Biofilm Formation And Mass Transport Characteristics In A Planar Membrane Bioreactor For Volatile Organic Compounds Treatment

The air pollution is one of the signficant prominent environmental issues. A largeamount of volatile organic waste gas generated in large-scale industrial processes aswell as daily life has not only seriously destroy the natural ecological environment butalso threatened human’s health. So far, physical, chemical, and biological are theprimary methods for organic waste gas treatment. Biological methods have manyadvantages: simple operation processes and equipment, less consumption of rawmaterial, low cost and energy consumption, safety and reliablilty, effectively reducingthe secondary pollution. Because of these advantages, Biological methods hasone of thehot research topics in the environment protection filed.In recent years, among the biological methods, a new technique of the membranebioreactor （MBR） that combines the membrane separation technology with biochemicaldegradation for waste gas treatmentis rapidily developed. In the MBR, the gas phaseand liquid phase are separated by a selective permeable membrane. Microorganismfirstly attach onto the membrane surface and grow into a biofilm which can directlydegradate the organic compounds penetrated from the gas phase through the membrane.This design can effectively reduce the liquid phase mass transfer resistance and thebiofilm blockage encountered intraditional bioreactors: During the working process ofMBR, there exist coupled multiphase flow and mass transfer as well as biochemicalreaction. Organic compounds driven by the concentration gradient transport from thegas phase through the membarne into the biofilm, in which organic compounds aresimutaneously transported and biodegraded by microorganism and the generatedmetabolites are transported to the mainstream of the liquid phase. As a result, thebiofilm structure and mass transport are of great importance for the biodegradationperformances of MBRs.From the perspective of engineering thermophysics, a visual planar biofilmmembrane bioreactor inoculated with Pseudomonas was designed and fabricated in thisthesis to study the morphology and structure of the formed biofilms. The variation ofthe biofilm structure in the star-up process of MBR was investigated. Effects of the inlettoluene concentration, gas flow rate and liquid flow rate on the biofilm structure werestudied. With the biofilms formed under different conditions, the mass transport andbiodedgradation performance of MBRs were explored. Finally, a new membrane fabricated by carbon cloth and nano-sized carbon powders was developed for the MBRto purify toluene. Main findings are as follows:1. Design and fabricate a visual planar membrane bioreactor. The pseudomonasbacteria were achieved through the screening and identification of bacteria, which wereused to investigate the characteristics of the biofilm in the membrane bioreactor. It isfound that the predominant bacteria in the biofilm is short rods and a few long rodsbacteria；2. At the beginning of the biofilm formation （2-6day）, the biofilm grew fast andthe biomass and dry weight were rapidly increased as well, facilitating the transfer ofboth toluene and inorganic nutrients and thus improving the removal efficiency. As thebiofilm further grew, the biomass and dry weight gradually increased, leading to anincrease in the biofilm thickness. The increased thickness in turn increased the masstransfer resistance so that the supply of toluene for bacteria on the top of biofilm isinsufficient, making the biofilm loose in this zone. Eventually, the supplied toluene wasbalanced by the toluene biodegraded by bacteria and the growth and death of bacterialwas also in dynamic equilibrium. As a result, a relatively high and stable toluenebiodegradation performance was achieved;3. The formation of PSB biofilm and the toluene degradation performance weregreatly influenced by inlet toluene concentrations. It was found that the dry weight ofbiofilm firstly increased and then decreased with the increase of inlet tolueneconcentration and the highest dry weight of0.134g appeared at the concentration of1.5g/m³. Because of high biomass, the elimination capacity of toluene in this flat-panelmembrane bioreactor also reached the maximal value of10.61gmin^-1·m^-3.Correspondingly, the maximal removal efficiency of87.55%was achieved at a tolueneconcentration of1.5g/m³during the running stage. This fact indicates that the inlettoluene concentration of1.5g/m³is the optimal condition for the biofilm formation;4. The structure of biofilm becomes sparse with the increase of gas flow rate. Theflow rate of the inlet toluene has a greater impact on biofilm film. The thickness ofbiofilm gets the max value when the gas flow rate is20ml/min. Meanwhile, themaximum dry weight is4.75g/cm²at this flow rate. The biofilm formed under thiscondition has a high biological degradation performance;5. The structure of biofilm becomes denser with the decrease of the liquid flow rate.The biofilm gets the maximum biomass and thickness when the liquid flow rate is20ml/min. The morphology and structure of the biofilm is fit for transport of toluene and nutrient.The biofilm formed under this condition have the highest degradationefficiency of toluene;6. The carbon cloth after treatment can be used into the MBR. Biofilm can beeasily attanched on the carbon cloth. MBR with this new membrane material has gooddegradation efficiency. The further application of this membrane need more researches.