Desalination: Membrane Materials for Reverse Osmosis Water

Abstract

TFC-PA appears to be the most utilized RO films during brackish H2O appliance as well as H2O desalinations. These membranes have low chlorine tolerance and fouling characteristics. This makes it necessary to develop fouling resistant and chlorine tolerant membranes to improve their performance. Studies indicate that the preparation of effective membranes presents the potential availability of water purified through desalination.

Key Words: Desalination, Nanofiltration, Membrane.

Introduction

The reverse osmosis membrane is among the established separation technology applications utilized in a variety of industries for the separation as a unit division process. The technology can also be used as a blend with other processes (Park, 2010). In fact, there is a growing demand for the preparation of membranes that will assist in the desalination of brackish and seawater considering that water is gradually becoming scarce in many places.

Stratagem for the production of high fluctuation fouling resistant and chlorine resistant polyamide membrane

Water flux and solute rejection of polyamide thin-film composite (TFC) membranes have gradually increased. However, the membranes that have been manufactured are not fouling resistant. According to Park (2010), internal fouling caused by compaction can be controlled to a level if macrovoids in the membranes are lowered through an increased molecular mass polymer. Simultaneously, organic and biofouling rejecting membranes must be in a way such that elements are not connected to the surface of the membrane effortlessly. Current research indicates that only one kind of fouling can be resolved at a time. To illustrate this, when a higher molecular mass polymer is utilized fouling caused by compaction can be dealt with to a particular level. However, it does not yield surface fouling rejection membranes. Additionally, in surface activation through substance alteration of polymer to achieve surface fouling rejection membranes, the breaking of polymer leads to reduced compaction rejection membranes (Park, 2010). At the same time, the mixing of active polymer with support polymer can be utilized in the production of organic and biofouling rejection membranes. However, very limited polymers are accustomed to the popularly utilized membrane polymers. The idea of a mixed-matrix membrane (the tiny packing material distributed through a bigger polymeric intermediate) presents new levels of liberty to the production of superior membrane materials with increased substance, perfunctory, temperature stability and enhanced separation potential (Park, 2010). The fluctuation can also be enhanced through the utilization of absorbent nano-materials in mixed-medium membranes that release superior flow path for water to run from the supply end to the saturated side of the membrane (Glater, 1994).

To prepare a substance rejecting polyamide compound membrane, the connection between the element arrangement of polyamides and film substance interaction require to be identified. Under circumstances where polyamide becomes generated from fragrant principal diamines complex N chlorination succeeded by the polymerization of scented sphere via Orton reorganization, this must be accountable for by modifying the substance characteristic of such a polyamide.

On the other hand, the reorganization occurs once an amide correlation appears to be openly related to the benzenes’ orb. Aliphatic polyamides after reacting with chlorine result in N-chlorinated amide. The amide can then be rejuvenated to the original amide via treatment with an oxidant. However, advanced polyamide appears to be dormant to the substance known as an oxidative. The plan for improved substance rejection hence are the fortification of the active spots on aromatic circles, the processing of polymers with substitute amide substances and the incorporation of aliphatic derivative as well as tertiary amide linkages not directly linked to aromatic spheres. Polysulfones, which are unlike polyamide, seem to possess improved substance rejection given that they have powerful compound correlations amid O2, sulfured substances and carbonized elements. However, polysulfone is hydrophobic. This means that reverse osmosis (RO) cannot be produced unless the substance arrangement of polysulfone is modified by introducing managed degrees of hydrophilicity while maintaining the physical characteristics (Glater, 1994).

Polysulfone is recognized in the manufacturing of nano-filtration and loose reverse osmosis membranes that are extremely lenient to aqueous chlorine liquids (Hilal, 2004). However, the membranes are asymmetrical hence, flux is expected to be lower than that of thin film composite (TFC) membranes. Several nano-materials including carbon nanotube (CNT) possess significant level of chlorine tolerance. They can therefore be fixed onto the surface to create chlorine rejection properties (Schaep, 1998).

Contemporary position in TFC RO membranes

Thin film composite polyamide membranes that consist of three layers dictate the modern reverse osmosis membrane industry. These are polyester nonwoven fabric act as structural support (100μm thick), micro-porous interlayer support of polymer-like polysulfone (40μm thick) and an ultra-thin polyamide barrier layer on the outer surface (0.25μm thick). Other polymeric materials excluding polyamide function well as good thin film composite reverse osmosis membranes. The procedures and precise chemistries for manufacturing successful marketable reverse osmosis membranes remain proprietary. Research on the physicochemical characteristics of the available membranes revealed some important information regarding viable membranes (Schaep, 1998). The study was conducted on two high-pressure seawater reverse osmosis membranes including SWC4, SW30HR and brackish water reverse osmosis membranes. The study concluded that bare aromatic reverse osmosis membranes have surface essential properties that are very near to the forecast values of a polyamide founded on the conventional inter-surface polymerization chemistry on phenylenediamine and trimesoyl chloride. However, the coated membranes possess high oxygen and low nitrogen levels. Analyses of the possible parameters indicate that zeta prospects of coated membranes is considerably negative compared to the bare membranes. This is consistent with ATR-FTIR and XPS outcomes (Schaep, 1998).

The top commercially existing thin film composite reverse osmosis membranes have extremely thin blocking layer composed of cross-linked aromatic polyamide manufactured through polymerization of phenylenediamine and trimesoyl chloride as illustrated below.

Intra-facial polymerizations used in preparing the fragrant polyamide
Fig 1: Intra-facial polymerizations used in preparing the fragrant polyamide

Novel combination of Polyamides RO Membrane

Viable membranes accruing from the TFC polyamide appear to be hardly and entirely foulant defiant despite depicting perfect porosity and being first-rated. Researchers have been trying to augment the permeability and organic fouling rejection through membrane surface hydrophilization in later-treatment (Park, 2010). However, the physical compaction, chlorine tolerance and biofouling levels remain unsatisfactory.

Ivanovic (2011) in his study on the application of biofilm membrane bioreactor (BT-MBR) for wastewater treatment indicates that by the year 2025, more than 60 percent of the global population will experience water scarcity if the contemporary consumption level remains the same. The development of human society will amplify the need for fresh water. That is, the reuse and reclamation of water will be inevitable in the foreseeable future (Khan, 2010). The contemporary wastewater tools require upgrading to the more advanced treatment technologies. The technologies must possess the capacity to provide high quality treated water. Besides, the technologies must follow sustainable practices, be cheaper to operate and meet the legal requirements (Ivanovic, 2011).

Membrane bioreactor (MBR) technology comprises of activated sludge process and membrane technology is modernly recognized as the technology for future wastewater treatment. The technology has diverse advantages over conventional technologies. These include high quality and hygienic waste matter production, diminished footprint, reduced sludge production and manageable biomass separation. In fact, MBRs offer improved nutrient deduction capacity. Other advantages include the ease of operation compared to the activated sludge, high biomass activity given the accumulation of highly advanced microorganisms and increased resistance of biomass to poisonous substances. The use of biofilm process as opposed to activated sludge process results in the creation of a new treatment configuration, BF-MBR that has the capacity to merge the finest characteristics of MBR technology and biofilm water treatment process (Ivanovic, 2011).

Water scarcity

In many parts of the globe, water scarcity is common. The problem is even more pronounced in arid and semi-arid areas. The scarcity consequently signifies the limited options to the provision of water in seawater desalination (Abuhabi, 2012). The rejection of sodium and potassium chloride is critical in determining the effectiveness of nano-filtration membranes in desalination of wastewater and seawater. In places where chlorine concentration is high it is important to reduce the concentration to meet the World Health Organization standards (Al-Agha, 2005). The use of RO desalination plants to provide populations with pure water is crucial. The challenge of desalination arises from the high operational costs and quality improvement.

The utilization of RO membrane is important in desalination of sea and brackish water. The nano-filtration technology has been used in treating brackish water since it is effective despite the consumption of energy. The technology has the potential to reject diverse salts dissolved in water. In fact, the technologies offer increased water flux, reduced operational costs and diminished energy consumption. Commercial membranes such as NF-1 and ASP30 possess the capacity to reject a wide range of chloride based sodium chloride and potassium chloride salts. Research indicates that the NF-! Membrane possesses high rejection ability for salts compared to ASP30. With water continuously becoming scarce it is important for researchers to develop reverse osmosis membranes that will facilitate the desalination of brackish and seawater (Abuhabi, 2010).

Suggestions

Emaciated and coated nano-compound membrane

From various study literature, the rejection of mono valence ions within varied ions resolutions appear to be significantly less than that of sterilized mixture of monovalence saline. Therefore, the filtration system relies on the size segregation and Donna segregation given the double coating stimulated by ions on the aperture. However, membranes having aperture dimensions below those possessed by the hydrated ion within the source mixture could fully reject an ion. Nanoparticles including LTA-type zeolite display 0.4 nm pores (Abuhabi, 2010). MFI-type display 0.56 nm pores that are less than the common ions present in foul water. However, they are larger than the size of water molecule. The LTA-type particles ranging from 49-149nm with a Si/Al quotient of 1.5 are useful in making inorganic-organic thin film nanocomposite membranes. This corresponds to the theory that nanoparticles generate a straight trail for the flow of uncontaminated water. This only maintains salt ions in water. Additionally, the nanoparticles appear to be highly hydrophilic while bearing unhelpful charges that materialize to be extremely resistant to any anion. The nanoparticles are in hexane solution of trimesoyl chloride prior to the inter-surface poly-condensation. Membrane that composes the TFN appears to be manufactured using several zeolite dimensions, packs as well as the resultant membranes properties alterations (Abuhabi, 2012).

Emaciated coated nanoparticle membranes have a fine texture. They are more hydrogen bonding and negatively electrified compared to normal TFC membranes manufactured under similar circumstances. Consequently, the TFN membranes possess less fouling propensity than the TFC membranes. Additionally, TFN membranes display two times more fluctuation and considerable enhancement in salt rejection compared to handmade TFC membrane devoid of the zeolite nanoparticles (Al-Agha, 2005).

Research indicates that membranes containing sub-nanometer can desalt water when applied as reverse osmosis membranes. The small pores effectively reject ions exceedingly well. However, they conduct water as 10-1000 times depending on the nanotube loads compared to viable TFC RO membranes (Ivanovic, 2011). This is founded on the fact that salt dismissal and water movement is constricted even non-polar characteristics of CNTs. Therefore, the separation performance is not always particular to the nature of the concentric walls. In order to achieve photocatalytic reaction and anti-biofouling characteristics in the membranes, anatase TiO2 nanoparticles are heavily coated with a synthetic polymerized TFC polyamide membrane. The surface coat is then made operational with carboxylate groups (Kim, 2003). The membranes have previously been studied using E. coli with feed water. It was confirmed that they possess superior anti-biofouling characteristics in particular. By using ultra violet excitation, it was confirmed that there was compromise on flux and salt rejection of the unique membrane.

Membranes such as TFN and TFC allowed by chlorine

From study literature, chloride lenient polyamides membrane appears to be manufactured using aliphatics polypiperazineamide. They can also be manufactured using complex polymeric systems if the level of water flux is low. On the other hand, aliphatics polypiperazineamide can hardly be utilized during raised heat or stress considering the condensed challenge. In order to realize increased fluctuation chlorine resistant membrane, the polymer requires a benzene sphere to offer structural inflexibility and physical strength. However, to prepare such a membrane, Ivanovic (2011) suggest a system that can be used. In TFC polyamide manufacturing, the amine element can be poly while the alkaline element may be oxalic acid. The poly requires processing from attached copolymerization of enamine above poly polymer. The reactivity of the source copolymer with the solution above the base membrane offers cross-connected polyamide film. In such a coat, the amides’ elements and scented spheres are present. However, the amides bonds are hardly linked unswervingly to the scented sphere.

 Graft polymerization of poly with enamine
Fig 2: Graft polymerization of poly with enamine

In the case illustrated in the above equation, the acid is soluble in water. However, challenges may arise if the poly copolymer is insoluble in water immiscible natural solvent. Lack of an appropriate H2O dissolver may mean that the targeted polymers are produced through the mixture polymerizations processes. The resultant polyamide polymer may be used in the manufacture of asymmetric membranes.

Recent study indicates that non-composite TFC reverse osmosis membranes with multi-walled carbon nanotube possess enhanced chlorine resistance compared to standard TFC membranes. Nano-compound coverings appear to be typically extra defiant to compaction compared to similar uncontaminated polymeric casing. By utilizing biofouling together with compression rejecting the prop up membrane, the manufacturing of permeable silica founded TFN technology as well as CNT will facilitate the improvement in attaining high flux. The resistance to fouling and tolerance to chlorine will be achieved in the resultant membranes.

Development in zeolite membrane synthesis

Polymeric membranes permit both high perm selectivity and increased permeation. The manufacturing of the membranes is permitted by the availability of advanced technology to develop thin polymeric membranes. The manufacturing is relatively cheap yet the properties of the membranes limit their application in membrane reactors (McLeary, 2006).The drawback arise from the fact that the membranes have limited resistance to solvents, temperature and corrosive surroundings. This limits the application of these membranes in bioreactor and liquid-phase reactor systems. The inorganic membranes including metals and ceramics guarantee considerable payoff in chemical processes considering the superior features of structural, mechanical and thermal stability in addition to the solvent and chemical resistance of the membranes. These membranes allow for the regeneration through the oxidative removal of carbonaceous class at 400-499 degrees (Corry, 2008). They hence have extended life expectancies compared to polymeric membranes.

Addition of different additives to biofilm – MBR

While using MBR technology in manufacturing membranes, the addition of inorganic coagulants and cationic polymers is reported to improve membrane quality and the performance of membranes. A study conducted by Ivanovic (2011) on the addition of additives using two iron and two alum based coagulants with modified cationic polymer revealed improved the quality of the polymer. The study sought to investigate the connection between reducing soluble organic matter and membrane fouling. The optimal quantity was determined founded on the maximum figures of a predetermined coefficient additive. The degree reducing the colloidal matter as retained by the polymer was connected to the measured membrane-fouling pace. Iron chloride was used for investigation. At high dosage, the test showed the best performance concerning reducing foul rates. In polymerized alum, a reduction of fouling was observed to be three times. However, increased basicity of the inorganic coagulant did not yield improved membrane functioning (Bruggen, 2008). The altered cationic polymer exhibited significant possibility of instant fouling reduction. Importantly, Ivanovic (2011) observed that increased dosage of inorganic coagulants did not seem to be applicable devoid of observing the polymer concentration within the reactor. The diminishing of fouling was observed to compare well with SMP reduction. FCOD appeared to be a probable limit for estimating the rate of foulants. Synergetic results of increased phosphorous removal and decreased fouling making the iron chloride coagulant to be preferable to any other type of additives. These observations are critical for the preparation of membranes that will be effective in future in terms of performance through reduced fouling (Arnot, 2011).

Another study conducted in addition to the previous research regarding the enhancement in performance of membrane reactor in BF-MBR exhibited the potential to improve the performance of membranes. The test involved the investigation of the reduction of submicron particles, the increase in suspended flocculent and the diminishing of organic colloidal substances. Solvent agent put in the membrane at the filtrate stage produced positive results on the improved mixed filtration resulting in reduced fouling rates. The rate was reduced similar to the reduced alum and iron dosages. At increased dosages, iron displayed better results (Bruggen, 2008).

Lower organic loads on biofilm reactors present better effluent feature with positive filterability and reduced fouling rates. However, optimal consumption of aeration in bio treatment presents researchers with the issue of desalination in future to provide pure water from wastewater and seawater (Hoffmann, 2009).

Conclusion

The result of aeration intensity in reverse osmosis membranes as a model for fouling control demands for detailed examination of the commercially available membranes. Effort must be put to ensure that the membranes are effective while ensuring reduced energy consumption. Back pulsing with increased backwash intensity is potentially a strategy for fouling control. It has the capacity to diminish fouling emanating from pore blocking and narrowing of submicron particles in seawater. However, the method has not been fully studied in BF-MBR arrangement. The method requires investigation to help mitigate water scarcity.

References

Abuhabi, A. (2010). Characterization of nano-filtration membranes for brackish desalination. International Conference on Membrane Science and Technology, 4(6), 23-49.

Abuhabi, A. (2012). Rejection of chloride salts by nano-filtration membranes in brackish desalination. Towards Engineering, 2(3), 1-5.

Al-Agha, M. (2005). Desalination in the Gaza strip: Drinking water supply and environmental impact. Desalination, 173(2), 157-171.

Arnot, K. (2011). A review of reverse osmosis membrane materials for desalination: Development to date and future potential. Journal of Membrane Science, 370 (12), 1-22.

Bruggen, B. (2008). Drawbacks of applying nano-filtration and how to avoid them: A review. Separation and purification technology, 6(3), 251-263.

Corry, B. (2008). Physics and chemistry. International Journal of Science, 2(12), 1427-1434.

Glater, J. (1994). Desalination. Science Journal, 2(3), 325-345.

Hilal, N. (2004). Comprehensive review of nano-filtration membranes: Treatment, pretreatment, modeling, and atomic force microscopy. Desalination, 1(7), 281-308.

Hoffmann, S. (2009). Planet water: Investing in the world’s most valuable resources. Hoboken, New Jersey: John Wiley and Sons.

Ivanovic, I. (2011). Application of biofilm membrane bioreactor (BF-MBR) for municipal wastewater treatment. Upper Saddle River, NY: Sage Publishers.

Khan, S. (2010). Performance of suspended and attached growth MBR systems in treating high strength synthetic wastewater. Bioresource Technology in press, 2(1), 1-20.

Kim, S. (2003). Membranes. Membrane Science, 2(11), 157-165.

McLeary, E. (2006). Zeolite based films, membranes and membrane reactors: Progress and prospects. Microporous and Mesoporous Materials Journal, 90(1), 198-220.

Park, J. (2010). Desalination and water treatment. Upper Saddle River, NY: Sage Publishers.

Schaep, J. (1998). Removal of hardness from groundwater by nano-filtration desalination. Science Journal, 119(1-3), 295-301.