## 2 Materials and Methods  
### Preparation of Membranes  
Porous alumina tubular supports (19-channel; outer diameter of 30 mm and the length of 1200 mm) was chosen for the foundation. The average pore size was about 4 um, and its porosity was 40%. The stable coating slurry was prepared as follows: the alumina powders (Luoyang, He'nan, China, Purity \(\geqq\) 99%) were used without further treatment; the particles size distribution of alumina powders is shown in Figure 1. The alumina was dispersed in pure water, Dolapix CE-64 as a dispersant was added, and the addition amount was 0.05 wt % of the powder mass. The suspension was mixed with \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Coating Process** & **Advantages and Disadvantages** & **Authors and Literature** \\ \hline Dip-coating process. The support is placed vertically, and the suspension is driven by a pump to maintain the flow inside the support’s inner cavity at a certain flow velocity, One-step coating and sintering is used. & It is easy to operate with high efficiency, but the performance of the ceramic membranes depends on the pump power and the support properties. However, this leads to different parts having uneven membrane thicknesses. & [PERSON] [17]; [PERSON]; [PERSON] “Modified dip-coating method for preparation of pinhole-free ceramic membranes” \\ \hline Dip-coating process, two-step coating and one-step sintering. A layer of organic material is coated, and then the separate layer of slurry is coated. & Low cost and high efficiency are the advantages of the process, but the viscosity of the organics and sintering system have a great influence on the bonding properties of the ceramic membranes. It is much harder to industrialize. & [PERSON] [17]; [PERSON]; [PERSON] “Modified dip-coating method for preparation of pinhole-free ceramic membranes” \\ \hline Dip-coating process, boehmite mixed with alumina particles as the membrane slurry, two-step coating and one-step sintering. & The process is highly efficient and has a low cost. However, the stability of the slurry is difficult to control, and the membrane slurry, two-step process is complex. It is easy to identify membrane defects, but hard to industrialize. & [PERSON] [19]; [PERSON]; [PERSON]; [PERSON] “One-step preparation of high-performance bi-layer \(\alpha\)-alumina ultrafiltration membranes via co-sintering process” \\ \hline Dip-coating process, sol-gel method, two-step coating, and two-step sintering. & The performance of the membrane is high, and the preparation has a high cost. It has low efficiency, and the process is complex. & [PERSON] [20]; [PERSON]; [PERSON] “Preparation and characterization of y-alumina ceramic ultrafiltration membranes for pretreatment of oily wastewater” \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Preparation process of ceramic membranes. a ball mill at 150 rpm for 3 hr with different solid contents of 5 wt %, 6 wt %, 7 wt %, and 14 wt %. Then, an aqueous solution of 30 wt % or 38 wt % Polyvinyl Alcohol (12% concentration, PVA-1799) was added as a binder. This suspension was ball milled for 2 hr to prepare the stable ceramic membrane slurry. In ball milling, the mass ratio of powder/alumina ball/alcohol was 1:1:1.8. The process is shown in Figure 2.  
(1) One-step coating process:  
The support was placed vertically and filled with suspension with 14 wt % solid content, 30 wt % PVA solution with a viscosity of 40 Pa.s. The coating time was 20 s, and the superfluous suspension was discharged from the bottom using the effect of gravity. (2) Two-step coating process:  
The preparation process was similar to that in the traditional coating process. The difference was that the coated membrane needed to be flushed again after the one-step coating process. The solid content was 5-7 wt %, the content of the PVA solution was 38 wt %, and the viscosity was 55 Pa.s. Then, the support was placed horizontally, and the suspension was placed in a 2 m high storage tank. The flow rate of the slurry was 0.8 m/s.