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 undesirable 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 virtually anywhere pure water is needed; common uses include:
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Drinking Water
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Humidification
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Ice-Making
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Car Wash Water Reclamation
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Rinse Waters
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Biomedical Applications
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Laboratory Applications
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Photography
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Pharmaceutical Production
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Kidney Dialysis
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Water used in
chemical processes
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Cosmetics
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Animal Feed
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Hatcheries
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Restaurants
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Greenhouses
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Metal Plating Applications
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Wastewater Treatment
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Boiler Water
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Battery Water
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Semiconductor production
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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 substances. 
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
24 and 35 gallons per day of water (Pure-Pro System offer 50-80 gallons
per day), Water purity can be as high as 95 percent of rejection. These
systems can be highly affordable, undersink units starting at about US $500.
These units produce water for a cost as low as five 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 PPW Home Reverse Osmosis System
to find products that have been successfully tested to industry performance
standards; and to Certified Water Specialists (CWS I-V), 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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