Improved flux and anti-fouling performance of a photocatalytic ZnO membrane on porous stainless steel substrate for microalgae harvesting

Photocatalytic self-cleansing ZnO membranes were developed and used for pre-concentrating microalgae solution prior to harvesting. The inorganic membranes were fabricated via two different methods: (i) direct deposition of ZnO onto a porous stainless steel substrate and (ii) post-growth oxidation of a deposited metallic Zn layer. Systematic surface characterisation revealed a thin layer of homogeneous, crystalline ZnO on the porous membrane. Direct ZnO deposition resulted in a thicker layer with higher UV light absorption capability compared with the post-growth oxidization method. The ZnO coating made the membrane surface highly hydrophilic, resulting in two-fold increase in water permeance compared to the base stainless steel substrate. The high hydrophilicity of the ZnO-coated membrane also led to an increase in the permeate flux of the microalgae solution by up to 100%, making it suitable for microalgae pre-concentration. Upon UV light irradiation, the ZnO membrane demonstrated self-cleansing capability due to the photocatalytic activity of the ZnO coating layer. After 30 min of UV irradiation, the ZnO membrane could achieve 60% permeance recovery after complete fouling. No recovery was observed with the base stainless steel membrane, which was used as the control.