New mathematical models of biomass viability and membrane fouling in a membrane bioreactor
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The optimized performance of a membrane bioreactor (MBR) for wastewater treatment depends not only on the biomass viability but also on the dynamic effects of biomass properties on membrane fouling. This research developed new conceptual mathematical models of biomass viability and fouling using biomass parameters and operational parameters of an MBR. It also presents, as outcomes, new simple and practical models for tracking biomass viability and fouling of an MBR system. The proposed models can be used to track instability in the operation of an MBR, and consequently, measures can be taken to act against instability in the oxygen uptake and for fouling control. The proposed conceptual models include parameters such as the specific oxygen uptake rate (SOUR) of microorganisms, the soluble or colloidal chemical oxygen demand (COD) of effluent along with the mixed liquor suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS) concentrations. The COD parameters of the models represent soluble microbial product (SMP) or bound extra-polymeric substances (bEPS) present within an MBR, offering the possibility of developing practical models with these easily measurable parameters. The experimental study investigated the effects of biomass parameters on SOUR in a lab-scale sponge submerged MBR (SSMBR) system. Statistical analyses of experimental results indicate that bEPS, SMP, MLSS and MLVSS had significant effects on SOUR and their relative influence on SOUR was EPS>bEPS>SMP>MLVSS/MLSS. The EPS is considered as a lumped parameter of SMP and bEPS. The progressive change of SMP and bEPS within the bioreactor consistently maintained a negative exponential correlation with SOUR, and two independent models of biomass viability were developed based on correlations among these parameters. Both the model simulations for biomass viability agreed well with experimental values of the SSMBR system. The simplified model of membrane fouling considered cake formation on the membrane and its pore blocking as the major processes of fouling. In the model, MLSS is used as a lumped parameter to describe the cake layer formation including the biofilm whereas SMP is assumed as the key contributor to pore fouling. The combined effects of aeration and backwash on detachment of membrane foulants, and new exponential coefficients are included to better describe the exponential increase of transmembrane pressure (TMP). With practical assumptions of these major processes, the new model successfully simulated the fouling phenomena with fairly accurate predictions of the rise of TMP for the operations of two lab-scale submerged MBR systems.
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