Mass Transport And Biodegradation Characteristics In A Plane Membrane Bioreactor For Organic Waste Gas Treatment

In recent years, air pollution has become one of the most prominent environmental issues in China. Many industrial processes, such as organic chemistry industry, paint spraying, rubber regeneration and wastewater treatment, etc., emit volatile organic compounds (VOCs), which not only seriously pollute the ecological environment but also harm human health. To resolve this problem, therefore, several purification technologies including condensation, catalytic combustion, activated carbon adsorption and biological treatment methods have been employed for waste gas treatment. Compared with those physical and chemical methods, the biological treatment technology towards VOCs is the most promising because of its high removal efficiency, low capital and operation cost, low energy consumption, and environmental friendliness etc.Among the biological treatment technologies, membrane bioreactor (MBR) which incorporates the membrane separation technique into the bioreactor design for waste gas treatment is a relatively new technology. In addition to the above mentioned advantages, this technology offers another two advantages over the conventional biological treatment technologies because the use of separation membranes enables the separation of the gas loop from the liquid loop. One is the removal of the mass transfer resistance resulting from liquid phase encountered in conventional ones. Another one is the blockage problem owing to the over-growth of biofilm. Although promising, the performance of the MBR is still limited by a transport issue that is coupled with bioreactions. Basically, the biodegradation process by is rather complex, in which the flow, mass transfer and bioreactions are inherently coupled and occur in the MBR consisting of liquid and gas phases, biofilm and permeation membrane. Under such a circumstance, the studies of the characteristics of the coupled transport process and bioreaction in MBRs merit the performance and are valuable in both engineering and academy.In this thesis, we experimental and theoretically investigated the performance of the MBR towards the treatment of low concentration toluene. We firstly investigated the start-up characteristics of the MBR. After that, the effects of different operating parameters on the MBR performance were explored. A new approach to improve the removal efficiency by enhancing the toluene transport was also proposed. Finally, a theoretical model was developed to shed light into the flow and mass transport process coupled with the bioreaction. By doing these works, main outcomes are obtained and summarized as follows.①A membrane bioreactor with a composite membrane consisting of a porous Polyamide (PA) support layer coated by a thin and dense polydimethylsiloxane (PDMS) layer for treating organic waste gas was designed and fabricated. The used microorganism was collected from sludge and then cultivated and domesticated to achieve high toluene degradation capability. With the microorganism, the experiments on the start-up of membrane bioreactors(MBR)were successfully performed and the stable and highly-active biofilm was obtained for toluene removal.②Based on the experiments on the start-up of MBR, a standard for evaluating the status of biofilm formation was proposed. It was found that the OD600nm of circulated solution, pressure drop of liquid phase over the reactor, removal efficiency and dry weight of biofilm could be used to judge the completion of the start-up.③The effect of different parameters on the removal efficiency was visually studied, including the inlet toluene concentration, gas residence time, liquid flow rate, operational mode and pH value. It is found that increasing the inlet toluene concentration resulted in a decrease in the removal efficiency, but the elimination capacity firstly increased and then became stable. The removal efficiency increased and the elimination capacity slightly decreased with increasing the gas residence time. The counter-flow mode yielded better performance than did the co-flow mode. It is also found that the MBR exhibited the highest removal efficiency at pH=7.④A new bioreactor with the added fins in the gas flow channel was designed and fabricated to enhance the gas transport. The experimental results show that the perturbation in the gas flow channel due to the added fins can increase the mass transfer rate of toluene in MBR, and thus improve the removal efficiency.⑤A mass transport theoretical model with the consideration of the bioreactor was developed to further investigate the effect of design and operating parameters on the removal efficiency, with which the reactor design and operating conditions can be optimized to maximize the performance. It is also found that numerical results are in agreement with the experimental data.