Thermodynamic Mechanism Underlying Fouling Behaviors Of Typical Foulants In Membrane Bioreactor

Membrane bioreactor(MBR)technology has been extensively researched and applied for wastewater treatment and reclaim in recent years mostly because of its tremendous advantages over conventional activated sludge process(CASP),steadily falling membrane price,intensive demand for water reclamation,and stricter emission standard.SMPs and other particulates including sludge flocs,bacteria and debris are simultaneously presented in the sludge suspension in MBRs.Hence,exploring the fouling mechanisms related with SMPs is critical important for MBR technology.In this study,a laboratory-scale submerged membrane bioreactor(SMBR)has been started and operated continuously.This MBR reactor has been used as a research object to study gel contaminants in membrane bioreactors(with SMPs).Therefore,this study was conducted to provide a unified thermodynamic framework for the fouling behaviors of SMPs.First of all,mechanisms based on Flory-Huggins and Flory-Rehner models were proposed to address gel SFR.Model simulation showed that SFR resulted from elastic chemical potential change accounted for about 13% of total SFR of agar gel during filtration under conditions in this study,suggesting important roles of gel elasticity in gel SFR.Meanwhile,Triangulation technique combined with AFM characterization successfully reconstructed a continuous membrane morphology which well represented the real one.This case study of the deduced method in the MBR realized numerical elucidation of the interfacial interactions with real membrane morphology for the first time.Finally,the revealed thermodynamic mechanism underlying SMPs fouling behaviors significantly deepened understanding of fouling,and facilitated to development of effective fouling control strategies.This work was performed as follows:(1)A case study of the proposed method was conducted.With surface characterization of the foulants and membrane,the interfacial interactions with real membrane morphology were approximated for the first time by computer programming according to composite Simpson's rule.The results showed that rough morphology prolonged the interfacial interactions,indicating the profound role of morphology in the interfacial interactions related with membrane fouling.The new method would provide significant insights into membrane fouling in MBRs.(2)A continuous real membrane morphology was reconstructed based on atomic force microscopy(AFM)characterization and triangulation technique.Thereafter,a method to calculate those interactions was established by incorporating the spatial relationship between a foulant and the reconstructed morphology into surface element integration(SEI)method.In this study,theoretical deduction and experiments were carried out to numerically elucidate interfacial interactions in MBR.(3)In this study,agar was used as a model polymer for gel formation.Mechanisms and roles of gel elasticity in extremely high specific filtration resistance(SFR)were investigated.It was found that,as compared with cake layer in a membrane bioreactor(MBR),real gel layer in the MBR and agar gel possessed extremely high SFR.Foulant characterization showed that foulants were easy to bind water,and agar gel possessed a network structure.(4)Mechanisms based on Flory-Huggins and Flory-Rehner models were deduced to describe the high SFR of agar gel.Model simulation showed that sum of SFR induced by the mixing chemical potential and the elastic chemical potential change is close to that of the agar gel,suggesting feasibility of the deduced models.Gel elasticity accounted for about 13% of total SFR of agar gel under conditions in this study.This study satisfactorily explained the extremely high SFR of gel,and addressed roles of gel elasticity in gel SFR.(5)In this study,the typical fouling behaviors of SMPs during initial operational period of MBR were characterized.It was found that,although being low content,SMPs rather than sludge particulates preferentially adhered to membrane surface to accumulate a gel layer,and moreover,specific filtration resistance(SFR)of SMPs was approximately 700 times larger than that of the sludge particulates at operational day 3.According to energy balance principle,a unified thermodynamic mechanism underlying these fouling behaviors of SMPs was proposed.Thermodynamic analyses demonstrated that,the attractive interaction energy strength in contact between SMPs and membrane was larger by around 3700 times than that between sludge particulates and membrane,well explaining the extremely high adhesive ability of SMPs over sludge particulates.(6)Meanwhile,filtration through a SMPs layer was modelled and simulated as a thermodynamic process.Simulation on an agar gel showed that,about 92.6% of SFR was originated from mixing free energy change during filtration.Such a result satisfactorily interpreted the extremely high SFR of SMPs layer over sludge cake layer.The revealed thermodynamic mechanism underlying SMPs fouling behaviors significantly deepened understanding of fouling,and facilitated to development of effective fouling control strategies.