Interfacial Thermodynamic Mechanisms And Their Control In Membrane Bioreactor

Membrane bioreactor(MBR)is an emerging wastewater treatment technology which integrates membrane separation with biological treatment.Although the development of MBR technology is not long,it has been developing rapidly,and the market potential is tremendous.Overall,the growth of MBR technology is driven by:(i)the high quality of produced water,(ii)increased water scarcity,(iii)the increasingly strict discharge and reuse quality legislation,(iv)the decreasing investment costs,(v)the acceptance of the technology and(vi)the potential for upgrading existing wastewater treatment plants.Although MBRs are currently a mature technology,membrane fouling remains the most important operational problem,hindering their universal and wide scale application.Membrane fouling reduces MBR productivity,increases the energy requirements due to air scouring and requires frequent cleaning of the membrane to restore its permeability;the latter shortens the membrane’s lifespan and results in higher membrane replacement costs.Therefore,it is necessary to search from the membrane fouling mechanism aspects and seek effective pollution control strategyA submerged membrane bioreactor(MBR)treating synthetic wastewater was continuously operated more than 400 days to investigate the fouling mechanisms and the key factors.This study linked the chemical potential change to high specific filtration resistance(SFR)of gel layer,and then proposed a novel membrane fouling mechanism regarding gel layer filtration,namely,Flory-Huggins based filtration mechanism.Further more,the interaction energies between foulants and membrane were systematically assessed based on thermodynamic methods.And then the impacts of floc size and membrane surface characteristics on membrane fouling had been investigated.This work was performed as follows:(1)This study linked the chemical potential change to high specific filtration resistance(SFR)of gel layer,and then proposed a novel membrane fouling mechanism regarding gel layer filtration,namely,Flory-Huggins based filtration mechanism.A mathematical model for this mechanism was theoretically deduced.The model was fit to the experimental results.And the model was feasible.(2)Agar was used as a model polymer for gel formation.Simulation of the mathematical model for agar gel showed that volume fraction of polymer and Flory-Huggins interaction parameter were the two key factors governing the gel SFR,whereas,pH and ionic strength were not related with the gel SFR.Filtration tests of gel layer showed that the total SFR value,effects of pH and ionic strength on the gel SFR well agreed with the results of model’s simulation,indicating the real occurrence of this mechanism and the feasibility of the proposed model.This mechanism can satisfactorily explain the extremely high SFR of gel layer,and improve fundamental insights into membrane fouling regarding gel layer filtration.Developing strategies that allow tuning anti-adhesion ability of membranes in membrane bioreactors is of primary interest in membrane fouling research.(3)A submerged membrane bioreactor(SMBR)treating synthetic wastewater was continuously operated to investigate the impacts of floc size on membrane fouling.Particle size distribution(PSD)analysis showed that almost all the sludge flocs in sludge suspension had size larger than 1 μm,which corresponded to low pore clogging fouling.(4)Thermodynamic analyses showed that adhesion of sludge flocs on membrane surface needed to overcome a repulsive energy barrier.(5)Decrease in floc size slightly increased the specific energy barrier,but highly increased the attractive specific interaction energy in contact,and as whole,facilitated adhesion of small flocs to the membrane surface.Presence of biopolymer matters in sludge suspension remarkably gave rise to the cake resistance.This result could be explained by the osmotic pressure mechanism.Decrease in floc size would greatly increase both of hydraulic cake resistance and osmotic pressure-induced resistance.The findings shed significant light on membrane fouling control.(6)In this study,interaction energies between foulants and membrane were systematically assessed based on thermodynamic methods,which involved gelling foulants-rough membrane,sludge floc-smooth membrane and sludge floc-rough membrane.It was found that,membrane surface electron donor tension(γ-)rather than surface hydrophilicity was a more reliable indicator to predict adsorptive fouling.The interaction energy would be continuously repulsive in the initial range of separation distance when membrane γ-is higher than a critical value,suggesting that designing membrane with γ-higher than a critical value would confer membrane with high anti-adhesion ability.It was also found that,zeta potential on the membrane surface exerted certain effects on adsorptive fouling.This study proposed a novel strategy regarding adjusting membrane γ-to tune anti-adhesion ability of membrane,and also offered a thermodynamic theoretical background to this strategy.