reverse osmosis definition meaning example

What is Reverse Osmosis (RO) Definition

Reverse osmosis (RO) is basially the reverse of the osmosis process. Scientists found that all that is required to reverse the process of osmosis is a suitable semipermeable membrane and applying a pressure to the concentrated salt solution above the applied and osmotic back-pressures, thereby forcing pure water through the semipermeable membrane. In other words, reverse osmosis is the process where water containing inorganic salts (minerals), suspended solids, soluble and insoluble organics, aquatic microorganisms, and dissolved gases (collectively called source water constituents or contaminants) is forced under pressure through a semipermeable membrane. Semipermeable refers to a membrane that selectively allows water to pass through it at much higher rate than the transfer rate of any constituents contained in the water. Learn more about pressure driven membranes here. If water of high salinity is separated from water of low salinity via a semipermeable membrane, a natural process of transfer of water will occur from the low-salinity side to the high-salinity side of the membrane until the salinity on both sides reaches the same concentration. This natural process of water transfer through a membrane driven by the salinity gradient occurs in every living cell; it is known as osmosis.

The hydraulic pressure applied on the membrane by the water during its transfer from the low-salinity side of the membrane to the high-salinity side is termed osmotic pressure. Osmotic pressure is a natural force similar to gravity and is proportional to the difference in concentration of total dissolved solids (TDS) on both sides of the membrane, the source water temperature, and the types of ions that form the TDS content of the source water. This pressure is independent of the type of membrane itself. In order to remove fresh (low-salinity) water from a high-salinity source water using membrane separation, the natural osmosis-driven movement of water must be reversed, i.e., the freshwater has to be transferred from the high-salinity side of the membrane to the low-salinity side. For this reversal of the natural direction of freshwater flow to occur, the high-salinity source water must be pressurized at a level higher than the naturally occurring osmotic pressure.

If the high-salinity source water is continuously pressurized at a level higher than the osmotic pressure and the pressure losses for water transfer through the membrane, a steady-state flow of freshwater from the high-salinity side of the membrane to the low-salinity side will occur, resulting in a process of salt rejection and accumulation on one side of the membrane and freshwater production on the other. This process of forced movement of water through a membrane in the opposite direction to the osmotic force driven by the salinity gradient is known as reverse osmosis (RO).

The rate of water transport through the membrane is several orders of magnitude higher than the rate of passage of salts. This significant difference between water and salt passage rates allows membrane systems to produce freshwater of very low mineral content. The applied feed water pressure counters the osmotic pressure and overcomes the pressure losses that occur when the water travels through the membrane, thereby keeping the freshwater on the low-salinity (permeate) side of the membrane until this water exits the membrane vessel.

reverse osmosis definition meaning example

Osmosis and Reverse Osmosis Process

The salts contained on the source water (influent) side of the membrane are retained and concentrated; they are ultimately evacuated from the membrane vessel for disposal. As a result, the RO process results in two streams—one of freshwater of low salinity (permeate) and one of feed source water of elevated salinity (concentrate, brine or retentate), as shown in the figure above. While semipermeable RO membranes reject all suspended solids, they are not an absolute barrier to dissolved solids (minerals and organics alike). Some passage of dissolved solids will accompany the passage of freshwater through the membrane. The rates of water and salt passage are the two key performance characteristics of Reverse Osmosis membranes.

reverse osmosis process drawing

Reverse Osmosis Process Drawing

pressure membrane process using feed or permeate pumps

What is a Pressure Membrane / Pressure-Driven Membrane

The pressure membrane processes are:

Reverse osmosis (RO)
Nanofiltration (NF)
Ultrafiltration (UF)
Microfiltration (MF)

A pressure membrane is a membrane that functions by applying a pressure. A pressure membrane is permeable to water but not to substances which are rejected and removed. All membranes including any pressure-driven membrane separate feedwater into two streams: permeate and concentrate streams. The permeate (for RO, NF, or UF) or filtrate (for MF) stream passes through the membrane barrier. The concentrate (or retentate) stream contains the substances removed from the feedwater after the pressure membrane barrier rejects it.

Actually, the driving force for these pressure membrane processes may come from (1) a pressurized feedwater source with the membranes installed in pressure vessels, called modules. Or (2) a partial vacuum in the filtrate/permeate flow stream caused by use of a filtrate/permeate pump or gravity siphon. The vacuum-driven processes typically apply to MF and UF only and have membranes submerged or immersed in nonpressurized feedwater tanks.

pressure membrane process using feed or permeate pumps

pressure membrane process using feed or permeate pumps

Pressure membrane processes are designed for cross-flow or dead-end operating modes. In the cross-flow mode, the feed stream flows across the pressure membrane surface. And permeate (or filtrate) passes through the pressure-driven membrane tangential to the membrane surface. Moreover, cross-flow operation results in a continuously flowing waste stream. A cross-flow system design sometimes contain a concentrate recycle. Also with a reject stream (feed-and-bleed mode). Many MF and UF systems treating relatively low turbidity waters are also designed to operate in a dead-end flow pattern where the waste concentrate stream is produced by an intermittent backwash. The figure below shows the relative removal capabilities for pressure-driven membrane processes and compares these processes with media filtration.

pressure membrane processes RO NF UF MF difference pore size rejection

pressure membrane processes RO NF UF MF difference pore size rejection

In fact, MF and UF separate substances from feedwater through a sieving action. Separation depends on the pressure membrane pore size and interaction with previously rejected material on the membrane surface. Furthermore, NF and RO separate solutes by diffusion through a thin, dense, permselective (or semi-permeable) membrane barrier layer, as well as by sieving action. The required pressure membrane feed pressure generally increases as removal capability increases.

electrodialysis ED membrane desalination technology schematic process system

What is Electrodialysis ED

In electrodialysis (ED)–based desalination systems, the separation of minerals and product water is achieved through the application of direct electric current to the source water. This current drives the mineral ions and other ions with strong electric charge that are contained in the source water through ion-selective membranes to a pair of electrodes
of opposite charges. As ions accumulate on the surface of the electrodes, they cause fouling over time and have to be cleaned frequently in order to maintain a steady-state Electrodialysis ED process. A practical solution to this  challenge is to reverse the polarity of the oppositely charged electrodes periodically (typically two to four times per hour) in order to avoid frequent electrode cleaning.

An Electrodialysis ED process that includes periodic change of the polarity of the system’s electrodes is referred to as an electrodialysis reversal EDR process. At present, practically all commercially available ED systems are of the EDR type. Electrodialysis ED systems consist of a large number (300 to 600 pairs) of cation and anion exchange membranes separated by dilute flow dividers (spacers) to keep them from sticking together and to convey the desalinated flow through and out of the membranes. Each pair of membranes is separated from the adjacent pairs above and below it by concentrate spacers which collect, convey, and evacuate the salt ions retained between the adjacent membranes. The membranes used for ED are different from those applied for Reverse Osmosis RO desalination—they have a porous structure similar to that of microfiltration and ultrafiltration membranes. Reverse Osmosis membranes do not have physical pores. Electrodialysis ED membranes are more resistant to chlorine and fouling and are significantly thicker than RO membranes.

electrodialysis ED membrane desalination technology schematic process system

electrodialysis ED membrane desalination technology schematic process system

It is important to note that a single set of EDR stacks can only remove approximately 50 percent of salts. As a result, multiple EDR stacks connected in series are often used to meet more stringent product water Total Dissolved Solids TDS targets. It should be pointed out that compared to brackish water RO membranes, which typically yield only up to 85 to 90 percent recovery, Electrodialysis Reversal EDR systems can reach freshwater recovery of 95 percent or more. The energy needed for ED desalination is proportional to the amount of salt removed from the source water. TDS concentration and source water quality determine to a great extent which of the two membrane separation technologies (RO or ED) is more suitable and cost effective for a given application. Typically, ED membrane separation is found to be cost competitive for source waters with TDS concentrations lower than 3000 mg/L. This applicability threshold, however, is a function of the unit cost of electricity and may vary from project to project.

The TDS removal efficiency of Electrodialysis ED desalination systems is not affected by non-ionized compounds or objects with a weak ion charge (i.e., solids particles, organics, and microorganisms). Therefore, ED membrane desalination processes can treat source waters of higher turbidity and biofouling and scaling potential than can RO systems. However, the TDS removal efficiency of ED systems is typically lower than that of Reverse Osmosis systems (15.0 to 90.0 percent versus 99.0 to 99.8 percent), which is one of the key reasons why they have found practical use mainly for brackish water desalination. In general, Electrodialysis Reversal EDR systems can only effectively remove particles that have a strong electric charge, such as mono- and bivalent salt ions, silica, nitrates, and radium. EDR systems have a very low removal efficiency with regard to low-charged compounds and particles—i.e., organics and pathogens. Table below provides a comparison of the removal efficiencies of distillation, ED, and RO systems for key source water quality compounds.

Contaminant Distillation (%) ED/EDR (%) RO (%)
TDS >99.9 50-90 90-99.5
Pesticides, Organics/VOCs 50-90 <5 5-50
Pathogens >99 <5 >99.99
TOC >95 <20 95-98
Radiological >99 50-90 90-99
Nitrate >99 60-69 90-94
Calcium >99 45-50 95-97
Magnesium >99 55-62 95-97
Bicarbonate >99 45-57 95-97
Potassium >99 55-58 90-92

One important observation from this table is that, as compared to distillation and RO separation, ED desalination only partially removes nutrients from the source water. This fact explains why EDR is often considered more attractive than RO or thermal desalination (which remove practically all minerals from the source water) if the planned use of the desalinated water is for agricultural purposes—i.e., generating fresh or reclaimed water for irrigation of agricultural crops.

Construction and equipment costs for brackish water reverse osmosis (BWRO) and EDR systems of the same freshwater production capacity are usually comparable, or EDR is less costly, depending on the Reverse Osmosis membrane fouling capacity of the source water. However, since the amount of electricity consumed by EDR systems is directly proportional to the source water’s salinity, at salinities of 2000 to 3000 mg/L the energy use of EDR systems usually exceeds that of BWRO or nanofiltration systems for source waters. Therefore, EDR systems are not as commonly used as RO systems for BWRO desalination and are never applied for seawater reverse osmosis (SWRO) desalination.

It should be pointed out, however, that salinity is not the only criterion for evaluating the cost competitiveness of EDR and BWRO systems. Often, other compounds such as silica play a key role in the decision making process. For example, at the largest operational EDR plant worldwide at present—the 200,000 m3/day Barcelona desalination facility in Spain—this technology was preferred to BWRO desalination because the brackish surface water source for this plant—the Llobregat River—contains very high level of silica, which would limit recovery from a BWRO plant to only 65 percent; the EDR system can achieve 90 percent recovery. In addition, the Llobregat River was found to have very high organic content, which was projected to cause heavy fouling and operational constraints on a BWRO plant of similar size.


Reference: “Desalination Engineering” by Nikolay Voutchkov

cellulose acetate membrane pores reverse osmosis desalination

What is a Cellulose Acetate Membrane

The thin semi-permeable film of the first Reverse Osmosis membranes was developed in the late 1950s at UCLA (University of California Los Angeles). It was made of cellulose acetate (CA) polymer. A Cellulose Acetate membrane have a three-layer structure similar to that of a Polyamide Thin Film Composite TFC membrane. The main structural difference is that the top two layers (the ultrathin film and the microporous polymeric support) are made of different forms of the same Cellulose Actetate polymer.

Cellulose is a polymer that is made up of repeating units (monomers) of C6H10O5. (Note: A monomer is a molecule which comes together with other identical monomers to form a chain of monomers, called a polymer). The number of acetates on the cellulose molecules affects the semi-permeability and other characteristics of the membrane. In general, the following are some of the most important differences between diacetate and triacetate membranes.

cellulose polymer cellulose acetate membrane

cellulose polymer cellulose acetate membrane

In a TFC membrane these two layers are made of completely different polymers. The thin semi-permeable film is polyamide, while the microporous support is polysulfone. Similar to a TFC membrane, a Cellulose Acetate membrane have a film layer that is typically about 0.2 um thick. But the thickness of the entire membrane (about 100 um) is less than that of a TFC membrane (about 160 um). One important benefit of a Cellulose Acetate membrane is that the surface has very little charge and is practically uncharged. As compared to a TFC membrane, which have a negative charge and can be more easily fouled with cationic polymers. If such polymers are used for source water pre-treatment.

cellulose acetate membrane pores reverse osmosis desalination

cellulose acetate membrane pores under microscope

In addition, a Cellulose Acetate membrane have a smoother surface than a TFC membrane. Which also renders them less susceptible to fouling. Cellulose Acetate membrane have a number of limitations. Including the ability to perform only within a narrow pH range of 4 to 6 and at temperatures below 35 C (95 F). Operation outside of this pH range results in accelerated membrane hydrolysis, while exposure to temperatures above 40 C (104 F) causes membrane compaction and failure. In order to maintain the Reverse Osmosis concentrate pH below 6, the pH of the feed water to the cellulose acetate membrane are reduced to between 5 and 5.5. This results in significant use of acid for normal plant operation and requires Reverse Osmosis permeate adjustment by addition of a base (typically sodium hydroxide) to achieve adequate boron rejection.

Cellulose Acetate membrane experience accelerated deterioration in the presence of microorganisms. Since they’re capable of producing cellulose enzymes and bioassimilating the membrane material. However, they can tolerate exposure to free chlorine concentration of up to 1.0 mg/L. Which helps to decrease the rate of membrane integrity loss due to destruction by microbial activity.

Since Cellulose Acetate membrane have a higher density than a Polyamide TFC membrane. They create a higher headloss when the water flows through the membranes. Therefore a cellulose acetate membrane operates at higher feed pressures, which results in elevated energy expenditures. Despite their disadvantages, cellulose acetate membrane have high tolerance to oxidants (chlorine, peroxide, etc.). As compared to a PA TFC membrane. Cellulose Acetate membrane are used in municipal applications for saline waters with very high fouling potential. Mainly used in the Middle East and Japan in seawater reverse osmosis plants, and for ultrapure water production in pharmaceutical and semiconductor industries.

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