Moderate pre-photocatalysis-enhanced coagulation alleviates ultrafiltration membrane fouling of algae-laden water

Abstract

Moderate chemical pre-oxidation of coagulation to remove algae without breakage of the algae cells and release of algal organic matter has been well reported, while little was known about moderate pre-photocatalysis followed by coagulation on the ultrafiltration performance and membrane fouling control during algae-laden water treatment. Herein, three pretreatment methods (photocatalysis, coagulation and moderate pre-photocatalysis-enhanced coagulation (PC)) were investigated to reduce membrane fouling during ultrafiltration of algae-laden water. Moderate photocatalysis by a composite of Bi2O3-TiO2/PAC (Bi-doped TiO2 nano-composites supported by powdered activated carbon) not only avoided algae cell breakage and controlled the release of intracellular organic matter, but also halved the aluminum sulphate (AS) coagulant dosage, since Bi2O3-TiO2/PAC acted as both a photocatalyst and flocculation core. Under optimal conditions (1.2 g/L Bi2O3-TiO2/PAC irradiated under visible light for 20 min and 0.1 mM AS), the removal of OD680, DOC and UV254 were 83.8 %, 60.3 % and 77.3 % respectively. Moderate photocatalysis degraded medium and high molecular weight organics (biopolymers, humic substances and building blocks) to low MW acidic and neutral organics, resulting in effective removals of humic substances and microbial metabolites typically present in algal-derived pollutant. Following PC pre-treatment with ultrafiltration, 87.5 % increase in the final specific flux, 96.1 % reduction in irreversible resistance, and a change in the fouling mechanism from cake layer blockage to intermediate blockage. In addition, the zeta potential was reduced and the interaction force between pollutant-pollutant/membrane became a repulsive force. This work demonstrates the potential benefits of moderate pre-photocatalysis-enhanced coagulation to remove algal contaminants and mitigate ultrafiltration membrane fouling.