Mechanism Of Multiphase Flow Mass Transfer And Wastewater Treatment In Aerobic Fluidized Bed Biofilm Reactor

Efficient wastewater treatment is of great significance to the utilization of wastewater resources and the sustainable development of society.However,due to the limitation of wastewater treatment technology and its mechanism understanding,the efficiency of wastewater treatment is low and the treatment cost is high.In this study,an aerobic fluidized bed biofilm reactor(AFBBR)was used as the research object.Coupling mathematical model of gas-liquid-solid three-phase flow,was constructed by using the Euler-Euler-Euler three-fluid model,population balance model(PBM)and other theories,was applied to obtain multiphase flow parameters in system.The oxygen mass transfer process of macro and micro in system was explored and applied to reveal the response mechanism of multiphase flow and oxygen mass transfer efficiency.The high-throughput sequencing technology,quantitative polymerase chain reaction(qPCR),and other methods were used to analyze the impact of flow mass transfer on the characteristics of microorganisms.Combining with the results of wastewater treatment efficiency,the mechanism of multiphase flow mass transfer and wastewater treatment was revealed.The main research results include:(1)The Euler-Euler-Euler-PBM three-fluid coupling mathematical model could obtain the gas-liquid-solid three-phase flow parameters.The gas-liquid-solid three-phase fluidization velocity,turbulence intensity,and gas volume fraction under the higher aeration rate were higher.The aeration distance obviously increased the degree of gas dispersion in the radial direction of the cylinder,and has little effect on the gas-liquid-solid three-phase fluidization velocity.The aeration aperture significantly changed the bubble diameter of the system.Under the condition of DS=0.16 mm,the number of small diameter bubbles(0.27?1.03 mm)in the system is significantly higher,reaching 74.8%.Additionally,when the carrier filling rate is 20-30%,the fluidized state of the suspended carrier is better.(2)The multiphase flow characteristics formed by suitable aeration mode and carrier filling rate improved the macro and micro mass transfer efficiency of the system.Additionally,the difference in carbon source affected the oxygen mass transfer efficiency and oxygen diffusion kinetics of the system.When the aeration rate,aeration distance,aeration aperture,and carrier filling rate was respectively 5.77 m3/(h·m3),10 mm,0.16 mm,and 20-30%,the system had a higher oxygen mass transfer efficiency between gas and liquid phase.The diffusion of oxygen concentration had a Gaussian distribution relationship with the biofilm thickness.There was a opposite trends between the oxygen transfer rate(OTR)in wastewater and oxygen diffusion in biofilms under different C:N and carbon source type.(3)The aeration mode,carrier filling rate,and carbon source changed the treatment efficiency of the AFBBR system.When the aeration rate,aeration distance,aeration aperture,and carrier filling rate was respectively 5.77 m3/(h·m3),10 mm,0.27 mm,and 20-30%,the nitrogen and phosphorus removal efficiency of the system was higher than other conditions.Simultaneous nitrification and denitrification under the high C:N was improved so that the nitrogen removal was increased.Under this condition,the treatment efficiency of TN and TP reach to 72.2%and 67.4%,respectively.Compared with glucose,sodium acetate,and starch,sodium propionate significantly improved the system's denitrification and phosphorus removal efficiency significantly higher than other carbon source conditions.The degradation kinetics of COD,NH4+-N,and TN in the AFBBR system met the Haldane kinetic model of suspended biomass substrates.The degradation rate(qs,max)of NH4+-N and TN in the system with high C:N and suitable carbon source(sodium propionate)was higher than other conditions.(4)Aeration mode,C:N,and carbon source type affected the physical and chemical composition and microbiological characteristics of biofilm.The microstructure of the biofilm in the surface of the suspended carrier was relatively uniform.Additionally,there are microorganisms with various morphological structures.The protein content in the extracellular polymeric substances(EPS)was significantly higher than that of polysaccharides and nucleic acids,which increases the degree of adhesion of the biofilm on the carrier surface.The fluorescent group-like substances in the EPS were mainly protein-like.The type of functional group(polysaccharide,carboxyl or hydrocarbon-based compounds,proteins,phosphate groups or sulfate groups,aliphatic groups)contained in EPS was similar to that of biofilm.Protrobacteria,Actinobacteria,and Bacteroidetes is the dominant flora of the AFBBR system.A total of 24 denitrification microorganisms and 11 phosphorus removal microorganisms were detected in the system.(5)The multiphase flow and mass transfer process of the AFBBR system under the conditions of aeration rate of 5.77 m3/(h·m3),aeration distance of 10mm,aeration aperture of 0.27 mm,and carrier filling rate of 20-30%increased the functional microorganisms(Zoogloea,Acidovorax,Ottowia,Dechloromonas)abundance and improved the functional genes(nitrite reductase gene(nirK/nirS),anammox gene(AMX))expression.The secretion of more extracellular polymers from biofilms was promoted.The CODcr,TN,NH4+-N and TP removal in system reached the best.Compared with glucose,sodium acetate,and starch,sodium propionate significantly improved the expression of nirS,nirK,AMX and other functional genes.These results improved the system's nitrogen and phosphorus removal efficiency higher.The results have perfected the mechanism of wastewater treatment in the AFBBR system from the perspective of engineering thermophysics,environmental engineering,microbiology,and other disciplines,and can provide technical and theoretical support for the design and promotion of the AFBBR system.