Photocatalytic dual layer hollow fiber membrane for colour pigment degradation of aerobically treated palm oil mill effluent

Abstract

Large amount of wastewater is produced during the processing of palm oil. The final form of effluent is aerobically-treated palm oil mill effluent (AT-POME), an odourless and oil-free brown solution. The brown pigments found in AT-POME pose high risk to eutrophication and water contamination if they are released into natural water bodies without further treatment. Current treatment methods to remove colour pigments such as nanofiltration face membrane fouling due to pore blockage. Therefore, a more efficient method to remove pigments from AT-POME has to be developed. Hence, this study focused on the fabrication of a photocatalytic dual layer ultrafiltration hollow fiber membrane for colour removal from AT-POME. Novel boron doped titania nanotubes (TNT-B) photocatalyst was incorporated on the outer layer of a polyvinylidene fluoride (PVDF) dual layer hollow fiber membrane (DLHFM). The molarity of boron doped into TNT was manipulated between the 0.25 M (TNT-B0.25), 0.5 M (TNT-B0.5) and 1.0 M (TNT-B1.0). TNT-B was prepared via a two-step hydrothermal method. The physicochemical properties of prepared TNT-B were characterized. The prepared photocatalyst were also tested for their photocatalytic activity under visible light irradiation for the photodegradation of lignin and tannic acid (TA), which are the two important constituents of AT-POME. TNT-B0.5 exhibited the best photocatalytic activity, where it was able to degrade lignin and TA up to 96.47% and 96.91%, respectively. High surface area (159.552 m2/g) and visible light absorption have contributed to remarkably enhanced photocatalytic activity. TNT-B0.5 was then used as the photocatalyst to prepare PVDF DLHFM at a loading of 1 wt% the outer layer of membrane. The membranes were spun using a triple orifice spinneret, while three important spinning parameters, i.e. bore fluid flow rate (BFFR), outer dope solution flow rate (OLFR), air gap (AG) were manipulated to study the effect of these parameters on membrane characteristics and filtration performance. All membranes were analysed to understand their morphology and physicochemical properties. It was deduced that a BFFR of 3 ml/min, OLFR of 3 ml/min and AG of 10 cm were the optimum spinning conditions to prepare DLHFM with high flux and high rejection. The optimised membrane was then loaded with different photocatalyst loadings (1 wt%, 2 wt%, 3 wt%) on the outer layer. Then, the effect of different loading towards dynamic photocatalytic filtration and antifouling properties using synthetic AT-POME as model pollutant in a submerged membrane photo reactor (SMPR) was evaluated. With 2 wt% TNT-B0.5, the optimised DLHFM exhibited the highest flux of 51.29 L/m2h and rejection of 79.42% when tested with synthetic AT-POME. The membrane also exhibited superior antifouling properties in which the flux was recovered by 95% after four filtration cycles of synthetic AT-POME. The optimised membrane was then used to treat real AT-POME for 20 days. The results showed that an increase in both flux and rejection over treatment time which was due to the synergistic effect of photocatalysis and membrane filtration.