Effect of anaerobic digestion broth composition on membrane permeability

Abstract

The effect of composition of anaerobic digestion (AD) broth on membrane permeability was explored through stirred-cell filtration of fractionated broths using various microfiltration (MF) and ultrafiltration (UF) membranes in order to provide insight into the membrane performance in the membrane bioreactor. In the filtration of original AD broth, the cake formation offered the highest hydraulic resistance to permeation but it led to greater rejection of volatile fatty acids. The fine colloids in the supernatant had the greatest responsibility for the cake layer resistance compared to other components in the broth, even though the quantity was relatively quite small. The flux behavior appeared to be irrelevant to the types of membranes used, but there was a pronounced discrepancy in the extent of fouling caused by adhesion and pore blocking. The extent of internal fouling for different membrane materials was evaluated in terms of surface free energy changes involved. The free energy of adhesion between substances in the broth and membrane surfaces was expressed as a function of the dispersion component of surface tension, which could explain why the fluoropolymer (PVDP) exhibited the lowest fouling tendency. The MF membrane with a pore size of 0.1 μm resulted in a minimal fouling tendency, suggesting that an optimal pore size exists due to the relationship between the sizes of membrane pore and broth constituents.