Process configurations and fouling in membrane bioreactors
MBR process consists of a suspended growth biological reactor combined with a membrane unit. The widespread of this system for waste water treatment is contained by membrane fouling, which is strongly influenced by three factors: biomass characteristics, operating conditions and membrane characteristics. Fouling control techniques mainly include low-flux operation (sub-critical flux operation) and/or high-shear slug flow aeration in submerged. configuration. Based on the concept of the critical flux (Jo), the flux-step method has been developed to more fully characterise transmembrane pressure (TMP) behaviour during constant-fluxoperation. A zero rate of TMP increase was never attained during the trial, such that no critical flux, in its strictest definition, could be defined in this study for a submerged MBRs challenged with real and simulant sewage. Under similar operating conditions, Jc was obtained around 18 and 10 L.m-2.h-1 for a submerged MBR fed by real and synthetic sewage respectively. Three TMP-based parameters have been defined, all indicating the same flux value at which fouling starts to be more significant (the weak form of Jo). Results from factorial experimental designs revealed the relative effect of MLSS levels, aeration rate and membrane pore size on J, The MLSS effect on Jc was generally around double that of the aeration effect. The calculation of mean sub-critical values for the different TMP-based parameters suggest lower short-term fouling resistance for large pore sized membranes. A direct comparison between the two MBR configurations revealed a greater J, for the submerged compared to the SS MBR (22 and 11 L.m-2.h-1 respectively) under similar hydraulic conditions. The fluid hydrodynamics has been studied for both configurations, leading to an accurate calculation of shear at the membrane surface in SS MBR and to the determination of the minimum gas velocity required for Taylor bubble formation in submerged MBR (around 0.1 m.s-1). Finally, the effect of operating conditions such as process configuration, feed nature, and aeration type on biomass characteristics has been assessed and link to membrane fouling. Key words: Fouling, MBR, critical flux, process configuration, biomass characterisation.