| Aerobic granular sludge technology is a new type of sewage treatment technology.Compared with traditional activated sludge technology,it has advantages of fast sludge separation,wide treatment range,and can withstand higher organic load.The granulation of flocculated sludge is the key point of this technology.The research shows that the granulation of flocculated sludge can only occur under strong hydraulic shear and suitable flow field conditions.At present,SBR-type cultivation process is widely used in aerobic granular sludge.This process divides a running cycle into five links:water intake,aeration,precipitation,drainage and idle.Among them,aeration is the main source of hydraulic shear,and has an important impact on the flow field characteristics.It is the core of the whole process.However,at present,the research on aeration mainly focuses on the oxygen transfer performance,which has great limitations in the application of aerobic granular sludge process.Therefore,the influence of aeration parameters on the gas-liquid two-phase flow field in the reactor was studied by computational fluid dynamics numerical simulation and flow field visualization technique.Firstly,the influence of bubble size on flow field and hydraulic shear was discussed under the condition of single bubble size model.The results show that the smaller the bubble diameter,the larger the swing range of the gas flow,and the more uniform the gas diffusion.The diameter of bubbles has a great influence on the turbulent kinetic energy and its dissipation rate.When the bubble diameter increases,the turbulent energy dissipation rate first increases and then decreases,and reached the maximum when the bubble diameter is about 2.5mm.Secondly,the flow visualization technology and numerical simulation are used to further study the bubble size and distribution.High-speed CCD camera was used to obtain high quality original image of bubbles,and a series of image processing techniques were used to accurately extract the size and distribution of bubbles.The PBM model is used to describe the coalescence and fragmentation of bubbles,and to simulate the change of bubble diameter and distribution.The results show that the bubbles in the reactor are divided into two groups:the micro-bubbles group and the main bubbles group.The micro-bubbles are concentrated near the bottom of the reactor and the walls on both sides,which accounts for a small proportion of the total gas volume.The main bubbles group is distributed in most of the space in the flow field,and the bubble size is approximately normal distribution.Thirdly,the VOF two-phase flow model is used to simulate the formation and movement of aerated bubbles,and to study the influence of aeration aperture and aeration volume on the size of bubbles and a User Defined Function programe is compiled to accurately extract it.The results show that the formation of-bubbles can be divided into two stages:expansion stage and detachment stage.Compared with the operating gas volume,the aeration aperture has a greater influence on the size of bubbles.The smaller the aperture,the smaller the size of bubbles.Fourthly,the enlargement effect of aerobic granular sludge reactor was analyzed by numerical simulation.The effects of enlargement effect on gas-liquid two-phase manifold,velocity field distribution,turbulent kinetic energy and dissipation rate were discussed in detail.The enlargement effect of reactor was modified by adjusting the aerator area and operating air volume.The results show that the enlargement effect of the reactor is obvious,the enlargement factor increases,the ratio of the rise of gas phase oscillation decreases,and the velocity and turbulent energy dissipation rate increase.The magnification effect can be reduced by adjusting the aerator area and aeration volume.In the end,a set of pilot-scale aerobic granular sludge experiment design scheme is proposed,including the overall process,operation mode,reactor structure,which provides a reference for future pilot-scale aerobic granular sludge experiment. |
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