Fouling mechanisms of deflocculated sludge, normal sludge, and bulking sludge in membrane bioreactor

In this paper, a submerged hollow fiber membrane bioreactor was used to compare the fouling behaviors of bulking sludge, deflocculated sludge, and normal sludge. The results showed that the bulking sludge could cause severe cake fouling due to the deposition of irregular shaped sludge flocs. The deflocculated sludge not only led to cake fouling, but also resulted in severe pore blocking, which resulted from the deposition of colloidal particles and soluble matter onto/into the membrane. Comparatively, the normal sludge had a slight membrane fouling tendency. The contributions of sludge fractions on membrane fouling of the three sludge suspensions were different from each other. The bulking sludge had a higher bound extracellular polymeric substances (EPS) concentration, however, the deflocculated sludge had a higher free EPS concentration, and the increase of free EPS concentration would do great harm to membrane bioreactor. Confocal laser scanning microscopy (CLSM) analysis also showed that the bulking sludge and deflocculated sludge could form a dense cake layer as compared with normal sludge. The turbidity of sludge supernatant and filament index of sludge suspension can be used as indicators for evaluating membrane fouling potential in membrane bioreactors.