Materials used for membrane fabrication

fabricating low-cost membranes. Emani et al. fabricated a disc-shaped ceramic membrane using kaolin as a key precursor for treating oil-in-water emulsion and successfully achieved 98.52% reduction in oil content (Emani et al., 2014). Research work of Fatimah et al. also mentions about the fabrication of tubular ceramic membrane using kaolin and its subsequent application in water filtration (Fatimah et al., 2015). Another cheap precursor that is being considered for fabricating low-cost ceramic membranes is Moroccan clays. Membrane prepared using Moroccan red clay and natural phosphate had an average pore size of 2.5 μm and hydraulic permeability of 928 L/(hm²bar). It displayed good turbidity removal efficiency for seawater, FeS tannery beamhouse effluent as well as synthetic salt water (Mouiya et al., 2018). Several other literatures also cite the use of materials such as Moroccan Perlite, bentonite, etc., for the fabrication of ceramic membranes to be implemented in the treatment of industrial wastewater (Majouli et al., 2012; Bouazizi et al., 2016). Another such clay material that is also being used for low-cost membrane fabrication is Tunisian clay. Khemakhem et al.

extruded a paste of Tunisian silty marls to obtain tubular ceramic membranes, which were further used for cuttlefish effluents treatment. The membrane successfully reduced the turbidity of the permeate to below 1 NTU, along with a significant decrease (around 65%) in the COD value (Khemakhem et al., 2009). Another microfiltration membrane, fabricated using similar clay material, was implemented in the water treatment plant of Sfax, Tunisia. The membrane showed outstanding rejection performance by completely removing Biochemical Oxygen Demand (BOD) and 99% turbidity in treated effluent (Kamoun et al., 2020). Besides these clay materials, another cheaper precursor that is being widely used for low-cost membrane fabrication is fly ash. Flat ceramic membranes with a pore size range of 1.30-1.44 µm were fabricated for the treatment of oil-water emulsion. The fabricated membranes well served their purpose by achieving rejection up to 97% (Suresh et al., 2016). A similar study carried out by another group of scientists reported the preparation of disk-type membrane using fly ash for

oil-water separation, where a rejection of 99.2% was observed with a membrane pore diameter of 1.2 µm (Singh and Bulasara, 2015). A mixture of fly ash and kaolin was also used for the preparation of circular ceramic membranes. The fabricated membranes were found to be very effective in separating humic acid from water, with an average rejection of 98.46% (Rawat and Bulasara, 2018).

Fig. 1.3 Precursors used for fabrication of low-cost ceramic membranes

Fig. 1.4 Number of journal publications on fabrication of fly ash-based membranes (Courtesy: Google Scholar) (Accessed on 7^th March, 2021)

The research on fly ash for membrane fabrication was initiated in 2006 and only very recently, publications on this topic have started to increase steadily, as depicted in Fig. 1.4. The growing importance over utilization of fly ash in membrane fabrication can be attributed to the generation of huge quantities of fly ash by different thermal power plants across the world.

Fly ash is considered as a great threat to the ecosystem as it can pollute water and air equally.

Fly ash generated from thermal power plants is one of the leading causes of respiratory health problems owing to its silica content that causes irritation to mucus membrane of the lungs, diseases like asthma, allergy, pulmonary fibrosis and even cancer (Cho et al., 1994; Dhadse et al., 2008). The fly ash is also a potential source of ground water contamination due to its heavy metal content (Gamage et al., 2011). India ranks second in the countries of world in fly ash generation and China is the largest producer with approximately 600 MT of fly ash generated every year (Yao et al., 2015). In India, the coal used for generating power in thermal power plants is of very poor quality with an ash content of almost 30-45%. Fly ash generation in India has increased significantly from 86 MT in 2000-01 to 217.04 MT in the year 2018-19. The exact scenario regarding the generation and utilization of fly ash in India is pictorially depicted in Fig. 1.5.

This huge quantity of fly ash generated is partially used in cement, bricks and tiles industries, filling of mines and low-lying areas, construction of flyovers and bridges, etc. However, as of 2018-19, 22.41% of fly ash generated remains unutilized and is dumped inappropriately in open places leading to air as well as water pollution (Yousuf et al., 2020). Therefore, it can be inferred that the use of fly ash in membrane fabrication is beneficial as it not only reduces the membrane cost but also overcomes the problem with improper dumping of the waste material (Goswami and Pugazhenthi, 2020b). Moreover, the use of fly ash in membrane fabrication is attractive as a process of converting waste into a valuable material that can further be utilized for various purification purposes. Observing the two-fold benefits associated with using fly ash

as key precursor in membrane fabrication, it can be said that preparation of fly ash-based ceramic microfiltration membrane can be a great area for researchers to focus on in the coming years.

Fig. 1.5 Fly ash generation (a) and utilization (b) in India (Yousuf et al., 2020)