## 3 Results and Discussion
### Alumina Microfiltration Membrane  
An important advantage of using membrane filtration is the retention of the microorganisms present in the water, such as bacteria. The most harmful bacteria in water are usually larger than 0.4 \(\upmu\)m [28]. To prevent the permeation of bacteria, the average pore size of the effective membrane layer must be smaller than the minimum size of the bacteria. Thus, it is necessary to apply a microfilter separation layer to a macroporous support prior to water filtration. Conventional dip-coating was used to deposit the alumina microfiltration membrane on the multichannel support. The ceramic suspension for the dip-coating was prepared using the composition given in Table 1. Darvan-CN was added as a dispersing agent to enhance the dispersion of alumina particles in the suspension. The relationship between the dispersant and viscosity was studied to optimise its concentration. The amount of dispersant varied between 0.05 and 1 wt%, as shown in Figure 2. A ViscoQC(tm) 100-L viscometer ([PERSON], Graz, Austria) was used to measure the viscosity at a shear rate of 30 s\({}^{-1}\). The results indicated that the optimal amount of Darvan-CN was 0.6 wt% as this concentration offered the lowest viscosity (13.87 mPa-s). The viscosity trend may be explained by the dissociation of Darvan-CN into negatively charged carboxylic groups that are absorbed on the positively charged alumina surface via attractive electrostatic forces. At the optimum concentration (0.6 wt%), the dispersant formed a monolayer on the alumina particles. As a result, the electrostatic repulsive forces generated and increased the fluidity of suspension [29; 30]. At a lower concentration of dispersant, the particles were not completely covered with dissociative ions that led to partial attractive and repulsive forces, causing flocculation. Beyond the optimum concentration, the polymeric layers of the dispersant tended to agglomerate and make the suspension more viscous [31]. The coated support layers were subsequently dried at room temperature and then heat-treated at 1300 \({}^{\circ}\)C. This produced a membrane layer without any cracks. The average pore size of this microfiltration membrane was 0.09 \(\upmu\)m, as determined by the mercury porosimetry. We observed from the SEM analysis that the thickness of the bottom part of the membrane was larger than the thicknesses of the top and middle parts. This resulted in a nonuniform membrane along the support length. During the dip-coating, the bottom  
Figure 2: Effect of increasing dispersant concentration on the viscosity of Al\({}_{2}\)O\({}_{3}\) suspension for microfiltration coating. part of the support layer remained in contact with the suspension for a much longer time compared with the top and middle parts, forming a relatively thick layer. Based on this morphological analysis, membrane inhomogeneity may be controlled by fabricating a double-layered microfiltration membrane on the subsequent substrate. During the second dip-coating, the substrate was inverted so that the bottom side became the top, forming a uniform coating layer across the support. Figure 3 shows a broad view of single- and double-coated microfiltration membranes. Figure 3a shows a non-uniform single-coated membrane with an average thickness of 10.2 \(\upmu\)m. After the second coating, a homogeneous \(\alpha\)-Al\({}_{2}\)O\({}_{3}\) membrane, tightly integrated with the substrate, formed. It had a uniform coating with an average thickness of 17.5 \(\upmu\)m, as displayed in Figure 3b. In Figure 4, a cross-sectional SEM micrograph of a double-coated microfiltration membrane is presented. It is apparent from the SEM image that the double-coated microfiltration membrane was successfully developed on the multichannel tubular support without any defects. Figure 4: Cross-sectional SEM image of the asymmetric multichannel cylindrical tube-type substrate; (1) alumina macroporous support; (2) double-coated alumina microfiltration layer.