The Development of Reverse Osmosis Membranes - Term Paper Example

Name Professor Institution Date Reverse Osmosis Introduction The mention of the term reverse osmosis drives one directly to the concept of how membranes work. In every living organism, membranes play a very vital role that ensures that the human body remains healthy. The skin as an example of a membrane helps control the amount of sweat to cool off the bodies through evaporation of tiny water droplets during hot weather (410). The lung also contributes to the passage of oxygen from the inhaled air and in turn releases carbon (IV) oxide in the same stream (410). This work is done by the fine cells present in the lungs. The lung ensures that these cells are permeable enough such that nitrogen is not inhaled despite its high percentage in the air that living things breathe. Apart from the lungs and the skin, the kidney is another very efficient membrane that once the body is healthy, only a quarter of it can control processes like regulating release of water, salt ions, proteins and other nutrients within the body (410). In this case, the kidney remains a very sensitive membrane in the body just like the lungs. In other words, the membranes are important in that they help maintain the cell’s contents to be in the exact place and go further to regulate the input and output rates of nutrients (410). The use of membranes does not just confine itself to living organisms only, artificial membranes have been made to help in desalination processes. Such processes are like reverse osmosis that help in daily life to produce clean drinking water. The process entails diffusive mechanism for effective and dependent separation of key components (Crittenden et al., 2005). An understanding of the concept of reverse osmosis can be drawn first from the general concept of osmosis. From the scientific perspective, osmosis is the movement of a solvent from a point of weaker concentration to a point of stronger concentration through a membrane (410). This process normally ends up diluting the solvent such that the concentrations of the solutions become equal on both sides of the membrane (410). The molecules pass across the membrane by the help of osmotic pressure, a force that is vital in dissolving water and nutrients in living organism (410). This same concept is borrowed by reverse osmosis. In reverse osmosis, pure water is produced by forcing saline or impure water through a semi permeable membrane. This membrane is normally one that cannot allow salts and impurities to pass through. Therefore, this process of reverse osmosis is normally used for water filtration, desalination of sea water and in kidney dialysis machines (American Water Works Association & Bergman 2). In this comprehensive introduction, it is clear that the focus of this paper is geared towards a detailed discussion of reverse osmosis as a membrane process of producing pure water. The discussion thus will involve a look at the elements of membrane separation processes, the principles of membrane separation and application and different membrane processes. The paper will further discuss the different construction materials and module configurations as well as the components of RO desalination process. A good approach to this would involve having a brief historical development of membranes, artificial membranes and reverse osmosis process integrated along the problem statement. Historical Development of RO Membranes The use of membranes started as early as the time of early agriculture where household sieves were used to separate fine grain from coarse grain particles and shells (410). This later extended into the manufacture of cheese; there was use of cheesecloth made from cotton fibers. This kind of separations was based on the size of particles. Later, the rise of technological advancements took the use of membranes to another level. Modern membranes emerged, membranes that were made on the basis of differences in solution and diffusion rates of various species across the membrane material (410). This is what led to the rise of these new artificial membranes. Researchers and practitioners in this field have successfully produced fresh water from seawater in the mid-1950s although the flux was too low to be commercially viable (Glater 297). The development started with a definition of osmosis which Dutrochet (1823) termed as movement of solvent across a membrane from low to high concentration and dialysis which he stated as movement from high to low concentration (411). The studies went on until late 1800s that a discovery was made that oxygen could be produced by arranging membranes between a reservoir of pressurized air and another of unpressurized air (411). This concept was later used in the recovery of NaOH through dialysis from waste water (411). The history of RO membranes started later in the 1950s with development of cellulose acetate RO membranes followed by the use of asymmetric cellulose acetate membrane that had the ability to resist salts (411). The development of RO membranes was as a result of wide research that later led to its commercialization. For example, the Gulf General Atomics and Aerojet General used such membranes to make spiral wound modules; Permasep-10 permeator able to produce portable water from sea water was also introduced; the development continued to a level whereby high level membranes were produced with ability to have high water influx and salt rejection (411). Today, the application of Ro membranes is in areas such as portable water production, waste recovery, food applications, kidney dialysis, high purity for boiler feed and ultrapure water electronics applications (412). The development of ROI membranes has been helpful for most companies that purify water in the world. Major investments are now done in this area because of the ever increasing technology of RO membranes. Membrane Module Configurations for Reverse Osmosis In the use of reverse osmosis, certain membrane module configurations are important. It is upon such configurations that the whole RO process and system is defined and built. Such include hollow fiber and spiral wound. The hollow fine fiber configuration is one that uses hollow fibers got from cellulosic or non-cellulosic material (418). This configuration is more asymmetrical in structure and its fineness can be found in 42 microns (418). Once millions of such fibers have been formed into a bundle, a perforated plastic tube, acting as a feed water distributor gets inserted in the center to extend the length of the bundle (418). Both ends are normally sealed to from a sheet-like permeate tube. In the cylindrical housing, one finds hollow fiber membrane of 10-20 cm in diameter (418). Once water is pressurized, it enters the permeator feed end and flows around the filter bundle toward the outer permeable pressure wall (418). This allows water to pas through the outside wall of the fibers into the hollow core and exits through product connection on the feed end of the permeator. One thing to note is that is such a configuration, the water flow is normally low. This explains why such hollow fibers operate with a minimum reject flow to minimize concentration polarization and maintain the flow (418). Such fibers are normally used in seawater applications. Also to note is that hollow fiber water modules require high quality feed water that is, with low concentration of suspendable particles than any other configuration (418). When it comes to spiral wound configuration, the membranes which are flat sheets, are set apart permeator collector channel material to form a leaf (419). This forms the assembly with a feed spacer material sheet added to it and wound around a central plastic permeate tube. This is such that the industrial spiral wound membrane has a size of 100-150 cm long and 10-20 cm in diameter (419). A three to six membrane element connected in series in a pressure tube is usually added to it for it to operate in an acceptable way (419). Such spiral wound elements must always comprise of CA blend or thin film composite. The composite in this case can include polyamide, polysulfone, polyurea or other polymers (419). A study of these configurations is thus a good step to discussing the components of the RO system. Components of the RO System When it comes to the components of the reverse osmosis, certain elements like feed water supply unit, pretreatment system, high pressure pumping unit, membrane element assembly unit, permeate treatment and storage unit and cleaning unit all comprise a reverse osmosis system. The way this components works to complete the desalination process is very vital as it helps in understanding RO. In this system, the mesh strainers normally help in removing large particles from the water (421). This is always after the surface feed water has been disinfected to help control the biological activity. For the waters from the wells, no much chlorination is always needed and therefore no biological activity takes place. Chlorination for other waters is always used to oxidize iron and manganese in the well water before filtration starts (421). In a case where the water has hydrogen sulphide, no chlorination should be made because in most cases oxidation will take place and in turn produce the element sulphur which blocks the membrane at some point (421). According to Malki (28), antiscalants can control acid soluble scales at a fraction of the dosage required to control the same scale using sulfuric acid. The pretreatment of well water normally involve a controlled amount of screening of sand, addition of scale inhibitor to the feed water and cartridge filtration (422). This is because it contains low concentrations of particles as opposed to surface water. Surface water should always be treated to control biological activity and removal of the many suspended particles. In this case the filtration process must be very efficient through addition of filtration aids. For the system to completely block water born particles, cartridge filters are normally used in all RO systems before the use of high pressure pump (422). To enhance such, new pretreatment equipments have been used for users of RO. The equipment has a back washable capillary microfiltration and ultra filtration membrane modules (422). This calls for use of higher micron filters which will not require frequent replacement. Defining the RO Process The study of RO process is vital as well in understanding the desalination process by use of reverse osmosis. Defining in details the RO process involves a study of variable such as Osmotic and operating pressure, salt rejection and permeate recovery. Also, this process is well defined by the mention of the system specifications which are closely tied to feed qualities like salinity and temperatures. A measure of the concentration of dissolved salts in a solution will help one get to know the osmotic pressure of that solution marked as π. In other words, a direct determination of osmotic pressure can be done using the following equation: π=R TI where π is the osmotic pressure (kPa) T is the temperature (K) R is the universal gas constant, 8.134 kPa m3/kdmol/m3 i is the concentration of all constituents in a solution (kgmol/m3) (415). In this case, an approximation is always made for π to be 75.84 kPa of osmotic pressure (415). When it comes to operating pressure, adjustments is normally done because of factors like osmotic pressure, friction losses, and membrane resistance and permeate pressure. If operating pressure is equal to all these factors, the flow would be minimal, almost to zero. This explains why the operating pressure needs to be higher. In terms of salt rejection, the values should be high as technological advancement has put it to 99% for both seawater and brackish water membranes (415). This parameter can as well not works without the concept of permeate recovery in RO systems. This is because the rate of recovery affects passage and product flow. As the recovery rate increases, the level of salt concentration across the membrane also increases thus causing an increase in salt flow rate across the membrane (415). High salt concentration as well, increases the osmotic pressure thus reducing the product water flow rate (415). High levels of membrane recovery thus can be achieved by use of spiral wound membranes within the same vessel of pressure. Characteristics of the RO Membrane The study of reverse osmosis and the membranes that facilitate the process during desalination is only very well established when one gets an oversight of the features of the RO membranes. First the membranes are made up of thin film of polymeric material that is thick and casted on polymeric porous material (416). The RO membranes that are considered commercial are normally made up of high water permeability and high degree of semi-permeability. This level is always achieved to ensure that the rate of water transport is higher than the rate of transport for dissolved ion (416). This therefore calls for stable membranes that have a variety of PH and temperature and have good mechanical integrity (416). On the life length for commercial membranes, one should go for those that last as they normally go for 3-5 years. Such membranes though should be replaced annually depending as well on the quality of the water, pretreatment conditions and stability operation (416). Examples of such commercial RO membranes include cellulose acetate (CA) and Polyamide (PA). When one is choosing a membrane, he/she should always look at factors like compatibility as opposed to separation performance and flux factors. The CA membrane is one made up of cellulose diacetate and triacetate through a process of film casting, cod bath leaching and high temperature annealing (417). While the cast process removes solvent through evaporation, the cod bath removes the remaining solvent and leachable compounds as the annealing step improves the semi permeability of the membrane (417). The membrane is asymmetrical in structure and exhibits a dense surface layer. The polyamide membranes have two layers and its manufacture normally ensures that membrane is like a salt rejecting skin. The membrane itself is very stable and also has a wide PH than the CA membranes. Unfortunately sometimes they succumb to oxidative degradation by free chlorine (417). The surface is always smooth, and really charges just like CA. in terms of performance, the CAS membranes are more preferred to Polyamide ones because of their free nature to chlorine and their neutral surface (417). Difference between RO and other Membrane-based Desalination Processes The desalination process can be done either through reverse osmosis, nalofiltration, ultra filtration and microfiltration. As this paper discusses RO, it is worth noting the difference between the above mentioned processes. For example, sieving mechanism is not used in RO but in filtration. In this mechanism, the membrane does the function of allowing only smaller particles to pass and retains larger. Contrary to this, in RO, the membrane permeates only the solvent and retains the solute. Secondly, the other processes are largely used to separate suspended material while reverse osmosis is normally used to separate dissolved solids (412) This explains why in reverse osmosis the direction of solvent is largely dependent on the chemical potential of the solvent. This duty is normally done by factors like pressure, temperature and concentration of dissolved solids (412). Take an example of pure water that is contact with both sides of a semi permeable membrane; when the temperature and pressure is equal then no flow will be witnessed. If a salt is added to one side, the chemical potential is reduced on that side leading to a flow until equilibrium is state is realized. In this case, the hydrostatic pressure is normally equal to the osmotic pressure (412). The difference between RO and other processes can also be in the issue of size particles. Microfiltration for example does operate on a particle size ranging from 0.15µm to 0.15 µm while ultra filtration operates in particle size range of 0.15 µm to 5× 10-2 µm. Nanofiltration on the other hand operates between 5×10-2 µm to 5×10-3 µm while reverse osmosis operates between 5× 10-3 µm to 5× 10-4 µm (412). This shows clearly that RO could be most preferred to other desalination processes in water purification. Conclusion Most, if not all living organisms depend on some level of membrane activity for proper functioning and other normal (metabolic) processes. As mentioned earlier, certain membrane activities detail the use of membranes in supporting normal coordination of the body. Ultimately however, such activities are not limited to living things and extend into explaining other artificial processes such as desalinization. For the most part, reverse osmosis plays a major role in production of pure water in the sense that it integrates elements of membrane separation, the processes as well as principles of application. A chronological account of reverse osmosis points to possible utilization of RO membranes as early as the agro revolution. Reverse osmosis is as old as the existence of agro industries. For instance, cheese manufacturing was part of the extended historical development of reverse osmosis membranes. Subsequent rise in technology saw the process jump a niche higher, marking the rise of artificial membranes to aid the process. Structurally, the RO systems contain elements like feed water, supply unit among other components. These form an integral part of the RO process and allows for the systems to adequately cover the desalination process. The economics of RO process as well as its operational commitment reflects its complexity in equal measures as its significance. The basic premise of understanding and/or employing reverse osmosis is to integrate important components along the process gradient. For instance, membrane module configuration remains as important as the process itself. As aforementioned, the configuration assumes an asymmetrical structure on the hollow fine fiber extracted from either cellulose or non cellulose material. Alternatively, configuration can be in form of spiral wounds which incorporates a pair of flat sheets separated by a permeate collector. Incorporating the RO model together with either configuration allows for smooth operation of the reverse osmosis. In it, reverse osmosis combines distinct system variables such as osmotic pressure, concentration polarization, permeate recovery and salt injection among others. The whole essence of integrating such variables is to ensure a system balance of each component. In the general sense, reverse osmosis serves an important role in water purification or desalination industries. One such (practical) application is the role it plays in military units where RO water purification units are used in battlefield training. The scope of the process extends into serving eco friendly and sustainable functions such as waste water purification in large quantities. Works cited: American Water Works Association & Bergman, Robert. Reverse osmosis and nanofiltration, 2nd ed. US: American Water Works Association. 2007. Class notes. Chapter 7: Reverse osmosis. Crittenden, John. Trussell, Rhodes. Hand, David. Howe, Kerry and Tchobanoglous, George. Water Treatment Principles and Design, 2nd ed. US, New Jersey, John Wiley & Sons. 2005. Glater, J. (1998). "The early history of reverse osmosis membrane development". Desalination 117: 297–309. 1998. Malki, M. Optimizing scale inhibition costs in reverse osmosis desalination plants, International Desalination and Water Reuse Quarterly 17 (4): 28–29. 2008. Read More

Today, the application of Ro membranes is in areas such as portable water production, waste recovery, food applications, kidney dialysis, high purity for boiler feed and ultrapure water electronics applications (412). The development of ROI membranes has been helpful for most companies that purify water in the world. Major investments are now done in this area because of the ever increasing technology of RO membranes. Membrane Module Configurations for Reverse Osmosis In the use of reverse osmosis, certain membrane module configurations are important.

It is upon such configurations that the whole RO process and system is defined and built. Such include hollow fiber and spiral wound. The hollow fine fiber configuration is one that uses hollow fibers got from cellulosic or non-cellulosic material (418). This configuration is more asymmetrical in structure and its fineness can be found in 42 microns (418). Once millions of such fibers have been formed into a bundle, a perforated plastic tube, acting as a feed water distributor gets inserted in the center to extend the length of the bundle (418).

Both ends are normally sealed to from a sheet-like permeate tube. In the cylindrical housing, one finds hollow fiber membrane of 10-20 cm in diameter (418). Once water is pressurized, it enters the permeator feed end and flows around the filter bundle toward the outer permeable pressure wall (418). This allows water to pas through the outside wall of the fibers into the hollow core and exits through product connection on the feed end of the permeator. One thing to note is that is such a configuration, the water flow is normally low.

This explains why such hollow fibers operate with a minimum reject flow to minimize concentration polarization and maintain the flow (418). Such fibers are normally used in seawater applications. Also to note is that hollow fiber water modules require high quality feed water that is, with low concentration of suspendable particles than any other configuration (418). When it comes to spiral wound configuration, the membranes which are flat sheets, are set apart permeator collector channel material to form a leaf (419).

This forms the assembly with a feed spacer material sheet added to it and wound around a central plastic permeate tube. This is such that the industrial spiral wound membrane has a size of 100-150 cm long and 10-20 cm in diameter (419). A three to six membrane element connected in series in a pressure tube is usually added to it for it to operate in an acceptable way (419). Such spiral wound elements must always comprise of CA blend or thin film composite. The composite in this case can include polyamide, polysulfone, polyurea or other polymers (419).

A study of these configurations is thus a good step to discussing the components of the RO system. Components of the RO System When it comes to the components of the reverse osmosis, certain elements like feed water supply unit, pretreatment system, high pressure pumping unit, membrane element assembly unit, permeate treatment and storage unit and cleaning unit all comprise a reverse osmosis system. The way this components works to complete the desalination process is very vital as it helps in understanding RO.

In this system, the mesh strainers normally help in removing large particles from the water (421). This is always after the surface feed water has been disinfected to help control the biological activity. For the waters from the wells, no much chlorination is always needed and therefore no biological activity takes place. Chlorination for other waters is always used to oxidize iron and manganese in the well water before filtration starts (421). In a case where the water has hydrogen sulphide, no chlorination should be made because in most cases oxidation will take place and in turn produce the element sulphur which blocks the membrane at some point (421).

According to Malki (28), antiscalants can control acid soluble scales at a fraction of the dosage required to control the same scale using sulfuric acid.

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