Effects of the acidogenic biomass on the performance of an anaerobic membrane bioreactor for wastewater treatment

Continuous flow experiments were performed to study the effects of acidogenic biomass development, induced by feeding with non-acidified substrate, on the operation and performance of an anaerobic membrane bioreactor (AnMBR). The AnMBR was operated at cross-flow velocities up to 1.5 m/s and fed with a gelatine–starch–ethanol mixture. A significant fraction of acidogenic biomass developed during reactor operation, which fully determined the sludge rheology, and influenced the particle size distribution. As a result, flux levels of only 6.5 l/m2 h were achieved, at a liquid superficial velocity of 1.5 m/s. Even though the soluble microbial products levels in the AMBR were as high as 14 g COD/l, the observed hydraulic flux was not limited by irreversible pore fouling, but by reversible cake layer formation. Propionate oxidation was the limiting step for the applied organic loading rate. The assessed specific methanogenic activity (SMA) with propionate as substrate was, however, similar to the values found by others during thermophilic treatment of non or partially acidified substrates in granular sludge bed reactors, indicating an appropriate level of the propionate oxidation capacity.