Relation between EPS adherence, viscoelastic properties, and MBR operation: Biofouling study with QCM-D

Abstract

Membrane fouling is one of the main constraints of the wide use of membrane bioreactor (MBR) technology. The biomass in MBR systems includes extracellular polymeric substances (EPS), metabolic products of active microbial secretion that adversely affect the membrane performance. Solids retention time (SRT) in the MBR is one of the most important parameters affecting membrane fouling in MBR systems, where fouling is minimized at optimal SRT. Among the operating parameters in MBR systems, SRT is known to strongly influence the ratio of proteins to polysaccharides in the EPS matrix. In this study, we have direct evidence for changes in EPS adherence and viscoelastic properties due to changes in the sludge removal rate that strongly correlate with the membrane fouling rate and EPS composition. EPS were extracted from a UF membrane in a hybrid growth MBR operated at sludge removal rates of 59, 35.4, 17.7, and 5.9 L day(-1) (corresponding SRT of 3, 5, 10, and 30 days, respectively). The EPS adherence and adsorption kinetics were carried out in a quartz crystal microbalance with dissipation monitoring (QCM-D) technology in several adsorption measurements to a gold sensor coated with Polyvinylidene Fluoride (PVDF). EPS adsorption to the sensor surface is characterized by a decrease of the oscillation frequency and an increase in the dissipation energy of the sensor during parallel flow of aqueous media, supplemented with EPS, above the sensor surface. The results from these experiments were further modeled using the Voigt based model, in which the thickness, shear modulus, and shear viscosity values of the adsorbed EPS layers on the PVDF crystal were calculated. The observations in the QCM-D suggested that the elevated fouling of the UF membrane is due to higher adherence of the EPS as well as reduction in viscosity and elasticity of the EPS adsorbed layer and elevation of the EPS fluidity. These results corroborate with confocal laser scanning microscopy (CLSM) image analysis showing thicker EPS in close proximity to the membrane surface operated at reactor conditions which induced more fouling at elevated sludge removal rates.