Flux criticality and sustainability in a hollow fibre submerged membrane bioreactor for municipal wastewater treatment

The critical flux concept has been widely used to investigate fouling mechanisms in membrane processes, including membrane bioreactors. In this field, the concept is commonly coupled to the operation time to evaluate the actual flux “sustainability”. Most researches have been carried out on bench or small pilot scale rigs. This study is focused on the optimisation of hydraulic/hydrodynamic conditions on a large pilot scale MBR to reasonably reflect operation of full scale systems. Short-term tests have been carried out according to the classical flux-step protocol with a view to evaluating the optimum membrane aeration rate. Long-term tests under constant aeration intensity (0.048 N m3 m−2 s−1) gave a linear relationship between the applied flux and the sustainability time. The effect of air-sparging was found not to affect the critical flux above a given aeration intensity, though a significant impact was observed in terms of membrane permeability recovery in stepping back operation from super-critical to sub-critical. Comparison with other studies seems to indicate the feedwater characteristics to be the most important factor in the sub-critical fouling rate. The role of extracellular polymeric substances in sub-critical fouling was confirmed by the removal efficiencies for proteins and carbohydrates, but the difference between EPS concentration in the sludge water phase and in the permeate was not a fair indicator to describe the TMP jump.