Effects Of Organic And Inorganic Matter On Membrane Biofouling In Treating Industrial Secondary Effluent

Membrane technology has been widely used in industrial wastewater treatment in recent years because of its advantages of good separation effect and simple operation.However,membrane fouling results in decreased membrane performance and service life with deteriorating water quality as well as increased operating and maintenance costs.Among them,biofouling is a very complicated process and cannot be completely eradicated,which is affected significantly by organic matter and inorganic salt from the biological treatment process.In China,the production of vitamin C ranks first,and it discharges huge amount of wastewater.Vitamin C secondary effluent contains high concentration of organic matter and inorganic salts,as well as residual microorganisms.This condition will lead to the formation of membrane biofouling.Therefore,based on the water quality of the Vitamin C secondary effluent,the effects of typical organic matter and inorganic salt on biofouling of nanofiltration membrane were systematically studied in this study.The result had theory instruction significance for controlling and slowing down membrane fouling in industrial wastewater treatment.Pseudomonas aeruginosa(PA)was selected as model bacterium,and sodium alginate(SA),bovine serum albumin(BSA),and humic acid(HA)were selected as model organics for polysaccharides,proteins,and humic substances in the wastewater,respectively.The effects of different organic matter under various Ca2+ concentrations on membrane permeate flux,the foulant constituents on membrane surface,and the fouled membrane characteristics(including surface morphology and roughness,hydrophobicity,surface charge,and adhesion force)in the conditioning and subsequent biofouling process were systematically investigated.In addition,the interfacial energies between membranes and foulants of the whole fouling process under different conditioning types and Ca2+ concentrations were quantificationally analyzed through XDLVO theory.The result shows the importance of different interfacial forces in membrane fouling and the dominant mechanism responsible for biofouling with different conditioning types and Ca2+ concentrations.The main results are as follows:In the absence of OM,the flux decline in the conditioning was affected slightly by the presence of Ca2+ and Ca2+ concentration.Approximately 7.5%of permeability is lost during organic-free conditioning in the absence and presence of Ca2+.However,subsequent biofouling Was reduced with the increase in Ca2+ concentration,because the biofilm properties(hydrophobicity,surface charge,and spatial distribution)and EPS production were influenced by high Ca2+ concentration.In the presence of SA,the presence of Ca2+ accelerated SA conditioning as Ca2+ concentration increased,which led to a severe flux decline.The relative flux reduction(RFR)in the conditioning was 28%at 8 mM Ca2+.SA coditioning inhibited subsequent biofouling because the conditioning films of SA-calcium made the membrane surface smoother and more hydrophilic,which was not in favor of bacterial adhesion.Similarly,the coexistence of HA and calcium enhanced organic conditioning but inhibited subsequent biofouling mainly because of the high surface charge and hydrophilicity of HA-calcium conditioning films.The presence of BSA resulted in less organic retention in the conditioning but more biomass retention in the biofoulng.BSA conditioning accelerated the biofouling and resulted in a severe flux decline with RFR of more than 44%.This result is attributed to the high hydrophobicity and roughness of BSA conditioned membranes and "valley clogging" in the biofilm.According to the result of interfacial free energy,hydrophobic force is a short-range interaction force and contributes most in the membrane fouling.BSA conditioned membranes have the strongest hydrophobicity and the lowest interfacial free energies with PA and BSA,especially at 2 mM Ca2+,with a△Gcoh of-40.32 mJ/m2 and △Gacdh of 3.49 and-26.36 mJ/m2,respectively.This condition is in favor of foulant adhesion and biofouling.However,SA conditioned membranes have the weakest hydrophobicity and the biggest interfacial free energies with PA and SA,especially at 5 mM Ca2+,with a △Gcoh of 42.96 mJ/m2 and △Gadh of 45.85 and 39.64 mJ/m2,respectively.This condition is not in favor of biofouling.The hydrophobicity and biofouling of HA conditioned membranes located between SA and BSA conditioned membranes.The △Gcoh and △Gadh changed significantly after PA biofouling with 74.8-85.6 mJ/m2 and 39.1-43.8 mJ/m2,respectively.The△Gcoh and △Gadh of PA-BSA biofouling at 2 mM Ca2+ increased with 95.4 mJ/m2 and 47.6 mJ/m2,respectively.In complex systems,interfacial energies between membranes and foulants change as fouling evolves.In general,except for XDLVO interactions,many other factors must be considered to quantify membrane biofouling.