Abstract
Photocatalysis with titanium dioxide semiconductor catalyst can effectively degrade recalcitrant organic pollutants present in biologically treated sewage effluents. Focusing on process efficiency and sustainability within a broader program, this study presents results obtained with a bench-scale hybrid treatment system. The process train comprised of a slurry (suspension) type continuous photocatalytic (CP) system and an immersed hollow fibre membrane micro-ultrafilter (MF/UF) unit. The CP reactor charged with 1 g/L P-25 catalyst removed 63% dissolved organic carbon (DOC) from a synthetic wastewater (representing biologically treated sewage effluent). The addition of 0.05 g/L of powdered activated carbon (PAC) increased DOC removal up to 76%. The start-up times to achieve 60% DOC removal were 31 min and 15 min, respectively. These results show a 16 times improvement in volumetric load over a comparable batch reactor system used in previous studies by our group. Slurry type photocatalytic reactors need subsequent particle separation to retain the catalyst in the system and allow the discharge of treated effluent. The immersed membrane module accomplished this without prior slurry settling step. Membrane feed pre-treatment with pH adjustment and particle charge neutralisation with aluminium chloride coagulant led to improved critical membrane fluxes, 15.25 L/m 2 h and 19.05 L/m 2 h, respectively. In each experiment MF/UF produced near zero turbidity permeate, completely retained the photocatalyst, and flocculation also improved the efficiency of DOC removal. Membrane fouling was controlled by particle aggregation rather than feed DOC levels, but the latter had significant impact on coagulant demand. The complete treatment train achieved up to 92% DOC reduction with 12 mg/L AlCl 3 dosage using in-line coagulation conditions. The results show that in-line coagulation offers a simple yet effective means to improve the performance of slurry type photocatalytic–MF/UF hybrid systems for advanced water and wastewater treatment applications.
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