Physical properties of activated sludge in a submerged membrane bioreactor and relation with membrane fouling

Abstract

The objective of this study is to establish some relations between the physical properties of the sludge (i.e., specific resistance to filtration, capillary suction time …) and membrane fouling in a submerged membrane bioreactor, for high solid retention time (SRT) conditions. The analysis of the membrane performances, carried out with an analysis of the depollution performances and of the activated sludge physical properties, is proposed in an experimental membrane bioreactor (MBR) unit working with complete biomass retention. It is first confirmed that high-SRT conditions involve a high degree of organic degradation, with low sludge yield. A continuous increase of the viscosity can be observed during time but the sludge dewatering behaviour can be considered as constant, despite the total suspended solids (TSS) increase. No direct correlation is established between the transmembrane pressure (TMP) evolution and the conventional parameters used to describe sludge dewaterability but it can be assumed that the sludge filtration is not possible when TSS and viscosity reach “critical” values. Soluble microbial substances and mean floc size diameter evolutions cannot explain the decrease of membrane permeability, although these compounds may enhance fouling. The characterisation of the bulk suspension seems to not provide enough informations to anticipate the evolution of the membrane fouling and some complementary methodologies, allowing the characterisation of the environment close to the membrane, have to be developed.