Principles of Operation
Reverse osmosis is a membrane
separation process for removing solvent from a solution. When a semi permeable
membrane separates a dilute solution from a concentrated solution, solvent
crosses from the dilute to the concentrated side of the membrane in an attempt
to equalize concentrations. The flow of solvent can be prevented by applying an
opposing hydrostatic pressure to the concentrated solution.
The magnitude of the pressure
required to completely impede the flow of solvent is defined as the "osmotic pressure". If the applied hydrostatic
pressure exceeds the osmotic pressure (see figure below), flow of solvent will
be reversed, that is, solvent will flow from the concentrated to the dilute
solution. This phenomenon is referred to asReverse
Osmosis. The figure illustrates the concepts of osmosis, osmotic
pressure and reverse osmosis schematically.
Overview of osmosis and reverse osmosis
In order to use reverse osmosis as a
water purification process, the feed water is pressurized on one side of a semi
permeable membrane. The pressure must be high enough to exceed the osmotic
pressure to cause reverse osmotic flow of water.
If the membrane is highly permeable
to water, but essentially impermeable to dissolved solutes, pure water crosses
the membrane and is known as product water. As product water crosses the
membrane, the concentration of dissolved impurities increases in the remaining
feed water (a condition known as concentration polarization) and, as a
consequence, the osmotic pressure increases.
A point is reached at which the
applied pressure is no longer able to overcome the osmotic pressure and no
further flow of product water occurs. Moreover, if the applied pressure is
increased in an attempt to gain more product water, a point is reached at which
the membrane becomes fouled by precipitated salts and other un dissolved
material from the water.
Therefore, there is a limit to the
fraction of feed water which can be recovered as pure water and reverse osmosis
units are operated in a configuration where only a portion of the feed water
passes through the membrane with the remainder being directed to drain
(cross-flow configuration).
The water flowing to drain contains
concentrated solutes and other insoluble materials, such as bacteria, endotoxin
and particles, and is referred to as the reject stream.
The product water to feed water ratio can range from 10% 50% for purification
of water depending on the characteristics of the incoming water as well as
other conditions.
Types of Reverse Osmosis
Membranes
A reverse osmosis membrane must be
freely permeable to water, highly impermeable to solutes, and able to withstand
high operating pressures. It should ideally be tolerant of wide ranges of pH
and temperature and should be resistant to attack by chemicals like free
chlorine and by bacteria.
Ideally, it should also be resistant
to scaling and fouling by contaminants in the feed water. There are three major
types of reverse osmosis membranes: cellulosic, fully aromatic polyamide and
thin film composite. A comparison of characteristics of these three membrane
types is given in the following Table.
|
Comparison of Reverse Osmosis Membranes
|
|
Feature
|
Cellulosic
|
Aromatic
Polyamide
|
Thin
Film Composite*
|
|
Rejection
of Organic
|
L
|
M
|
H
|
|
Rejection
of Low Molecular Weight Organics
|
M
|
H
|
H
|
|
Water
Flux
|
M
|
L
|
H
|
|
pH
Tolerance
|
4-8
|
4-11
|
2-11
|
|
Temperature
Stability
|
Max
35 deg C.
|
Max
35 deg C.
|
Max
45 deg C.
|
|
Oxidant
Tolerance(e.g. free Chlorine
|
H
|
L
|
L
|
|
Compaction
Tendency
|
H
|
H
|
L
|
|
Biodegradability
|
H
|
L
|
L
|
|
Cost
|
L
|
M
|
H
|
|
L
= Low; M = Medium; H = High
|
|
*Thin
film composite type having polyamide surface layer
|
Cellulosic Membranes: The concept of reverse osmosis was first demonstrated in the
late 1950s with cellulose acetate membranes. These membranes are asymmetric,
composed of a thin dense surface layer (0.2 to 0.5 ~m ) and a thick porous
substructure. Solute rejection is accomplished by the thin dense layer and the
porous substructure provides structural strength. Cellulose acetate membranes
can be cast in sheets or as hollow fibers.
Cellulose acetate membranes are
inexpensive and easy to manufacture but suffer from several limitations. Their
asymmetric structure makes them susceptible to compaction under high operating
pressures, especially at elevated temperatures.
Compaction occurs when the thin
dense layer of the membrane thickens by merging with the thicker porous
substructure, leading to a reduction in product flux.
Cellulose acetate membranes are
susceptible to hydrolysis and can only be used over a limited pH range (low pH
3 to 5 and high pH 6 to 8, depending on the manufacturers). They also undergo
degradation at temperatures above 35°C.
They are vulnerable to attack by
bacteria.
Cellulose acetate membranes have a
high water permeability but reject low molecular weight contaminants poorly.
Cellulose triacetate membranes have
been developed with improved salt rejection characteristics and reduced
susceptibility to pH, high temperature and microbial attack. However, cellulose
triacetate membranes have a lower water permeability than cellulose acetate
membranes. Blends of cellulose triacetate and cellulose acetate have been
developed to take advantage of the desirable characteristics of both membranes.
Caution: Both CA and CTA membranes may contain 1,4 Dioxane,
a chemical known to cause cancer and banned in California by Proposition 65.
Manufacturers of CA and CTA membrane Reverse Osmosis systems are required by
State Law to place warning labels on the product package to alert consumers(and
dealers) of this fact. If you purchase a CA or CTA system and it does not have
these designations, it is not in compliance with State Law.
The 1,4, Dioxane is used
to create the membrane porosity features and portions of that chemical may
remain in the product following manufacture. Manufacturers with whom we
discussed this issue readily admit the use of 1,4 Dioxane but are unable to
specify the number of gallons of water which must initially be run through the
system to purge this chemical from the system.
To our knowledge, TFC
membranes do not use this chemical in manufacturing process.
Aromatic polyamide
membranes: Aromatic polyamide membranes were
first developed by DuPont in a hollow fiber configuration. Like the cellulosic
membranes, these membranes also have an asymmetric structure with a thin (0.1
to 1.0 ,um ) dense skin and a porous substructure.
Polyamide membranes have better
resistance to hydrolysis and biological attack than do cellulosic membranes.
They can be operated over a pH range of 4 to 11, but extended use at the
extremes of this range can cause irreversible membrane degradation. They can
withstand higher temperatures than cellulosic membranes. However, like
cellulosics, they are subject to compaction at high pressures and temperatures.
They have better salt rejection
characteristics than cellulosic membranes as well as better rejection of water
soluble organics.
A major drawback of polyamide
membranes is that they are subject to degradation by oxidants, such as free
chlorine.
Thin film composites: As the name indicates, these membranes are made by forming a
thin, dense, solute rejecting surface film on top of a porous substructure. The
materials of construction and the manufacturing processes for these two layers
can be different and optimized for the best combination of high water flux and
low solute permeability.
The water flux and solute rejection
characteristics are predominantly determined by the thin surface layer, whose
thickness ranges from 0.01 to 0.1 micrometers.
Several types of thin film composite
membranes have been developed, including aromatic polyamide, alkyl-aryl
polyurea/polyamide and polyfurane cyanurate. The supporting porous sub layer is
usually made of polysulfone.
Polyamide thin film
composites, like polyamide asymmetric membranes, are highly susceptible to
degradation by oxidants, such as free chlorine. Consumers must be consistent in
their maintenance of the TFC systems, particularly the carbon pre filtration
element which is present to remove free chlorine(and other oxidative organics)
and prevent damage and premature destruction of the TFC membrane
Although the stability of these
membranes to free chlorine has been improved by modifications of the polymer
formulation and the processing technique, exposure to oxidants must be
minimized.
Applications: Reverse osmosis membranes reject dissolved inorganic
solutes, larger organic solutes (molecular weight greater than 200), a portion
of microbiological contaminants such as endotoxin, viruses and bacteria, and
particles. Because of this broad spectrum of solute rejection, reverse osmosis
is an important process in a wide variety of water treatment processes.
NOTE: the following
section is provided to emphasize the variability of the performance of reverse
osmosis insofar as time and input contaminant characteristics are concerned.
Removal of inorganic
contaminants: The removal of inorganic contaminants by reverse osmosis
membranes has been studied in great detail by many researchers using a variety
of membrane types. Complex interactions occur in feed waters containing
mixtures of ionic species. Nevertheless, general guidelines for the rejection
of inorganic contaminants by reverse osmosis membranes can be given:
Ionic contaminants are more readily
rejected than neutral species. For most membrane types, polyvalent ions are
rejected to a greater extent than monovalent ions. If the polyvalent ion is
strongly hydrated, rejection is even higher.
Because electrical neutrality must
be preserved, ions diffuse across the membrane as a cation-anion pair. As a
consequence, rejection of a particular ion depends on the rejection of its
counterion.
IMPORTANT: An example of this interaction is that of sodium.
Sodium as sulfate (Na2SO,), has a higher rejection than when present as sodium
chloride (NaCl), because the divalent sulfate ion is rejected to a greater
extent than the monovalent chloride ion.
When a home-use reverse
osmosis system is combined with a water softener/conditioner, an increasing
amount of sodium chloride(or potassium chloride if used) is allowed thorough
the membrane. In hard water areas, where several grains of hardness are
present, or where large amounts of calcium and magnesium are found, the water
softener exchanges a certain amount of sodium(Click to see water
softener section for specific calculations of these amounts) and these salts are then sent through the house
plumbing.
The reverse osmosis system
progressively lets more and more of these sodium salts thorough into the
drinking water. For those on sodium restricted diets or who experience other
health problems such as diabetes(large water consumption) or hypertension, this
issue may preclude the practical use of reverse osmosis in the home.
We recommend you determine
how much additional sodium is being added to your home by the water softener
and then estimate the residual sodium after a hypothetical reverse osmosis
units ---and then determine if such a system is allowing more sodium than you
can tolerate. If you find such levels are unacceptable for your health
condition, we recommend you consider a steam distillation system where all sodium ions
are removed.
Variations in pH influence the water
flux and rejection characteristics of reverse osmosis membranes exposed to a
mixture of monovalent and polyvalent solutes. This effect of pH varies with
membrane composition and ionic species.For example,
fluoride rejection increases from 45% to 90% as pH increases from 5.5 to 7.2,
whereas nitrate rejection decreases slightly as pH increases from 5.2 to 7.0.
The pH of municipal water has been
recently increased in some areas in anticipation of the newly proposed lead
regulations (see Section 4.3). In instances when pH has exceeded 9, and the
water contained chloramines, a decreased rejection of solutes by polyamide thin
film composite membranes has been observed.
It is thought that the high pH
causes chloramines to dissociate into ammonium and hypochlorite ions. The ammonium ions, which are poorly removed by
activated carbon, interact with the polyamide membranes, causing their
rejection characteristics to deteriorate. The
decrease in rejection can generally be reversed by lowering the pH of the water
supply.
NOTE: most larger municipal water systems are now using
chloramines to treat water(versus free chlorine). This dramatically reduces
membrane performance(and lifetime).
Inorganic contaminants with higher
molecular weights (greater than 200) are rejected to a greater extent than
small molecular weight inorganic solutes.
We have selected not to illustrate
rejection percentages for inorganic contaminants since each manufacturer uses
different types of
"challenge" inorganics to demonstrate the better characteristics of
their individual membranes. A common set of test conditions is virtually
impossible to identify.
The variability of local
water conditions, particularly where a municipal water system relies on a
variety of water sources during the course of the year, thus creates a
virtually unpredictable performance specification for home-type reverse osmosis
units.
Although Total Dissolved Solids(TDS)
measurements will indicate a gradual degradation of the overall inorganic
performance of the reverse osmosis system, short of an expensive quantitative
and quantitative laboratory test it is virtually impossible to tell if specific contaminant removal
percentages are achieved under these highly variable conditions.
The purpose of the above
discussion is to caution the homeowner (and dealer) as to variability of the
performance of an in-home reverse osmosis system.
While general inorganic performance
can be measured by conventional conductivity meters(for TDS), specific
performance specifications which a manufacturer depicts in a product brochure
may be considerably different from what is actually achieved in home-use
conditions.
In general TFC membranes do better
when total dissolved solids are the sole measure of system performance, albeit
they must be carefully maintained as to chlorine intolerance as noted above.
Removal of organic
contaminants: While reverse osmosis membranes have a wide spectrum of
removal of organic contaminants, the nature and extent of rejection will depend
upon the nature of the organic solute. However, some general guidelines
regarding rejection of organic contaminants can be given:
Reverse osmosis is effective in
rejecting organic solutes with molecular weights greater than 200 to 300, such
as fulvic acids, lignins, humic acids and detergents. Low molecular weight, non polar, water soluble
solutes (for example, methanol, ethanol, and ethylene glycol) are poorly
rejected.
Un dissociated organic acids and
amines are poorly rejected while their salts are readily rejected. For example,
phenol is poorly rejected by reverse osmosis membranes, but when converted to
its salt, rejections as high as 95 to 99% are observed. Also, rejection of
acetic acid is only of the order of 50% but that of sodium acetate is as high as
90 to 95%.
The variable(and in some cases poor)
removal characteristics of reverse osmosis membranes dictates the use of
auxiliary carbon filtration components either before or after(or both) the
membrane. As in steam distillation, which has similar problems with organic
materials, both reverse osmosis and distillation require some type of organic
removal mechanism such as replaceable carbon filters.
The placement of carbon filters in
reverse osmosis systems depends on the type of membrane in use: for cellulose
acetate or cellulose triacetate membranes the carbon element is usually placed
AFTER the membrane and captive air tank, and just before the dispensing faucet.
For thin film membranes, a carbon
filter is usually placed before AND after the membrane. The carbon filter
placed in front of the membrane is necessary since various types of organic
materials and chlorine are detrimental to the structure of the thin film
membrane. Extra caution must be taken to regularly replace the carbon pre
filter so as to ensure reasonable performance and lifetime for the TFC
membrane.
Removal of
microbiological contaminants:
Reverse osmosis manufacturers claim to reduce levels of bacterial and viral
contamination in the feed water by factors of 10(3rd power) to 10(5th power).
However, in reality reverse osmosis
should not be relied upon to produce sterile, much less water with reduced
bacterial levels.
Using the biological
process called MITOSIS, Bacteria and viruses may rapidly penetrate the reverse
osmosis membrane through defects and imperfections in the membrane as well as
through tiny leaks in seals of the membrane module. In order to prevent colonization of the product water side
with bacteria and proliferation of these bacteria, regular disinfection
procedures are necessary(unfortunately most of which
are never explained to consumers, and still fewer undertaken by owners of home
use reverse osmosis systems).
In general, because of this marked
deficiency in system capabilities, most of the reverse osmosis industry(dealers
and salespersons included) doggedly try to steer the discussion away from this
sensitive topic.
Contrary to what most if not all of
the industry consultants and manufacturers are saying about this subject,
controlled, clinical studies have been done which indicate massive bacterial
re-growth problems in PROPERLY MAINTAINED in-home reverse osmosis units. What
this Canadian government-sponsored study showed was an incredible increase in
gastrointestinal illnesses which were directly correlated with the higher
levels of bacteria appearing in the test reverse osmosis systems.
Comparisons were made with neighbors
who drank straight tap water. The neighbors did not experience the types of
illnesses which were occurring in their neighbor's homes who owned the reverse
osmosis units.
One can easily see why the US
reverse osmosis industry has been strangely silent on these studies---studies
which expose one of the more dangerous aspects of employing reverse osmosis in
home use situations.
Much like the salt refining industry
and water softener manufacturing and sales organizations, reverse osmosis
industry representatives and their paid consultant organizations are
continually attempting to ally fears of such microbiological contamination problems.
As seen in another part of this web
site, reverse osmosis is a terrific performer in industrial applications, when
combined with other technologies such as mixed bed de ionization and where
microbiological problems can be dealt with through the use of high powered
ultraviolet and ozone systems.
Endotoxin aggregates have a high
molecular weight of the order of 2 million and are well rejected by reverse
osmosis membranes. However, endotoxin fragments may penetrate reverse osmosis
membranes. These fragments may carry toxic components to the home drinking
water and may endanger specifically those with reduced immune system
characteristics.
Placement of a reverse osmosis
system(without auxiliary processing capabilities such as ultraviolet or ozone)
in a rural environment which is naturally prone to a wider and greater
concentration of microbiological hazards is also cautioned.
Individuals who purchase point of
use systems such as reverse osmosis need to be aware of both the capabilities
and deficiencies of these systems.
TYPICAL HOOKUP
CONFIGURATION
COMPARATIVE PERFORMANCE
OF REVERSE OSMOSIS, STEAM DISTILLATION AND CARBON FILTRATION VERSUS TIME.
QUALITATIVE PERFORMANCE
COMPARISON OF ALL TYPES OF HOME-USE WATER TREATMENT TECHNOLOGIES BY HYDROTECH CORPORATION
