Effect of gradient profile in ceramic membranes on filtration characteristics: Implications for membrane development

A proper gradient structure in ceramic membranes can reduce the thickness of the top active layer and therefore enhance the filtration performance and enable a good mechanical stability simultaneously. We designed and fabricated different gradient ceramic membranes including 1-, 2-, and 3-layer ceramic membranes using a simplified single sintering process. To the best of our knowledge, the preparation of the ceramic membrane with gradient porous structure within the top layer and the effect of gradient profile on filtration performance have not been investigated to date. The gradient profile in the top layers can be easily controlled by choosing the slurries with different alumina grain sizes. The 3-layer membrane exhibited the highest water flux and the lowest membrane resistance due to the advantages of gradient porous structure. It retarded internal fouling because foulants smaller than the surface pore size in the gradient porous structure could be easily swept through the membrane by permeate flow as demonstrated by particle size retention tests. Fouling mechanisms were largely size dependent and were similar between different membranes. Transitionary stage between pore blocking and cake filtration was observed from bovine serum albumin (BSA) and sodium alginate foulants but not from activated sludge. Rate and severity of membrane fouling decreased with increasing gradient profile in ceramic membranes. The gradient membrane has the lowest total resistance. This study demonstrated a breakthrough in ceramic membrane development, i.e., developing the gradient porous membrane to enhance the filtration performance.