Simulation And Experiment Study On Biofilm Mass Transfer Based On Percolation Theory

The process of the substrates mass transfer in the biofilm plays a very important role in wastewater treatment technology using biofilm, which is the essence or basis of biofilm theory. With technology development and theory updates, this research field is constantly given new contents. Basing on previous work in this area, this thesis added percolation theory to mass transfer model within biofilm porous medium and make an intensive research of mass transfer process in biofilm by means of numerical simulation and experiment. The results of numerical simulation agree well with experimental results.The contents of this study include:1. Introduce percolation theory to the process of mass transfer within biofilm and build a "convection-diffusion-reaction" model. The model was stratified so that physical parameters of biofilm can be closer to actual situation, and numerically calculated by Runge-Kutta method. The influence of substrate concentration and seepage velocity, tortuosity and porosity to mass transfer within biofilm was investigated accordinglly. The numerical results showed that as the value of each parameter increased, the substrate penetration depth increased. Besides, most of these parameters had significantly influence on mass transfer within biofilm except the tortuosity. The substrate penetrates deeper into stratified biofilms than it does into homogeneous biofilms, the stratified biofilm model shows the lower mass transfer resistance to the substrate penetrate.2. A fixed-bed aerobic biofilm reactor of 10 L was established. The COD of effluent was monitored with HRT,COD of influent and aeration rate changing. The results show that smaller liquid flow rate has little effect on mass transfer within biofilm and higher COD influent concentration will increase the oxygen consumption rate of aerobic biofilm. When the value of COD reached to 800 mg/L, the COD was no longer the limiting factor for microbial growth.The best operating conditions in this experiment are listed as follows: HRT=6-8 h,COD in influent=600 mg/L, the aeration rate Qg=0.1m3/h.3. According to batch test data, the aerobic biofilm reaction rate equation was fitted, namely the largest oxygen consumption is 0.0099 min-1, half-saturation 6.94 mg/L, Glucose maximum degradation rate 0.0095 min-1, half-saturation 485.71 mg/L. Substrate concentration distribution within biofilm can be determined by numerically calculating mass transfer equcation with above-mentioned parameters. The error of the result between that obtained by stratified biofilm model and the convection-diffusion-reaction equation is 1.91%-18%. It was also verified that the stratified biofilm model is more suitable for actual mass transfer than the homogeneous biofilm model, thus can be used to predict the mass transfer process within the biofilm.