Research On Mathmatical Modeling Of Wastewater Treatment And Fouling Process In A Membrane Bioreactor

Submerged membrane bioreactor （MBR） is considered to be a promising biologicaltreatment technology that uses membrane to replace the conventional sedimentationbasin for the solid-liquid separation of mixed liquor. However, membrane fouling is stilla major problem that hinders MBR’s more widespread application, thus the mitigation ofmembrane fouling has become a challenge of membrane technology. In this work, theproduction-degradation approach of SMP and their fouling potentials were elucidated.Thebilogical process in the MBR and the organic interception by cake layer were wellsimulated. The XDLVO interaciton energy between extracellular polymeric substance（EPS） and membran were investigated. The morphological parameter of sludge withimage analysis was developed, and a fouling model including pore blocking and cakelayer formation was proposed. These compensated the deficiency of MBR research suchas the numerical simulation, enabled in-depth understanding the characteristics ofmembrane fouling, provided valuable information for MBR operation, and promoted thesustainable development of water quality and environment.Characterization and modeling of the SMP were conducted towards a betterunderstanding of the production and degradation of SMP in a MBR. Based on thechemical analysis, both utilization associated products （UAP） and biomass associatedproducts （BAP） exhibited the biodegradability and had a large fraction of molecularweight greater than20kDa, thus the approach of production–hydrolysis-utilization wereproposed for the formation and metabolism of SMP. With respect to the membranefouling, a series of stirred dead-end filtration tests were conducted to investigate thefouling potential of BAP and UAP, showing that the respective mean MFI-UF valueswere estimated to be4.37×103and1.63×104s/L2. Modeling work well indicated that themain fouling mechanisms for BAP and UAP filtration were cake filtration and completeblockage. These results showed that the UAP produced in the cell proliferation phaseexhibited the stronger fouling propensity than BAP. With respect to the biological process in MBR, extended ASM3-SMP model wasproposed based on the platform of Activated Sludge Model3（ASM3） with two mainimprovements:（i） adopting the concept of simultaneous growth and storage of organicsubstrates;（ii） introducing the formation and degradation of soluble microbial products.the model showed an acceptable agreement with MLSS, COD, SMP and S_NO, and thecorrelation coefficients between measured and simulated data were0.83,0.51,0.58and0.74, respectively. A complete global sensitivity and uncertainty analysis of the fullmodel is performed using the variance-based extended FAST method, which showed thatthe model discussed herein generally appeared to be much additive as SRT extended andoutputs were predominantly dependent on first-order effects. Additionally, a mechanisticmodel was developed to demonstrate the formation of dynamic cake layer by sludgelower than critical diameter, and the modeling results showed that the external cake layerof rejected sludge particles played an important role in COD removal before the physicalmembrane filtration. Therefore, the removal of organic substance in the MBR systemwas due to in-series phenomena: biological degradation with biomass, interception in thecake layer and filtration of primary membrane.Comparison of sludge morphology was made to elucidate the different filtrationcharacteristics, and image analysis was successfully developed in monitoring sludgestatus and gaining morphological parameters. The combinations of EFLI/FAI, FFand the floc elongation related parameter （AR or R） were the preferred performinginput candidates for ARX model. The performance of the ARX （[EFLI/FAI, FF, AR]）showed theR_adj²were0.81and0.86（respective for MBR I and MBR II） inidentification;0.65and0.57（respective for MBR II and MBR I） in validation. Mostoften, the inputs gave rise to a delay value, differing from zero, thus improving the intime prediction of the sudden rise in DSVI values.The interaction energy, represented by XDLVO potential, was calculated, allowingexploring the interaction energy profiles for EPS-reconstructed membrane surface. Thefree energy of BS-EPS was lower than that of NS-EPS with respect to PVDF in an aquatic environment, suggesting BS-EPS has higher fouling tendency in terms ofadsorption onto the membrane surface. Additionally, the XDLVO interaction energybetween the EPS and a smooth membrane surface was significantly altered by membranesurface topology （roughness）. The accurate assessment of membrane-EPS interactionsallowed identification of critical flux for effective membrane process operation, resultingin more than0.27and0.58decrement of critical flux for NS-EPS and BS-EPS with theincorporation of roughness effect.A mathematial model was developed to model membrane fouling in the MBR. Themodel for pore fouling was based on the organic interception by cake layer, and the massbalance, force balance and collapse effect was considered in the model of cake layerformation. The results of the simulation compared fairly well with the experimentalresults that were obtained with lab-scaled MBRs, which validated the applicability of thefouling model. The effects of operational variables on the membrane fouling wereconducted with the combination of extended ASM3-SMP and membrane fouling model.The simulation results demonstrated that filtration flux was the most significant factorcausing fouling problems. Based on the simulation results for this study, the hydraulicretention time had an optimum range of7-9h, solid retention time of20-40d andaeration rate of1.2-1.8m³/h from the fouling control and economic point of view.