not degradable by
microorganisms and hold their flux rates over long periods of
time. Like hollow fiber membranes, they do not withstand
chlorine well, so chlorine removal is needed as a pretreatment
step. Most TFC membranes are produced in a spiral wound module
configuration.
Cellulose
Acetate
Another
commonly used membrane is made of cellulose acetate (CA). These
membranes are asymmetric. This means they consist of a thin
dense salt barrier attached to a thicker micro-porous layer
manufactured in one step so it is essentially one layer.
Polyaramid
Hollow Fiber
A
third membrane used in the past and used occasionally is the
hollow fiber (HF) membrane. These membranes have been developed
in the form of bundles of thousands of tiny hollow filaments.
These hollow fibers are approximately the diameter of a human
hair in the form of a tiny tube that can take the high pressure
of RO operations. Like CA membranes, they are asymmetric. A
dense skin on the outside serves as the salt barrier to reject
mineral salts, and a porous inner layer allows the water to
pass through to service.
Polyaramid membranes normally operate at
higher flow volumes, have good temperature and pH stability,
good corrosion resistance and are not degradable by
microorganisms. Due to the tiny sixe, they are more prone to
turbidity fouling than spiral wound types. Hollow fiber
membranes have low resistance to chlorine. Pretreatment of the
water to remove chlorine is required for successful application.
All
three membranes perform essentially the same task…they
allow purified product water to pass through the membrane while
stopping the passage of dissolved and suspended matter. RO
membranes also have excellent rejection of organic matter, colloids
and turbidity (although, turbidity can foul them). The percent
rejection of each impurity varies somewhat according to the
type of impurity and the membrane. Rejection tables are
available for each membrane.
Design
Considerations – Ro units do not deliver
to service all of the water that is fed to them. During
operation, some of the incoming water is used to wash down the
membrane and only part becomes finished product water. Product
water is referred to as permeate, and waste water is referred
to as concentrate. The percent of water delivered as permeate
is called the recovery and depends on the membrane and total RO
unit design considerations.
Ro membranes are volume rated at 77 º F (25 º
C) incoming water temperature. Conversions must be made if the
incoming water temperature varies. For optimum RO unit
performance, mixing valves or heaters are often used to
maintain deed water at the rated temperature.
Pretreatment
– Pretreatment of water prior to the RO
process is almost always required. Chlorine has been mentioned,
but high hardness minerals should also be controlled by a
softener or other suitable methods. For example, hard water
scale build-up causes membrane hydrolysis and impairs RO unit
performances. Turbidity, iron and other impurities must be
controlled for optimum RO performance.
System
Engineering – RO units are often used to
provide low dissolved solids feed water to deionizers. This
extends the deionizer service cycle and lowers regeneration
frequency. Considerable costs can be saved through reduction of
regenerant chemicals. Systems engineering of water treatment
problems takes on added significance as RO and DI processes are
put into use together.
CA
membranes are usually fabricated in spiral wound module configurations
with a fabric support to provide a great deal of membrane
surface area in a small space. As water is forced against the
barrier layer, the dissolved salts are rejected and low
dissolved solids product water passes through to an inner
cylinder or tube and then to service.
Cellulose triacetate (CTA) is also used in RO
applications. It has a higher rejection of salt than regular
cellulose acetate, is more resistant to chlorine and can
operate at higher pH values (up to 8.5). Blends of cellulose
diacetate and cellulose triacetate are also used. This blend
has good resistance and salt rejection, but with higher flux
than cellulose acetate. Flux is the rate at which water is
transported through the membrane.
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