Development Of Nanofiltration Hollow Fibers Biology Essay

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Water hardness is one of the common issues in many countries. Predominant contributors of water hardness are as calcium, Ca2+ and magnesium, Mg2+. Hence, total hardness of water is often defined as the sum of calcium and magnesium. [1]

TH = Ca2+ + Mg2+

The term hardness is used to characterize water that does not lather well, cause a scum in the bath tub, and leaves hard, white, crusty deposits on coffee pots, tea kettle and hot water heaters. The failure to lather well and the formation of scum on bath tub is the result of the reactions of calcium and magnesium with the soap as follows:

Ca2+ + Soap Ca(Soap)2 ( a form of precipitate)

Despite such water will possess some inconveniences such as scaling in heating elements and galvanic corrosion in pipeline, presence of these dissolved ions in water is not harmful to one's heath according to the World Health Organization. [2] The objective of water softening is to reduce such dissolved ions in water so that an acceptable concentration is achieved.

Some of the conventional water softening methods that can be used for water softening are ion-exchange, chemical precipitation using lime or lime-soda and membrane-based approaches. [3]

1.1.1 Application of Chemical Softening

Chemical softening is often called as lime-soda softening. This is because lime (calcium hydroxide, Ca(OH)2 ) and soda ash (sodium carbonate, Na2CO3) are chemicals used to precipitate Ca2+ as CaCO3 and Mg2+ as Mg(OH)2. Lime is added to neutralizes any free acids and subsequently raise the pH of water until it is high enough to precipitate Ca2+ and Mg2+. Chemical precipitation using lime will remove carbonate hardness. If soda ash is added as well as lime, both carbonate and non-carbonate hardness can be removed.

Chemical precipitation is an effective softening process, but it does have some disadvantages. The process requires a lot of operator control to get an efficient result, which may make lime softening too operator-intensive for small treatment plants. The high pH used in lime softening can set colors in water and make them difficult to remove. Finally, lime softening produces large quantities of sludge which can create disposal problems. [3]

1.1.2 Application of Ion-exchange

Ion-exchange is one of the conventional approaches to soften water as a wide variety of dissolved solids, including hardness can be removed effectively. Resin and zeolite are often used in this process. Ion-exchange usually involves replacing of magnesium and calcium with non-hardness cation, usually sodium that are loosely bond to zeolite or resin. [4]

Hard water that is contacted with resin, in an ion exchanger follows a reaction as follows:

After the ability if the ion-exchanger to produce soft water has been exhausted, the resin must be regenerated. This is accomplished by passing a concentrated brine solution, NaCl through the ion-exchanger, causing the above reaction to be reversed. It is logical to expect that the effluent from the regeneration cycle will contain the hardness accumulated during the softening process as well as excess sodium chloride. With respect to that, such effluent must be disposed properly to avoid damaging to the environment.

On the other hand, less sludge and softener water is produced in ion-exchange softening as compared to chemical precipitation approach. Lastly, a recent research shows that the combination of ultrasound and ion exchange as a new method is able to improve the removal efficiency of hardness.[5]

1.1.3 Application of Membrane Filtration

A membrane is a thin layer of material that is capable of separating materials as a function of their physical and chemical properties when a driving force is applied across the membrane. Particle with size that is less than the membrane pore size can pass through the membrane while the larger size particles are retained. Membrane can be classified according to their pore size, namely microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (OS).[3] Nanofiltration

In water softening, NF is often chosen for softening water as it lies between RO and UF in term of selectivity of the membrane. RO is not favourable due to its high retention of macro and micro elements which is not necessary for softening water.[1] [6] Unlike other conventional softening methods, NF is able to produce purified soft water as it does not alter the composition of dissolved salts in water. Moreover, no regeneration is required in NF.

Generally, NF membranes are mainly in the configurations of composite flat sheet, spiral-wound and tubular membranes. Hollow fiber membranes have been widely used for liquid phase separation because of their high surface area to volume ratio and self-support capability than flat sheet. [7]

1.2 Objectives

This FYP project aims to assist in the development of high performance of NF hollow fiber membranes suitable water softening by means of layer-by-layer deposition of polyelectrolytes, and conduct a series of characterization.

1.3 Scope of works

2. Literature Review

2.2 Layer-by-layer (LbL) Deposition

In the early of 1991, a new technique to fabric membrane termed ass the layer-by-layer deposition method was proposed by Dexher. It is an attractive and easy technique for preparing composite polyelectrolyte multilayer membrane. This method involves the alternating immersion of a charged substrate into solutions containing cationic and anionic polyelectrolyte, which is often called as "static assembly method". [8] To improve this method, a novel kind of self-assembly method called as cross-flow dynamic static assembly was proposed recently. [8] The charged substrate can be any substrate that can support the adsorption of an initial layer of polymer.

Layer-by-layer deposition method was applied in this study due to its advantage to develop ultrathin skin of composite membranes through controlling of film thickness at nanometer scale to achieve minimal thickness for high flux. The layer-by-layer method enable film properties be optimized by varying polyelectrolyte or deposition condition. The charge near the surface of the film can be either positive or negative depending on whether films are terminated with a polycation or polyanion.[9]

Figure Schematic diagram showing the formation of thin organic films using layer-by-layer adsorption of polyelectrolytes [10]

2.3 Adsorption Process

2.4 Polyelectrolyte Multilayer (PEM)

[1] World Health Organization., Calcium and magnesium in drinking-water : public health significance. Geneva, Switzerland: World Health Organization, 2009.

[2] J.-C. Lou, W.-L. Lee, and J.-Y. Han, "Influence of alkalinity, hardness and dissolved solids on drinking water taste: A case study of consumer satisfaction," Journal of Environmental Management, vol. 82, pp. 1-12, 2007.

[3] S. W. Brett, M. R. Gaterell, G. K. Morse, and J. N. Lester, "A cost comparison of potable water softening technologies," Environmental Technology, vol. 20, pp. 1009-1018, Oct 1999.

[4] B. I. D. Sharon O. Skipton, Shirley M. Niemeyer, "Drinking Water Treatment: Water Softening (Ion Exchange)," Revised October 2008.

[5] M. H. Entezari and M. Tahmasbi, "Water softening by combination of ultrasound and ion exchange," Ultrasonics Sonochemistry, vol. 16, pp. 356-360, Mar 2009.

[6] P. Eriksson, M. Kyburz, and W. Pergande, "NF membrane characteristics and evaluation for sea water processing applications," Desalination, vol. 184, pp. 281-294, 2005.

[7] J. Mulder, Basic Principles of Membrane Technology: Springer, 1996.

[8] S. U. Baowei, T. Wang, Z. Wang, X. Gao, and C. Gao, "Preparation and performance of dynamic layer-by-layer PDADMAC/PSS nanofiltration membrane," Journal of Membrane Science.

[9] O. Y. Lu, R. Malaisamy, and M. L. Bruening, "Multilayer polyelectrolyte films as nanofiltration membranes for separating monovalent and divalent cations," Journal of Membrane Science, vol. 310, pp. 76-84, Mar 5 2008.

[10] M. L. Bruening, D. M. Dotzauer, P. Jain, L. Ouyang, and G. L. Baker, "Creation of Functional Membranes Using Polyelectrolyte Multilayers and Polymer Brushes," Langmuir, vol. 24, pp. 7663-7673, 2008/08/01 2008.