THEORY OF OPERATION REVERSE OSMOSIS

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

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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