Introduction and Literature Review
1.4 Possible scope for further research
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Alternative approaches to reduce the high cost of ceramic membranes refer to the utilization of cheaper inorganic precursors (raw clay (Saffaj et al. 2005), Moroccan clay (Saffaj et al., 2004), powdered apatite (Masmoudi et al., 2007), dolomite (Zhou et al., 2010), kaolin (Bouzerara et al. 2006), Tunisian clay (Khemakhem et al., 2006), sepiolite clay (Weir et al., 2001) and Algerian clay (Khider et al., 2004)) with which the sintering temperature can be significantly lowered to 900 – 1100 oC. Further, Xia and Liu (2001) indicated that uni-axial dry compaction method is an inexpensive method and can be used to prepare membranes on a laboratory scale (Xia and Liu, 2001). While reported compositions of inorganic materials in the literature for inexpensive ceramic membrane preparation refer to the utilization of materials such as quartz, CaCO3, kaolin and feldspar, ceramic membranes have not been reported till date with both conventional materials (quartz, calcium carbonate, etc.) with waste materials such as fly ash. Therefore, the preparation, characterization and application of ceramic membranes that consist of the said materials is to be targeted first to judge upon the utility of fly ash for membrane preparation. Based on the membrane performance, further improvement in inorganic material composition can be targeted.
1.4.2 Preparation, characterization, optimization and application of low cost fly ash based ceramic membranes
In the previous sub-section, research efforts have been dovetailed towards the verification of fly ash as an important constituent in inexpensive ceramic membrane fabrication. However, a key feature of the mixed clay membranes is that the membranes consist of significantly higher quantities of materials other than fly ash. In this regard, it is important that fly ash constitutes the highest composition in the ceramic membranes and only in such a scenario, the membrane cost would be significantly lower due to the enhanced utilization of waste material for value added product development. Therefore, the thesis need to eventually target
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the preparation and characterization of inexpensive fly ash based ceramic membranes, which need to have significantly different composition than those reported for mixed clays in the literature (Abadi et al., 2011; Yang et al., 1998; Cui et al., 2008). The fly ash based ceramic membrane cost is expected to be significantly low due to three reasons, namely significant utilization of fly ash waste material whose value is insignificant, lower sintering temperature (1100 oC) and simpler fabrication method such as uni-axial dry compaction method.
Thereby, the thesis primarily targets the resolution of environmental issues associated to fly ash by producing a value added product such as ceramic membrane with applications such as treatment of oily wastewater streams.
1.4.3 Preparation, characterization and application of TiO2–Fly ash composite membrane
Generally, ceramic clay and polymeric membranes are characterized to be hydrophobic. Due to this feature, these membranes require higher transmembrane pressure (in the order of several bars) to separate water from other constituents (Chakrabarty et al., 2008). Hence, quicker flux decline, greater fouling and lower flux are apparent. To counter such effect, surface modification techniques are adopted to enhance the hydrophilicity of the membranes (Li et al., 2009; Ahmad et al., 2011; Yi et al., 2011; Sadeghian et al., 2015). The hydrophilic surface modification refers to the deposition of hydrophilic nanoparticles of various materials such as γ-Al2O3 (DeFriend et al., 2003), ZrO2 (Zhou et al., 2010) and TiO2 (Chang et al., 2014) in the ceramic membrane porous structure. These nanoparticles enable an enhancement in the hydroxyl groups existent on the membrane surface and hence greater hydrophilicity and reduction in transmembrane flux decline (Tsuru et al., 2008; Pan et al., 2012). Among several mentioned materials, titania and alumina are inexpensive materials for the alteration of morphological and separation characteristics of ceramic membranes. Among
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these two, titania is inexpensive compared to alumina and therefore the first choice in this section is to fabricate titania composite membrane.
Generally, hydrothermal method was extensively used to deposit TiO2 and Al2O3 particles on the ceramic clay supports (Chou et al., 1999; Wang et al., 2007) but not on fly ash membrane supports. In this method, usually, alkoxide precursors of alumina are used which are expensive and significantly toxic (Patterson et al., 2006). However, for the same method, the utilization of TiCl4 instead of titanium butoxide, titanium-isopropoxide is promising due to its lower cost and relatively non-toxic nature (Zhang et al., 2001).
In the available prior-art, titania composite membranes have been prepared utilizing mixed clay (Chou et al., 1999), α-Al2O3 (Chowdhury et al., 2003; Li et al., 2006) and Cordierite (Saffaj et al., 2004) as supports. However, TiO2-fly-ash composite membrane has not been reported in the literature till date (Dong et al., 2010; Fang et al., 2011; Jedidi et al., 2011).
Therefore, there is scope to fabricate a new low cost and non-toxic TiO2 composite membrane. Needless to convey, the ceramic membranes shall also have better separation efficiencies towards the separation of oil-in-water emulsions. Hence, the next area of research that needs to be addressed in this work is with respect to the preparation, characterization and application of low cost TiO2-fly ash composite membranes.
Also, the application of TiO2-fly ash composite membrane needs to target the cross-flow microfiltration of oil-in-water emulsions as these studies would ensure the actual performance of the membranes for synthetic emulsions. This may not be the case for dead- end microfiltration studies.
1.4.4 Response Surface Methodology for the MF performance of membranes
Till date, RSM based parametric analysis and optimization have been studied for various processes including nanofiltration (Salahi et al., 2013; Abadikhah et al., 2014), microfiltration (Jokic et al., 2010, Rastegar et al., 2011), biosorption (Ghaedi and Kokhdan, 2015),
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adsorption (Savasari et al., 2015; Santos and Boaventira, 2008), electrocoagulation (Tir and Moulai-Mostefa, 2008; Chavalparit and Ongwandee, 2009; Gengec et al., 2012; Un et al., 2014; Olmez et al., 2009), electrochemical (Thirugnanasambandham et al., 2015; Kushwaha et al., 2010; Yue et al., 2015; Mohajeri et al., 2010; Korbahti and Tanyolac, 2008), coagulation-flocculation (Wang et al., 2014; Wang et al., 2011; Wang et al., 2007; Ghafari et al., 2009; Tringh and Kang, 2011; Jadhav and Mahajan, 2014), ion exchange (Bashir et al., 2010) and wet peroxide oxidation (Shi et al., 2014). In the field of microfiltration, very few studies were conducted and only one study carried out RSM analysis for oil-in-water emulsion treatment using ceramic membranes. Hence, it can be concluded from the prior-art that while RSM is a useful technique for the optimal analysis of flux and rejection characteristics with variation in process parameters. The RSM based oily wastewater treatment using ceramic membrane based microfiltration process has not been reported till date and is the primary objective of this work.
Thus, there is enough scope to conduct research and examine the optimality of dead-end and cross flow MF process parameters for oil-in-water emulsion treatment using several membrane data obtained in this work. Based on further analysis of experimental data, the research has been confined to fly ash and titania-fly ash composite membranes.
1.4.5 Preparation, characterization and application of TiO2–clay, γ-Al2O3– clay and clay membranes
After a thorough examination of available prior-art, it is also apparent that the additional research in the preparation, characterization and application of titania, γ-alumina-clay composite membranes is also beneficial for a comparative outlook with respect to titania-fly ash membranes. Also, while literature refers to experimental investigations for higher oil feed concentration, studies are scarce with feed concentration as low as 250 mg/L. Therefore, there is enough possible scope for the preparation, characterization and application of clay
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and titania/γ-alumina clay composite membranes so as to infer upon the performance efficiency of fly ash membranes from both experimental and theoretical point of view. The titania and γ-alumina layer on the clay membranes are to be targeted using hydrothermal method. Further, the membranes have been investigated for their separation characteristics during dead-end MF operation.
1.4.6 Cross flow microfiltration studies for TiO2–clay and clay membranes
Till date, several investigations targeted the microfiltration of titania-clay (Chou et al., 1999;
Saffaj et al., 2004) and clay membranes (Potdar et al., 2002; Benito et al., 2005; Nandi et al., 2010; Fang et al., 2013). However, it is well known that the clay membrane performance is a strong function of its composition. Therefore, the clay membranes prepared in this work have to be judged so as to understand upon their real performance. This is also due to the fact that membranes prepared with uni-axial dry compaction method may not have exactly similar morphology than those prepared with other methods such as extrusion, slip casting, etc.
Therefore, while data is available in the literature for the said membranes, cross flow microfiltration has to be conducted for these membranes to infer upon the optimality of fly ash based composite membranes for oil-in-water emulsion treatment.