Membrane fouling characterization and cleaning performance in anaerobic digestion effluent treatment by AGMD: Effect of feed pH

Water recovery and nutrient enrichment are crucial in anaerobic digestion (AD) effluent treatment from both economic and sustainable perspectives. Air gap membrane distillation (AGMD) is proposed as an efficient strategy, primarily because of its low energy requirements. However, membrane fouling and cleaning efficiency during AD effluent treatment via AGMD have rarely been investigated. Our study explored the impact of feed pH value on membrane fouling and flux recovery ratio during AD effluent treatment using AGMD. When the AD effluent was concentrated to 5 times, the normalized water fluxes of AD effluent at pH = 5, 7, and 9 decreased to 54 %, 42 %, and 30 % of the initial water fluxes, respectively. The characterization results of fouled membranes illustrated that the membrane fouling included inorganic scaling (e.g., calcium-, phosphorus-, and silica-) and organic fouling (e.g., polysaccharides, humic substance, and protein). The hydrophobicity of fouled membranes decreased with an increase in feed pH value. This is because the acidification of AD effluent reduced inorganic scaling and mitigated the inorganic-organic fouling, as observed in the changes in the component proportion of organic and inorganic foulants on fouled membranes. According to correlation analysis, the interaction between Mg2+/Ca2+ and organic components might be the key factor contributing to organic-inorganic membrane fouling. Furthermore, the mechanisms of membrane fouling development during AD effluent treatment at different pH values were identified through regression analyses using blocking filtration model. Finally, the flux recovery ratios reached 86 %-90 % after using the cleaning strategy of deionized water, sodium hypochlorite, and citric acid in sequence.