The effect of bubble plume on oxygen transfer for moving bed biofilm reactor*

CHENG Wen (程文), LIU Hu (劉鵠), WANG Meng (王蒙), WANG Min (王敏)

State Key Laboratory of Eco-Hydraulic Engineering, Xi'an University of Technology, Xi’an 710048, China, E-mail:wencheng@xaut.edu.cn

The effect of bubble plume on oxygen transfer for moving bed biofilm reactor*

CHENG Wen (程文), LIU Hu (劉鵠), WANG Meng (王蒙), WANG Min (王敏)

State Key Laboratory of Eco-Hydraulic Engineering, Xi'an University of Technology, Xi’an 710048, China, E-mail:wencheng@xaut.edu.cn

(Received June 1, 2013, Revised June 20, 2014)

The movement of the bubble plume plays an important role in the operation of a moving bed biofilm reactor (MBBR), and it directly affects the contact and the mixture of the gas-liquid-solid phases in the aeration tank and also the oxygen transfer from the gas phase to the liquid phase. In this study, the velocity field is determined by a 4-frame PTV as well as the time-averaged and timedependent velocity distributions. The velocity distribution of the bubble plume is analyzed to evaluate the operating efficiency of the MBBR. The results show that the aeration rate is one of the main factors that sway the velocity distribution of the bubble plumes and affect the operating efficiency of the reactor.

aeration tank, bubble plume, moving bed biofilm reactor (MBBR), image processing, particle tracking velocimetry, oxygen transfer

The moving bed biofilm reactor (MBBR), with high efficiency and low energy consumption, is one of biological wastewater treatment technologies, to make the suspension filler turn into a fluidized state by aeration and flow promotion in a reaction tank. The core part in the process is the suspension filler with a density close to the water, which is added to the aeration tank to promote the microbial activity of the carrier. The aeration process can supply oxygen for microbial degradation, improve the degree of turbulence, and ensure the effect of oxygen transfer. The aeration device is widely applied in engineering, especially in the sewage treatment, as one of energy-intensive industries. In order to provide some guidance for the sewage treatment to improve the oxygen filling ability of the reactor, an aeration simulation device should be constructed, to conduct the research of the movement distribution of the bubble plume flow, and analyze the influence of the bubble plume flow on the oxygen transfer[1].

This paper discusses the fundamental structure of the bubble plume via the image processing and the PTV techniques. The data come from the bubble velocity distribution obtained in laboratory experiments. The distribution of the bubble plume movement might provide some food for thought to raise the aeration efficiency in the sewage treatment. In addition, the oxygen transfer distribution, which depends on the behavior of the bubble plume flow movement, is evaluated and analyzed to understand the relationship between the water dynamics and the oxygen transfer in the MBBR.

The 4-Frame PTV methods are employed to evaluate the smoothness of the particle movement path, as is calculated by means of the particle deviations of both displacement and direction.

Figure 2 is the schematic diagram of the PTV algorithm, and in the first frame,x is set as the starting point for tracing one particle. The search areaS is determined by examining the maximum speed Umof the particle in the current frame,d is the maximum distance and θis the possible deviation angle, and then the same procedure is repeated for the next frames. In this process, the probable path of the particle is obtained by calculating the total variance, including:

Length variance

in which, the particle movement length and angle are

Through the path of the particle confirmed by 4-Frame PTV, the particle velocity is calculated by:

where Sxand Syare the displacements of the particle inx -and y -directions, respectively, anddtis the time interval.

Figure 3 is the bubble plume flow fields at the different aeration rates, where at the aeration rate of 1.39×10–5m3/s, the swing cycle of the spiral structure formed by the bubble plumes is long, while the swing amplitude is small. The bubble plume structures are stable at the bottom of the flow field without attraction among plume columns. In the process of rising, the bubble plumes are slightly mixed and the liquid turbulence is not obvious. At 2.08×10–5m3/s, the bubble groups are subject to a combined action of the pressure and the flow shear stress after breaking away from aerators. The bubble plumes swing periodically and steadily at the bottom of the flow field and they are mixed only in the upper part, which would improve the contact between the gas and the liquid phases and increase the liquid turbulence. If the aeration rate becomes 2.78×10–5m3/s, the bubble plumes are in a very dense distribution, but the plume columns are slightly influenced by each other in the flow field. In this state, the recurrent swing of bubble plumes is not obvious but the mixture is significant, the structure is unstable and fails to make a steady liquid circulation, which is against the cyclic motion of the liquid and the action between the gas and the liquid phases.

The increase of the aeration rate influences the regular distribution of the bubble plume movement and the velocity distribution, which lead to a nonlinear change of KLaand EA. Thus the appropriate aeration rate is important to improve the operating efficiency of the MBBR.

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[2] CHENG Wen, HU Bao-wei and YANG Chun-di et al. The velocity field of multiphase flow and efficiency of biological aeration filter[J]. Journal of Hydrodynamics, 2010, 22(2): 260-264.

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10.1016/S1001-6058(14)60073-1

* Project supported by the National Natural Science Foundation of China (Grant No. 51076130).

Biography: CHENG Wen (1968-), Female, Ph. D., Professor