Understanding Reverse Osmosis: Functionality And Durability Of RO Membrane

Write about the effects of variation of pressure and flow rate in RO membrane.

What is Reverse Osmosis?

Reverse osmosis (RO) is defined as the process by which a solvent is able to pass through a porous membrane in the direction which is opposite of the natural osmosis (Gupta, 2012). This usually happens when the process is subjected to high hydrostatic pressure than the osmotic pressure. In terms of filtration, RO is the finest level of filtration. Them RO membrane is able to act as the main barrier to all the dissolved salts available, organic molecules and organic molecules, which have higher molecular weight than the approximation of 100 (Khurana and Sen, 2010). The available water molecules on the other hand are able to pass freely through the membrane and therefore able to create a purified product. The rejection rate of the dissolved salts is usually about 95% and goes to more than 99%. RO has some of major applications such as desalination of sea water, brackish water for drinking, waste water recovery, in food and processing industry, biomedical separations as well as purification of drinking water and industrial water (In Valladares et al., 2017). The principle through which RO happen is when the pure water flows from the dilute saline solution through the membrane to a level of higher concentrated saline solution point (Khurana and Sen, 2010). In the RO situation, the water molecules move from the saline solution to the area where the water concentration is high. Mostly, the RO process is considered a water purification method, which removes ions, molecules and large particles in water through the use of semipermeable membrane. Most of dissolved and suspended particles are removed by the RO process. Reverse osmosis membrane is a critical parameter which helps the functionality of the RO process (Gupta, 2012). As the most reliable, cost effective and energy efficient measure of producing fresh water, RO membrane is a critical element which must be able to function well at all times.  

The RO membrane comes after the pressure feed. This means that the functionality of the membrane will depend and be affected by the changes in pressure from the pump. The pressure system is able to force molecukle4s to the membrane edge (Berge, Gad, Khaled  and Rayan, 2009). Therefore the pressure and flow rate will increase and decrease at the same time making them directly related. The reverse osmosis membrane is made up of flat sheet of reverse osmosis membrane. The RO membrane are classified according to different parameters such as flow rate, salt rejection, water feed concentration, temperature, pressure and porosity. These parameters are able to play critical roles in determining the durability of the membrane (Mohammadi, Moghadam & Madaeni , 2002). The RO membrane functionality is able to depend on the balancing on the different parameters.  

RO Membrane Functionality and Barrier

The reverse membrane functionality is affected by different factors. Analysis of the effect when the parameters change is important to help to determine the efficiency of the reverse osmosis (Salls & University of Nevada, Reno, 2017). Flow rate and pressure are some of two key parameters which will be analyzed in this report. Changing the pressure and flow rate is able to lead to different functionality of the Rom membrane. Some of the functionality parameters which are affected include the salt rejection, recovery rate; permeate concentration (TDS) and also the minerals concentration in permeate (Berge, Gad, Khaled and Rayan, 2009).  Pressure change is able to affect the force which the membrane is able to experience as the molecules are able to move from one point to another. The pressure change is able to change the functionality of the osmotic pressure, which in RO is acting in the negative direction.

The RO membrane is porous in structure and this means that the flow rate will determine the amount of molecules which are able to pass through the membrane. The porosity is able to determine the amount of water molecules which are able to go across the membrane at any given moment. The RO membrane durability is affected by the flow rate. Increasing the flow rate increases the amount of water molecules which are available and fighting to go through the membrane at any given moment (Mohammadi, Moghadam & Madaeni , 2002). Thus the flow rate and pressure at the membrane are directly related. This is because the molecules available are able to provide the pressure change within the membrane edge. The flow rate is affected by the amount of pressure which is inserted to the membrane. Additionally, the pressure pushes the molecules to the membrane and this dictates the amount of stress inserted to the membrane (In Figoli, In Hoinkis, In Altinkaya, & In Bundschuh, 2017). This is able to directly affect the durability of the membrane. Increased flow rate means that many molecules are available to pass through the RO membrane. For the RO membrane to produce water, the pressure provided should be able to exceed the osmotic pressure. First, the osmotic pressure must be understood to depend on two factors which include the concentration of feed water and temperature (Gilau, and Small, 2008). The osmotic pressure (tt) is the product of feed water concentration C, temperature of feed water T and ideal gas constant R.

Applications of RO Process

Since the pressure has to be more than the osmotic pressure for RO to happen, increased pressure is usually required to make sure that the RO membrane has to function. These key factors which influence the osmotic pressure must be exceeded for the RO to happen (Gilau, and Small, 2008). The increase on pressure will therefore have key effects on the molecules and durability of reverse osmosis membrane. Increasing the pressure will increase the tension and thus durability (Voutchkov, 2017). Variation on pressure means different tensional forces are created and thus affecting the functionality. High pressure creates high tension and thus increasing the porosity of the membrane.

Pressure is one of the important parameter which affects the different parameters in RO. The functionality as well as durability of the RO is as well affected by the pre4ssure variation. As seen earlier, the pressure has to be more than the osmotic pressure for the RO membrane to function (Alexiadis, Bao, Fletcher, Wiley and Clements, 2006). In order to determine the effect of variation of pressure, all other parameters are kept at constant when the reverse osmosis happens. The factors are studies with effects to the change on pressure. The effects can be used to study the durability of the RO membrane according to the effect of the tension created on the membrane.

One of the factors which is affected due to the change on pressure is the percentage of salt rejection. Increasing pressure level is able to increase the salt rejection rate. The RO process helps to separate the salts in water and the water molecules and thus purifying the water. The increase of the pressure helps to increase the amount of water molecules reaching the membrane (Alexiadis, Bao, Fletcher, Wiley and Clements, 2006). This in turn increases the functionality of the RO membrane and thus increasing rate of salts rejection. Salt rejection means that the salts are prohibited from passing the membrane and thus purification process is achieved. The increased pressure means that high level of salt rejection is achieved since increase of molecule passing the membrane are available. The salt rejection means that high percentage of salt buildup within the membrane is attained (Staff, 2006)). This increases the pressure build up at the membrane point. The long term effect of the buildup means that the membrane durability will be reduced. The high pressure of the salts is able to reduce the porosity level of the RO membrane (Fritzmann, Lowenberg, Wintgens and Melin, 2007). Looking at the pressure increase and salt rejection rate, the pressure will have negative impact in the long run for the RO membrane. The salts are able to block the porous areas of the membrane and therefore in the long run the salt reject will reduce. This will negatively affect the functionality and durability of the RO membrane.

RO Membrane Parameters for Durability

Increasing pressure feed in RO membrane is able to bring more molecules for recovery. The pressure increase pressure increases the movement of molecules and thus increasing the molecule recovery rate. Supplying high pressure is able to increase the pressure for the functionality of the membrane (Fritzmann, Lowenberg, Wintgens and Melin, 2007). The molecules present for recovery are increased and they fight to go through the membrane. For the long term functionality and durability, the increased and high pressure is not recommended. The increase of the recovery is a good factor but in the long term, the recovery will reduce (Hu & Dickson, 2015). In order to maintain the durability, moderate pressure is required to supply minimal amount of stress for the RO to take place.

The pressure increase increases the stress to the RO membrane and therefore able to lead to the increase in recovery rate. The increased stress is not recommended for the long term durability of the RO membrane. The feed pressure creates increased porosity when high recovery rate is able to happen (Gilau, and Small, 2008). This will in the long term reduce the effectiveness of the recovery rate by the membrane. Thus increasing pressure will increase the recovery rate but it will reduce the effectiveness and durability of the reverse osmosis membrane in the long run. High pressure is able to have the highest rate of recovery of the water molecules. This is simply because a lot of molecules are supplied by the pressure to be recovered. As seen, in the long run, this affects the functionality and durability due to the high stress supplied to the membrane.

The different salts in water are filtered during the RO process. Their concentration is usually reduced when the pressure is increased (Warsinger et al., 2016). This is because the increase of pressure is able to increase the rate at which these salts are filtered away from the membrane. The pressure feed is able to have great impact on the water molecule movement through the RO membrane and this leads to reduction of salts filtered. The amount of salts such as fluoride in the permeate water is least when the pressure is maximum. This is because the fu8nctionality of the RO membrane is increased when the pressure is high and this leads to little amount of fluoride or salts passing. The long run effect of fluoride concentration is favorable by the high pressure (Cui, & Muralidhara, 2010). In the permeate water, the amount of salts required need to be low and this means that the membrane porosity will be maintained. The amount of salts or fluoride meant to pass through the RO membrane need to be lower to assure proper functionality. This will be able to assure high durability or the membrane for long.  Moreover, it is important to have the maintaining of low salts in the permeate water (Cadotte, 2010). This shows an increase in the effectiveness of the RO membrane functionality. This means that the high pressure feed will be important for the process and achieving the effectiveness. Then effectiveness level will reduce with time due to the pressure being supplied (Kucera, 2015). Although the high pressure feed is required for the effectiveness of the membrane functionality, the durability will reduce with time. Nevertheless, the durability status is not more important than the amount of fluoride and salts reduction on the permeate water. Therefore supply of minimum highest pressure which will provide the minimum amount if salts and fluoride in water is required.

The Effect of Pressure and Flow Rate

The pressure and water flow rate feed are able to affect the permeate salinity in terms of total dissolved solids (TDS). The permeate water should have the minimum amount of TDS. From analysis, the pressure increase is able to lead to the least amount of TDS and salinity of the permeate water (Singh, 2017). The increase in pressure is able to make sure more water molecules are able to pass through the RO membrane and therefore increasing its functionality. Varying the pressure therefore has a positive impact on the functioning of the membrane in terms of reducing the TDS level. Then pressure increase of pressure feed is crucial in reducing the salinity of permeate, which is a positive remark on the functionality of the RO membrane (Singh, 2017). Due to the porosity of the RO membrane, the increased pressure is able to produce tension to the membrane as many molecules are fighting to pass the membrane. The porous nature is affected when the increased pressure is supplied for long term. This affects the durability of the RO membrane and thus reduces its lifespan. Although the increased pressure feed has positive observation in terms of reduced TDS, the effect of the pressure is able to reduce the long term durability of the membrane (Warsinger et al., 2016 and Basile, and Catherine, 2015). For the long term effects, it is important to supply the required amount of pressure which will not strain the membrane more. The consideration for the reduce TDS has to take the forefront other than the pressure feed since this is able to measure the effectiveness of membrane functioning.

On the other hand, the increase in pressure feed to the RO membrane is able to result to an increase in the flow rate feed to the membrane. This means that the same effects which are realized due to the increase in the pressure feed will be experienced when considering the effects of increased water flow rate feed to the membrane (Cadotte, 2010). The increase of the flow rate feed is able to increase the amount of molecules at the edge of the RO membrane. The flow rate like the increase in pressure feed is able to pressure the molecules for filtering and increasing the tension of the membrane. These tensional forces are not fit when considering the durability of the RO membrane. Nevertheless, the consideration for the effective functionality when considering the functioning of the RO membrane is more important than the durability. The tensional forces due the increase in the flow rate feed are able to reduce the durability of the RO membrane. Likewise, the membrane functionality leads to reduction of the salts passing through the membrane as well as the salinity level in terms of TDS (Bucs, Vrouwenvelder, & Picioreanu, 2017)). Increasing the flow rate will lead to the increase of parameters such as the salt rejection and the recovery rate percentage. These parameters are used to determine the effectiveness of the membrane functionality. Therefore supplying the RO membrane with increase water flow rate feed will increase the effectiveness will reducing the durability in the long term.

Salt Rejection and High Pressure

In conclusion, both the flow rate and pressure feed increase are able to contribute to an increase in effectiveness of the RO membrane. On the other hand, these parameters increase are able to provide tensional forces which reduce the durability of the membrane. Precisely, the increase of the pressure feed increases the flow rate feed. The increase is seen to increase the recovery percentage as well as the salt rejection percentage. Moreover, their increase also leads to reduction of the salt or fluoride concentration and permeate concentration in terms of TDS. These are measures of effectiveness of the RO membrane. Therefore the increase in the flow rate feed and pressure feed to the systems helps to create more purified water. The long term durability is reduced due to the tension which is supplied by the increase of the molecules in the edge of the RO membrane.

References

Alexiadis, A., Bao, J., Fletcher, D.F., Wiley, D.E. and Clements, D.J. (2006), ‘Dynamic response of a high-pressure reverse osmosis membrane simulation to time dependent disturbances’, Desalination, 191, 397-403.

Basile, A., and Catherine C. (2015). Integrated Membrane Systems and Processes. Newark, NJ: Wiley. Retrieved from: https://public.eblib.com/choice/publicfullrecord.aspx?p=4529174.

Berge D, Gad H, Khaled I, and Rayan MA (2009). An experimental and analytical study of RO desalination plant. Mansoura Engineering Journal 34: 71-92.

Bucs, S. S., Vrouwenvelder, J. S., & Picioreanu, C. (2017). Biofouling in reverse and forward osmosis membrane systems. Delf: Technische Universiteit Delft.

Cadotte, J. E. (2010) "Interfacially synthesized reverse osmosis membrane" Retrieved from:  https://www.researchgate.net/publication/281347360_Reverse_osmosis_membranes_prepared_by_interfacial_polymerization_in_n-heptane_containing_different_cosolvents 

Cui, Z. F., & Muralidhara, H. S. (2010). Membrane technology: A practical guide to membrane technology and applications in food and bioprocessing. Amsterdam: Butterworth-Heinemann.

Fritzmann, C., Lowenberg, J., Wintgens, T. and Melin, T. (2007), ‘State-of-the-art of reverse osmosis desalination’, Desalination, 216, 1-76.

Gilau, A.M. and Small, M.J. (2008), ‘Designing cost-effective seawater reverse osmosis system under optimal energy options’, Renewable Energy, 33, 617-630.

Gupta S (2012). Drinking Water Quality: A Major Concern in Rural India, Journal of Barnolipi 1: 2249-2666.

Hu, K., & Dickson, J. (2015). Membrane Processing for Dairy Ingredient Separation. New York : John Wiley & Sons, Incorporated.

In Figoli, A., In Hoinkis, J., In Altinkaya, S. A., & In Bundschuh, J. (2017). Application of nanotechnology in membranes for water treatment. Boca Raton : CRC Press.

In Valladares, L. R., In Li, Z., In Elimelech, M., In Amy, G. L., & In Vrouwenvelder, J. S. (2017). Recent developments in forward osmosis processes. London : IWA Publishing.

Khurana I, and Sen R (2010). Drinking water quality in rural India: issues and approaches, Water Aid India.

Kucera, J. (2015). Reverse osmosis: Industrial processes and applications. Hoboken, New Jersey : John Wiley and Sons, Inc.

Mohammadi T, Moghadam KM & Madaeni SS (2002). Hydrodynamic factors affecting flux and fouling during reverse osmosis of seawater. Desalination 151: 239-245.

Salls, K. A., & University of Nevada, Reno. (2017). Transport of semi-volatile and non-volatile contaminants in direct contact membrane distillation. Reno, Nev. : University of Nevada, Reno, Ann Arbor, Mich. : ProQuest.

Singh, G. (2017). "Implication of Household Use of R.O. Devices for Delhi's Urban Water Scenario". Journal of Innovation for Inclusive Development. Retrieved from: https://jiid.in/2017/02/jiid21032429.pdf 

Staff, A. W. W. A. (2006). Reverse Osmosis and Nanofiltration. Denver: American Water Works Assoc.

Voutchkov, N. (2017). Pretreatment for reverse osmosis desalination. Amsterdam: Elsevier.

Warsinger, D. M.; Tow, E. W.; Nayar, K. G.; Maswadeh, L. A.; & Lienhard V, J. H. (2016). "Energy efficiency of batch and semi-batch (CCRO) reverse osmosis desalination". Water Research. pp. 272–282.