Study On Bacterial Locomotion And Attachment In Biofilm Reactor

Human society highly depends on fossil fuels, causing global warming and air pollution issues. It is of great importance to develop environmentally friendly and economic renewable energy for the sake of China’s sustainable development. Among all types of renewable energy, biological energy is promising to be a substitute for fossil fuels, since it is renewable, natural, abundant in energy and pollution-free. Microbial energy conversion is competitive in biological energy technologies because it has the advantages of mild reaction conditions, less environmental impact and low energy consumption. Biofilm reactor is one of the most widely used in biological energy conversion due to its stability and recyclability.The formation of biofilm in biofilm reactor undergoes three steps: locomotion of bacteria in suspension and their attachment on surface, stable adhesion and the growth of biofilm. Current studies on biofilm formation focus on bacterial adhesion and biofilm growth. While the studies on bacterial locomotion concentrate on basic mechanism of flagellar motion, in which flagellar propulsion, Brownian motion and tumbling are not studied comprehensively. Besides, these studies did not consider the operating conditions of biofilm reactor, including reactor structure, flow field and chemical concentration. It is obvious the performances of these biological systems are critically dependent of the biofilm, which is affected by bacterial locomotion and attachment. Thus, a mathematical model combining flagellar propulsion, Brownian motion, tumbling, fluid flow and chemotaxis is proposed for biofilm reactors. Bacterial locomotion and attachment in the early stage of biofilm formation is analyzed based on the model. Main studies and conclusions in this thesis are as follows:①First, forces applied on swimming microorganism is analyzed based on the physiological structure of the bacteria. Calculation of the bacterial motion is derived from the resistive force theory, which is related to the low Reynolds number flow over bacteria. Besides, Brownian motion and tumbling is combined in the mathematical model. The proposed model is validated with the experimental observation of bacterial trajectory in literatures.②Based on the proposed mathematical model, the effect of bacterial size, tumbling, bacterial suspension viscosity and temperature on bacterial locomotion are analyzed. The numerical results demonstrate: larger cell size leads to lower swimming velocity; the time spent to complete certain distance for bacteria with the ability of tumbling is much larger than that disable of tumbling; higher suspension viscosity results in lower bacterial swimming velocity; higher suspension temperature in reasonable range leads to higher bacterial swimming velocity.③To study the effects of reactor structures on bacterial swimming and attachment, packed bed reactor is studied. Bacterial locomotion and attachment in non-uniform interspaces is analyzed. It is found that higher specific area by changing stacking forms or reducing packing size can result in faster bacterial attachment on surface. Besides, smaller bacteria size leads to faster bacterial attachment. Attachment of bacteria without the ability of tumbling is much faster. For the bacteria with the ability of tumbling, viscosities of the suspension slightly affect the bacterial attachment.④It is typical that biofilm formation takes place under flow condition. A mathematical model with the consideration of fluid flow is proposed for bacterial locomotion and attachment in such conditions. It is demonstrated that various velocity profiles result in different attached bacterial density. The locomotion of bacterial in flow consists of bacterial motion(including Brownian motion and tumbling) and the motion with fluid flow. In the boundary layer where fluid velocity can be neglected, bacterial motion is predominant, which is usually much slower than the fluid flow. Thus, reducing boundary layer thickness can enhance the bacterial attachment in flow.⑤A bacterial chemotaxis model is proposed, since there is usually chemical gradient in biofilm reactors. Bacterial chemotaxis is a consequence of bacterial self-accommodation to its ambient environment. The bacteria chemotaxis in different chemical concentrations and gradient forms is studied.⑥For further analysis of bacterial chemotaxis and its application to biofilm formation enhancement, microfluidic oscillator is designed and fabricated aiming at cellular signal pathway. The microfluidic oscillator has the function of outputting oscillatory flow with constant inputs. Besides, automatically synchronized microfluidic oscillators are developed. Finally, single-valve microfluidic oscillator with simplified structure is designed.