What
is Reverse Osmosis?
Anyone
who has been through a high school science class will likely be
familiar with the term osmosis. The process was first described
by a French Scientist in 1748, who noted that water spontaneously
diffused through a pig bladder membrane into alcohol. Over 200
years later, a modification of this process known as reverse osmosis
allows people throughout the world to affordably convert undesireable
water into water that is virtually free of health or aesthetic
contaminants. Reverse osmosis systems can be found providing treated
water from the kitchen counter in a private residence to installations
used in manned spacecraft.
Reverse
Osmosis is a technology that is found virutally anywhere pure
water is needed; common uses include:
-
Drinking
Water
-
Humidification
-
Ice-Making
-
Car
Wash Water Reclamation
-
Rinse Waters
-
Biomedical
Applications
-
Laboratory Applications
-
Photography
-
Pharmaceutical
Production
-
Kidney
Dialysis
-
Water
used in chemcial processes
-
Cosmetics
-
Animal
Feed
-
Hatcheries
-
Restaurants
-
Greenhouses
-
Metal
Plating Applications
-
Wastewater
Treatment
-
Boiler
Water
-
Battery
Water
-
Semiconductor
production
-
Hemodialysis
How
Reverse Osmosis Works:
A semipermeable membrane,
like the membrane of a cell wall or a bladder, is selective about
what it allows to pass through, and what it prevents from passing.
These membranes in general pass water very easily because of its
small molecular size; but also prevent many other contaminants
from passing by trapping them. Water will typically be present
on both sides of the membrane, with each side having a different
concentration of dissolved minerals. Since the water i the less
concentrated solution seeks to dilute the more concentrated solution,
water will pass through the membrane from the lower concentration
side to the greater concentration side. Eventually, osmotic pressure
(seen in the diagram below as the pressure created by the difference
in water levels) will counter the diffusion process exactly, and
an equilibrium will
form.
The process
of reverse osmosis forces water with a greater concentration of
contaminants (the source water) into a tank containing water with
an extremely low concentration of contaminants (the processed
water). High water pressure on the source side is used to "reverse"
the natural osmotic process, with the semi-permeable membrane
still permitting the passage of water while rejecting most of
the other contaminants. The specific process through which this
occurs is called ion exclusion, in which a concentration of ions
at the membrane surface from a barrier that allows other water
molecules to pass through while excluding other supstances.
Semipermeable
membranes have come a long way from the natural pig bladders used
in the earlier osmosis experiments. Before the 1960's, these membranes
were too inefficient, expensive, and unreliable for practical
applications outside the laboratory. Modern advances in synthetic
materials have generally solved these problems, allowing membranes
to become highly efficient at rejecting contaminants, and making
them tough enough to withstand the greater pressures necessary
for efficient operation.
Even
with these advances, the "reject" water on the source
side of a Reverse Osmosis (RO) system must be periodically flushed
in order to keep it from becoming so concentrated that it forms
a scale on the membrane itself. RO systems also typically require
a carbon prefilter for the reduction of chlorine, which can damage
an RO membrane; and a sediment prefilter is always required to
ensure that fine suspended materials in the source water do not
permanently clog the membrane. Hardness reduction, either through
the use of water softening for residential units or chemical softening
for industrial use, may also be desirable in hard water areas.
Low
Pressure (Residential) Systems:
Low pressure
RO systems generally refer to those systems with a water feed
pressure of less than 100 psig. These are the typical countertop
or undersink residential systems that rely primarily on the natural
water pressure to make the reverse osmosis process function; a
typical system is shown schematically below.
Typical Point of
Use Reverse Osmosis System
Countertop
units typically have an unpressurized storage tank; Undersink
units typically have a pressurized accumulator storage tank where
the water pressure tends to increase as the tank fills. This pressurized
system provides sufficient pressure to move the water from the
undersink storage tank to the faucet. Unfortunately, this also
creates a back pressure against the membrane, which can decrease
its efficiency. Some units overcome this by using unpressurized
tanks with a pump to get the treated water where it is needed.
Low pressure
units typically provide between 2 and 15 gallons per day of water,
with an efficiency of 2-4 gallons of reject water per gallon of
treated water. Water purity can be as high as 95 percent. These
systems can be highly affordable, with countertop units starting
at about US $150, and undersink units starting at about US $500.
These units produce water for a cost as low as ten cents per gallon
once maintenance and water costs are factored in. Maintenance
usually requires replacing any pre- or postfilters (typically
one to four times per year); and the reverse osmosis cartridge
once every two to three years, depending on usage. Look for the
WQA Gold Seal (S-300) to find products that have been successfully
tested to industry performance standards; and to Certified Water
Specialists (CWS I-VI), Certified Sales
Representatives (CSR), and Certified Installers (CI) for advice
on your water needs, and equipment installation.
High
Pressure (Commercial/Industrial) Systems:
High pressure
systems typically operate at pressures between 100 and 1000 psig,
depending on the membranes chosen and the water being treated.
These systems are usually used in industrial or commercial applications
where large volumes of treated water are required at a high level
of purity.
Most
commercial and industrial systems use multiple membranes arranged
in parallel to provide the required quantity of water. The processed
water from the first stage of treatment can then be passed through
additional membrane modules to achieve greater levels of treatment
for the finished water. The reject water can also be directed
into successive membrane modules for greater efficiency (see diagram
below), though flushing will still be required when concentrations
reach a level where fouling is likely to
occur.
High
pressure industrial units typically provide from 10 gallons to
thousands of gallons per day of water with an efficiency of 1-9
gallons of reject water per gallon of treated water. Water purity
can be as high as 95 percent. These systems tend to be larger
and more complicated than low pressure systems, and this is reflected
in their costs, which range from US $1000 through tens of thousands
of dollars for a large, multi-module unit capable of providing
desalinated drinking water for a resort facility or water bottling
plant.
What
Reverse Osmosis Treats:
Reverse osmosis
can treat for a wide variety of health and aesthetic contaminants.
Effectively designed, RO equipment can treat for a wide variety
of aesthetic contaminants that cause unpleasant taste, color,
and odor problems like a salty or soda taste caused by chlorides
or sulfates.
RO can
also be effective for treating health contaminants like arsenic,
asbestos, atrazine (herbicides/pesticides). fluoride, lead, mercury,
nitrate, and radium. When using appropriate carbon prefiltering
(commonly included with most RO systems), additional treatment
can also be provided for such "volatile" contaminants
as benzene, trichloroethylene, trihalomethanes, and radon. Some
RO equipment is also capable of treating for biological contaminants
like Cryptosporidium. The Water Quality Association (WQA) cautions,
however, that while RO membranes typically remove virtually all
known microorganisms and most other health contaminants, design
consderations may prevent a unit from offering foolproof protection
when incorporated into a consumer drinking water system.
When
looking for a product to treat for a given health contaminant,
care should be used to find products that have been tested successfully
for such purposes at a quality testing laboratory.
Conclusion:
Reverse osmosis
is a relatively new, but very effective, application of an established
scientific process. Whether it is used to meet the needs of a
typical family of four, or the needs of an industrial operation
requiring thousands of gallons per day, it can be a cost effective
to provide the required quantity of highly treated water. With
continual advances in system and membrane design that boost efficiency
and reliability, RO can be expected to play a major role in water
treatment for years to come.
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