bqua Structure of a reverse osmosis membrane - RO membrane

RO Membrane – Reverse Osmosis Membrane Materials, Types and Structures

Reverse osmosis RO membrane differs by the material of the membrane polymer and by structure and configuration. Based on its structure, RO membrane can be divided into two groups: conventional thin-film composite and thin-film nanocomposite. Based on the thin-film material, conventional reverse osmosis RO membrane at present is classified into two main groups: polyamide and cellulose acetate. Depending on the configuration of the membrane within the actual membrane elements (modules), the reverse osmosis membrane materials is divided into three main groups: spiral-wound, hollow-fiber, and flat-sheet (plate-and-frame).

Conventional Thin-Film Composite Membrane Structure

The reverse osmosis RO membrane most widely used for desalination at present are composed of a semipermeable thin film (0.2 um), made of either aromatic polyamide (PA) or cellulose acetate (CA), which is supported by a 0.025- to 0.050-mm microporous layer that in turn is cast on a layer of reinforcing fabric (Fig. 1.1 for a membrane with an ultrathin PA film). The 0.2-um ultrathin polymeric film is the feature that gives the RO membrane its salt rejection abilities and characteristics. The main functions of the two support layers underneath the thin film are to reinforce the reverse osmosis membrane structure and to maintain membrane integrity and durability.

bqua Structure of a typical reverse osmosis RO membrane

Fig 1.1: Structure of a typical reverse osmosis RO membrane

The dense semipermeable polymer film is of a random molecular structure (matrix) that does not have pores. Water molecules are transported through the membrane film by diffusion and travel on a multidimensional curvilinear path within the randomly structured molecular polymer film matrix. While the thin-film RO membrane with conventional random matrix-based structure shown in Fig. 1.1 is the type of membrane that dominates the desalination industry, new thin-film membrane of more permeable structure are currently under development in research centers worldwide.

Thin-Film Nanocomposite RO Membrane Structure

Thin-Film Nanocomposite TFC membrane either incorporate inorganic nanoparticles within the traditional membrane polymeric film structure (Fig. 1.2) or are made of highly structured porous film consisting of a densely packed array of nanotubes (Fig. 1.3). In Fig. 1.2, part A shows the thin film of a conventional PA membrane, supported by the polysulfone support layer. Part B shows the same type of membrane with embedded nanoparticles (labeled “NP”).

bqua polyamide reverse osmosis ro membrane with nanoparticles

Fig 1.2: Polyamide reverse osmosis RO membrane with nanoparticles

bqua reverse osmosis ro membrane with carbon nanotubes

Fig 1.3: Reverse osmosis RO membrane with carbon nanotubes

Nanocomposite reverse osmosis membrane material reportedly has higher specific permeability than conventional RO membrane at comparable salt rejection. Which is the ability to transport more water through the same surface area at the same applied pressure). In addition, thin-film nanocomposite membrane have comparable or lower fouling rates in comparison to conventional thin-film composite RO membrane operating at the same conditions. And they can be designed for enhanced rejection selectivity of specific ions. If membrane material science evolved to a point where the membrane structure could be made of tubes of completely uniform size, theoretically the membrane could produce up to 20 times more water per unit surface area than the RO membrane commercially available on the market today. As membrane material science evolves toward the development of membrane with more uniform structure, the further development of RO desalination membrane technology has the potential to yield measurable savings in terms of water production costs.

Cellulose Acetate CA Membrane

The thin semipermeable film of the first membrane element – developed in the late 1950s at the University of California, Los Angeles – was made of cellulose acetate (CA) polymer. While CA membrane has a three-layer structure similar to that of PA 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 CA polymer. In PA membrane these two layers are of completely different polymers – the thin semipermeable film’s made of polyamide, while the microporous support’s made of polysulfone (see Fig. 1.1). Similar to PA membrane, CA membrane has 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 PA membrane (about 160 um).

One important benefit of CA membrane is that the surface has very little charge and is considered practically uncharged, as compared to PA membrane, which have negative charge and can be more easily fouled with cationic polymers if such polymers are used for source water pretreatment. In addition, a CA membrane have a smoother surface than the PA membrane, which also renders them less susceptible to fouling.

CA membrane has 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. Significant use of acid for normal plant operation requires reverse osmosis RO permeate adjustment by adding a base (typically sodium hydroxide) to achieve adequate boron rejection; in order to maintain the RO concentrate pH below 6, the pH of the feed water to the CA membrane has to be reduced to between 5 and 5.5.

CA membrane experiences accelerated deterioration in the presence of microorganisms 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 CA membrane has a higher density than PA membrane, it creates a higher headloss when the water flows through the membrane. Therefore they have to be operated at higher feed pressures, which results in elevated energy expenditures. CA membrane is used in municipal applications for saline waters with very high fouling potential (mainly in the Middle East and Japan) and for ultrapure water production in pharmaceutical and semiconductor industries. That is despite their disadvantages and mainly because of their high tolerance to oxidants (chlorine, peroxide, etc.) as compared to PA membrane.

Aromatic Polyamide Membrane

Aromatic polyamide (PA) membrane is the most widely used type of RO membrane at present. They have found numerous applications in both potable and industrial water production. The thin polyamide film of this type of semipermeable membrane is formed on the surface of the microporous polysulfone support layer (Fig. 1.1). It is formed by interfacial polymerization of monomers containing polyamine and immersed in solvent containing a reactant to form a highly cross-linked thin film. PA membrane operates at lower pressures and have higher productivity (specific flux) and lower salt passage than CA membrane. Which are the main reasons they have found a wider application at present.

While CA membrane has a neutral charge, PA membrane has a negative charge when the pH is greater than 5. Which amplifies co-ion repulsion and results in higher overall salt rejection. However, when pH < 4, the charge of PA membrane changes to positive and rejection reduces significantly to lower than that of a CA membrane. Another key advantage of PA membrane is that they can operate effectively in a much wider pH range (2-12). This allows easier maintenance and cleaning. In addition, PA membrane is not biodegradable and usually have a longer useful life – 5-7 years versus 3-5 years. Aromatic polyamide membrane is used to produce membrane elements for brackish water and seawater desalination, and nanofiltration.

Comparison between PA and CA Membrane

It should be noted that PA reverse osmosis membrane material is highly susceptible to degradation by oxidation of chlorine and other strong oxidants. For example, exposure to chlorine longer than 1000 mg/L-hour can cause permanent damage of the thin-film structure and can significantly and irreversibly reduce membrane performance in terms of salt rejection. Oxidants are widely used for biofouling control with RO and nanofiltration membranes. Therefore, the feed water to PA membrane has to be dechlorinated prior to separation. Table 1.4 below presents a comparison of key parameters of polyamide and cellulose acetate RO membrane in terms of their sensitivity to feed water quality.

Parameter Polyamide Membrane PA Cellulose Acetate CA Membrane
Salt rejection High (> 99.5%) Lower (up to 95%)
Feed pressure Lower (by 30 to 50%) High
Surface charge Negative (limits use of cationic
pretreatment coagulants)
Neutral (no limitations on pretreatment coagulants)
Chlorine tolerance Poor (up to 1000 mg/L-hours);
feed de-chlorination needed
Good; continuous feed of 1 to 2 mg/L of chlorine is acceptable
Maximum temperature of source water High (40 to 45°C; 104 to 113°F) Relatively low (30 to 35°C; 86 to 95°F)
Cleaning frequency High (weeks to months) Lower (months to years)
Pretreatment requirements High (SDI < 4) Lower (SDI < 5)
Salt, silica, and organics removal High Relatively low
Biogrowth on membrane surface May cause performance problems Limited; not a cause of performance problems
pH tolerance High (2 to 12) Limited (4 to 6)

Table 1.4: Comparison between Polyamide PA Membrane and Cellulose Acetate CA Membrane materials

Polyamide PA membrane is the choice for most RO membrane installations today. Mainly because of their higher membrane rejection and lower operating pressures. Exceptions are applications in the Middle East, where the source water is rich in organics. Thus cellulose acetate membrane offers benefits in terms of limited membrane biofouling and reduced cleaning and pretreatment needs. CA membrane provides an acceptable tradeoff between lower fouling rates and chemical cleaning costs. Also higher operating pressures and power demand on the other. Because of the relatively lower unit power costs in the Middle East. There are newer generations of lower-fouling PA membranes today on the market. The use of CA membrane elements is likely to diminish in the future.

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

bqua semipermeable RO membrane element separate dissolved solids tds suspended solids tss

Semipermeable Membrane Definition

Semipermeable membrane is a thin, soft, pliable sheet of material that allows certain substances to freely pass through it while restricting the passage of other substances. In the water treatment field, a semipermeable membrane allows water to pass freely while restricts the passing of dissolved materials.

Water passes through a semipermeable membrane into a solution of higher salt concentration. This phenomenon is called Osmosis. The semi-permeable membrane is a material through which water may pass, but prohibits the traveling of nearly everything else. Because of pressurized feed water enters each pressure vessel, a portion of the incoming water goes through the semipermeable membrane. The semi-permeable membrane selectively restricts the passage of suspended and dissolved elements in the raw water.

bqua semipermeable membrane element separate dissolved solids tds suspended solids tss

Semipermeable membrane separating TDS and TSS 

The incoming supply water and the dissolved and suspended substances which don’t pass throughout the semipermeable membrane go to waste. One stream enters an RO system and two streams exit. The permeate is the water stream and small amount of TDS which goes through the Semipermeable Reverse Osmosis membrane. While the concentrate is the waste stream containing the water and elements which don’t go through the semipermeable membrane. The concentrate is simply concentrated incoming stream of water. The concentrate contains the TDS and the TSS that are not allowable to pass through the semipermeable membrane.

Read more about the RO Membrane.

Reverse Osmosis RO Membrane Element

 

BQUA provides RO membrane element manufactured by major market leaders of the industry. From tap water, brackish and seawater RO membrane elements. Of the RO Membranes BQUA proudly offers: Dow Filmtec, Hydranautics and Toray. BQUA only provides its customers with the highest quality and top-rated membrane materials for a lifetime operation without any problem. Below are the elements provided by the company. Please feel free to browse the brand names and you can always contact our technical sales department for more information regarding a specific application or solution.

 

Hydranautics RO Membrane

bqua hydranautics ro membrane element

bqua hydranautics ro membrane element

Hydranautics RO Membrane

Hydranautics RO membrane is known for its performance and productivity and for continuously meeting the increasing demands of the water treatment industry. It has an excellent salt rejection  and savings.  These membranes set high performance standards for the reverse osmosis membrane elements which deliver the highest salt rejection rates available.

Hydranautics RO membrane lower fouling in waste and surface water while having a high fouling potential for reuse and reclaimed applications. These membranes offer superior membrane technology to lower membrane fouling and treatments to treat difficult feed and municipal wastewater. To this point, these applications have significantly required a pretreatment before exposing them to a composite polyamide membrane.

The Hydranautics RO Membrane can range in size from 4” to 8” elements, delivering the highest levels of salt rejection and an always pure end product. They are designed to accommodate varying levels of seawater salinity worldwide with dependable field-proven performance. The Hydranautics RO membrane is manufactured with a thicker brine spacer which reduces the Delta P, meeting the increased demand for reduced fouling membranes. Which maintains a higher permeate flow and permits a lesser need for frequent cleaning.

Hydranautics RO Membrane Features

  • High Rejection
  • Sea Water Membranes
  • LFC Membranes for Low Fouling
  • SanRO Membranes for Sanitary or Full Fit

Hydranautics RO Membrane Elements

ESPA:  Low Energy/High Flow Tap & Brackish Water Thin Film Membranes
Energy required to pressurize Reverse Osmosis feed water is the largest contributor to the total energy consumption of the Reverse Osmosis System. As a result, chlorine tolerant membranes have helped the membrane technology become affordable and cost effective by reducing energy consumption required to operate Reverse Osmosis water treatment system. ESPA membranes are the choice for applications demanding high-energy efficiency, with un-compromised productivity and salt rejection.

Product Name Size Download
ESPA4-4040 4″ diameter & 40″ length
ESPA4-LD-4040 4″ diameter & 40″ length
ESPA4 LD 8″ diameter & 40″ length
ESPA4 MAX 8″ diameter & 40″ length

CPA (Composite Polyamide):  High Rejection Tap & Brackish Water Thin Film Membranes
Ultrapure water is essential in several industrial applications. CPA RO membrane is regarded as the industry standard for all critical high purity applications – from pharmaceutical to power industry. CPA lines of spiral-wound Reverse Osmosis membrane is available in a variety of sizes and deliver unmatched performance with the highest salt rejection rates.

Product Name Size Download
CPA7-LD 8″ diameter & 40″ length

SWC (Seawater Composite):  Seawater Membranes
As the world is facing freshwater scarcity, Hydranautics brings a range of SWC RO membranes to meet the demands of desalination industry. SWC membranes have improved the productivity and salt rejection for more than two decades while reducing their environmental impact. SWC membranes come in a range of innovative formulations depending on the level of seawater salinity required.

Product Name Size Download
SWC4-LD 8″ diameter & 40″ length
SWC4 MAX 8″ diameter & 40″ length
SWC4-1640 16″ diameter & 40″ length

LFC:  Low Fouling Membranes
LFC3-LD from the LD Technology™ innovative low fouling membranes combines the attributes of a neutrally charged surface with hydrophilicity to achieve the lowest organic and colloidal fouling in the most demanding feed water conditions. Combining the attributes of a neutral surface charge and hydrophilicity, LFC3-LD provides significant reduction in fouling rates increasing the membrane’s efficiency by restoring nominal performance after cleaning.

Product Name Size Download
LFC3-LD-4040 4″ diameter & 40″ length
LFC3-LD 8″ diameter & 40″ length

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