Comparison of external and submerged membranes used in anaerobic membrane bioreactors: Fouling related issues and biological activity

•Anaerobic sludge was filtered in side-stream tubular and submerged hollow fiber membranes.•Particle size distribution, EPS, fouling rate and chemical cleaning efficiency were compared.•Fouling of the external membrane was notably greater than of the submerged one.•Shear stress associated to sludge recirculation caused a reduction in the particle size and VSS.•The effectiveness of the physical and chemical cleaning processes were also different. Anaerobic sludge filtration tests were performed with two experimental set-ups equipped with either submerged hollow-fiber membranes or external tubular membranes operated with low crossflow velocity and gas sparging in a gas-lift mode. Particle size distributions, extracellular polymeric substance concentrations, methanogenic activity, reversible and irreversible fouling rates, and chemical cleaning efficiency were all studied and compared. An increase in the percentage of smaller particles and a decrease in the concentration of volatile suspended solids in the external membrane set-up were observed. Both soluble protein and soluble polysaccharide concentrations were also higher in the external filtration. Operating at 12–15 L/m2 h for 7 days, the total resistance of the external membrane reached 9.3 × 1012 m−1, while the resistance of submerged membrane reached 1.1 × 1012 m−1 despite operating at 15–25 L/m2 h for 12 days. Finally, the type of fouling and the efficiency of chemical cleaning also differed in both configurations.