Publication: Templating superhydrophobic polyvinylidene fluoride membrane with 3d-surface microstructure to treat aquaculture wastewater using membrane distillation
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Date
2022-05-01
Authors
Teoh, Guang Hui
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Abstract
Fish farming industry is an important sector to provide food for human beings. However, its wastewater has negatively affected the environment which required to be treated. During membrane distillation (MD) process, organic and microalgae in the wastewater can lead membrane wetting and fouling. Hence, membrane with macro-and micro-structures were introduced to induce eddies for reducing foulant adhesion and enhance its wetting resistance. Principally, crystalline polymorphs phase of PVDF membrane strongly affects its wettability. However, it was found that surface roughness has greater impact on membrane hydrophobicity rather than the polymorphs. This work evaluated the performance of surface-printed PVDF membrane using templated substrates of different structures through non-solvent induced phase inversion (NIPS) method. During NIPS process, ethanol coagulation bath can be used to trigger nano-roughness of the membrane. The surface-printed membranes have showed impressive improvement in hydrophobicity. Among all the membranes, Lining A and Lining B membrane with macro-structures and micro-valleys created on the surface presented excellent wetting resistance. However, Lining B membrane demonstrated the best anti-wetting properties with high contact angle of 153.3°, low contact angle hysteresis of 8.5° and low sliding angle of 8.9°. Lining B membrane showed the
highest flux of 27 kg/m2.h which was 125% higher than the non-surface-printed membrane when treating high saline water in 3-hours DCMD process. This is due to the surface structures created on the surface provided extra evaporation surface area. Next, modelling based on heat and mass transfer was performed to predict vapor transport behavior in DCMD operation. Indeed, reliability of the model has been verified because predicted flux and experimental results only showed a 10% difference. Among all MD parameters, feed temperature has the most significant effect on MD operation. In actual separation of fish farm wastewater, Lining B membrane has also proved its effectiveness with inorganics rejection as high as 92% and completely rejected microalgae. The Lining B membrane also demonstrated a stable flux throughout 3- and 30-hours DCMD separation. Under 48-hours batch DCMD feed concentration process, water recovery rate reached 86.3%. The final concentration of fish farm wastewater was at least five times the initial concentration of ammonia (16.4 to 82.2 mg/L), phosphate (18.0 to 99.8 mg/L) and potassium (68.0 to 384.8 mg/L), which are the primary nutrients in liquid fertilizer. In overall, the use of surface-printed membrane in DCMD process has successfully treated fish farm wastewater by simultaneously concentrating nutrients and recovering clean water.