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The effect of bubble plume on oxygen transfer for moving bed biofilm reactor
664
2014,26(4):664-667 DOI: 10.1016/S1001-6058(14)60073-1
The effect of bubble plume on oxygen transfer for moving bed biofilm reactor* CHENG Wen (程文), LIU Hu (刘鹄), 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: [email protected] (Received June 1, 2013, Revised June 20, 2014) Abstract: 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. Key words: 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 * Project supported by the National Natural Science Foundation of China (Grant No. 51076130). Biography: CHENG Wen (1968-), Female, Ph. D., Professor
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.
Fig.1 Experimental setup of MBBR
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Figure 1 is the experimental set-up, and its main body is a cylindrical tank of 0.7 m in height and 0.25 m in inner diameter. A rectangular tank of 0.8 m in height and of a square cross-section of 0.28 m in each side covers the cylindrical aeration device to eliminate the bubble distortion from the optical refraction. The aeration distribution tray is placed at the bottom, where there are 24 aerators surrounding the center aerator with radii of 0.0417 m, 0.0625 m and 0.0833 m, respectively. The air is injected into the bottom of the tank through capillary tubes, and the locations and the intensity of bubble plumes can be precisely controlled. In order to study the properties of the movement and velocity distribution, 36 test cases are considered at the temperature of 13oC-15oC. A CCD camera is used to record the spatio-temporal structure of bubble plumes. The tap water is used as the liquid phase with the kinematic viscosity of 10−6 m2/s and the density of 1 000 kg/m3, while the gas phase is the air in the laboratory room with the density of 1.28 kg/m3. Table 1 shows the experimental conditions, where Q is the gas flow rate, H / W is the aspect ratio of the container, and R is the mean bubble radius in the tank. Table 1 Experimental conditions No.
Q (L/h)
R (m)
H /W
p (kPa)
1
50
0.0625
1.5
6.5
2
75
0.0625
1.5
7.0
3
100
0.0625
1.5
7.5
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.
distance and θ is 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
(
2 2 1 dij − d m + d jk − d m + d kl − d m 3
σl =
2
)
(1)
Angle variance
σθ =
(
2 1 θ jk − θ m + θ jl − θ m 2
2
)
(2)
Total variance
σt =
σ l2 d m2
+ σ θ2
(3)
in which, the particle movement length and angle are 1 1 d m = (dij + d jk + d kl ) , θ m = (θ jk + θ jl ) 3 2
(4)
Through the path of the particle confirmed by 4Frame PTV, the particle velocity is calculated by: u ( x, y ) =
Syk Sx k , v ( x, y ) = dt dt
(5)
where S x and S y are the displacements of the particle in x - and y - directions, respectively, and dt is the time interval.
Fig.3 Bubble plume flow fields at aeration rates Fig.2 Schematic diagram of PTV algorithm
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 area S is determined by examining the maximum speed U m of the particle in the current frame, d is the maximum
The oxygen transfer coefficient ( K La ) and the oxygen transfer efficiency ( E A ) are the indexes to evaluate the change of the oxygen transfer in the water, and the computational formulas could be found in literatures[2,3]. Under the existing experimental conditions, the forms of the bubble plume are affected by several factors, such as the aeration rate, which is related to
666
the gas-phase velocity, the bubble size, the movement distribution and the process of controlling the aerobic biochemical treatment[4].
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 2.78×10–5 m3/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.
Fig.4 Time-dependent bubble flow velocity field at aeration rates
Figure 3 is the bubble plume flow fields at the different aeration rates, where at the aeration rate of 1.39×10–5 m3/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 2.08×10–5 m3/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
Fig.5 Time-average bubble flow velocity field at aeration rates
Figures 4 and 5 are the time-dependent and timeaverage bubble flow velocity fields, respectively. When the aeration rate is 1.39×10–5 m3/s, the swing of the bubble plumes is obvious in the upper part, and the aerator-caused gas-phase velocity in the lower part, about 0.3336 m/s, is lower than that in the upper part.
667
When the aeration rate is 2.08×10–5 m3/s, the bubble plumes in the lower part rise steadily with a slight swing, the plume columns in the upper part begin to concentrate to the plume flow center, and the distribution of the bubble plume velocity is uniform, being about 0.3475 m/s. If the aeration rate is 2.78× 10–5 m3/s, the amplitude of the swing becomes large at the bottom, the bubble plume begins to spread around in the upper part, and the gas-phase velocity increases up to about 0.3614 m/s. In the aeration process, the aeration rate directly affects the bubble size and number, the gas-phase velocity, the liquid turbulence, and then the oxygen transfer in the aeration containers. Figures 6 and 7 show the relationships between Q and K La and E A , respectively.
velocity will be improved continually. To a certain extent, the movement distribution and the stay time of the bubble plume flow change, the distribution in the flow field tends to be dispersed, and the gas-phase velocity distribution becomes asymmetrical. These are detrimental for the bubbles to form a stable movement distribution in the liquid phase, so as to further improve the oxygen transfer. Therefore, with the further increase of aeration rate, K La does not increase in a simple linear relationship. These phenomena mean that the aeration rates play an important role in rising aeration[6]. The bubble plume flow distribution affects the oxygen transfer. The velocity distribution of the bubble plume varies with the change of the aeration rate, when the aeration rate is 0.0000139 m3/s, the attractive structure caused by the single bubble plume is beneficial to the formation of the steady liquid circulation and the turbulent fluctuation. The velocity distribution of the gas-phase is uniform. 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 K La and E A . Thus the appropriate aeration rate is important to improve the operating efficiency of the MBBR.
Fig.6 Relationship between aeration rate and K La
References [1] [2]
[3]
[4] Fig.7 Relationship between aspect ratio and E A
It could be noticed from Figs.6 and 7 that the oxygen transfer coefficient K La increases, while the oxygen transfer efficiency E A decreases with the increase of the aeration rate. The variation of either K La or E A is nonlinear with the aeration rate. With the increase of the oxygen transfer, the gas-phase velocity in the flow field increases, so that the number of bubbles grows in the liquid flow field, and the contact areas increase between the liquid and the gas phases, and then the oxygen transfer rate speeds up[5]. However, when the aeration rate increases, the gas-phase
[5]
[6]
LI Shi-rong, CHENG Wen and WANG Meng et al. The flow patterns of bubble plume in an MBBR[J]. Journal of Hydrodynamics, 2011, 23(4): 510-515. 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. DEEN N. G., WILLEMS P. and Van SINTANNALAND M. On image pre-processing for PIV of single- and two-phase flows over reflecting object[J]. Experiments in Fluids, 2010, 49(2): 525-530. ZHANG Kai, BRANDANI S. CFD simulation of particle-fluid two-phase flow in fluidized beds[J]. Journal of Chemical Industry and Engineering, 2010, 61(9): 729-733(in Chinese). XIA Guo-dong, CUI Zhen-zhen and LIU Qing et al. A model for liquid slug length distribution in vertical gasliquid slug flow[J]. Journal of Hydrodynamics, 2009, 21(4): 491-498. LIU Wen-hong, WAN Tian and CHENG Wen-juan et al. Analysis on steady structure of bubble plume in the basis of image binarization[J]. Journal of Hydraulic Engineering, 2008, 40(11): 1369-1372(in Chinese).
2014,26(4):664-667 DOI: 10.1016/S1001-6058(14)60073-1
The effect of bubble plume on oxygen transfer for moving bed biofilm reactor* CHENG Wen (程文), LIU Hu (刘鹄), 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: [email protected] (Received June 1, 2013, Revised June 20, 2014) Abstract: 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. Key words: 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 * Project supported by the National Natural Science Foundation of China (Grant No. 51076130). Biography: CHENG Wen (1968-), Female, Ph. D., Professor
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.
Fig.1 Experimental setup of MBBR
665
Figure 1 is the experimental set-up, and its main body is a cylindrical tank of 0.7 m in height and 0.25 m in inner diameter. A rectangular tank of 0.8 m in height and of a square cross-section of 0.28 m in each side covers the cylindrical aeration device to eliminate the bubble distortion from the optical refraction. The aeration distribution tray is placed at the bottom, where there are 24 aerators surrounding the center aerator with radii of 0.0417 m, 0.0625 m and 0.0833 m, respectively. The air is injected into the bottom of the tank through capillary tubes, and the locations and the intensity of bubble plumes can be precisely controlled. In order to study the properties of the movement and velocity distribution, 36 test cases are considered at the temperature of 13oC-15oC. A CCD camera is used to record the spatio-temporal structure of bubble plumes. The tap water is used as the liquid phase with the kinematic viscosity of 10−6 m2/s and the density of 1 000 kg/m3, while the gas phase is the air in the laboratory room with the density of 1.28 kg/m3. Table 1 shows the experimental conditions, where Q is the gas flow rate, H / W is the aspect ratio of the container, and R is the mean bubble radius in the tank. Table 1 Experimental conditions No.
Q (L/h)
R (m)
H /W
p (kPa)
1
50
0.0625
1.5
6.5
2
75
0.0625
1.5
7.0
3
100
0.0625
1.5
7.5
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.
distance and θ is 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
(
2 2 1 dij − d m + d jk − d m + d kl − d m 3
σl =
2
)
(1)
Angle variance
σθ =
(
2 1 θ jk − θ m + θ jl − θ m 2
2
)
(2)
Total variance
σt =
σ l2 d m2
+ σ θ2
(3)
in which, the particle movement length and angle are 1 1 d m = (dij + d jk + d kl ) , θ m = (θ jk + θ jl ) 3 2
(4)
Through the path of the particle confirmed by 4Frame PTV, the particle velocity is calculated by: u ( x, y ) =
Syk Sx k , v ( x, y ) = dt dt
(5)
where S x and S y are the displacements of the particle in x - and y - directions, respectively, and dt is the time interval.
Fig.3 Bubble plume flow fields at aeration rates Fig.2 Schematic diagram of PTV algorithm
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 area S is determined by examining the maximum speed U m of the particle in the current frame, d is the maximum
The oxygen transfer coefficient ( K La ) and the oxygen transfer efficiency ( E A ) are the indexes to evaluate the change of the oxygen transfer in the water, and the computational formulas could be found in literatures[2,3]. Under the existing experimental conditions, the forms of the bubble plume are affected by several factors, such as the aeration rate, which is related to
666
the gas-phase velocity, the bubble size, the movement distribution and the process of controlling the aerobic biochemical treatment[4].
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 2.78×10–5 m3/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.
Fig.4 Time-dependent bubble flow velocity field at aeration rates
Figure 3 is the bubble plume flow fields at the different aeration rates, where at the aeration rate of 1.39×10–5 m3/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 2.08×10–5 m3/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
Fig.5 Time-average bubble flow velocity field at aeration rates
Figures 4 and 5 are the time-dependent and timeaverage bubble flow velocity fields, respectively. When the aeration rate is 1.39×10–5 m3/s, the swing of the bubble plumes is obvious in the upper part, and the aerator-caused gas-phase velocity in the lower part, about 0.3336 m/s, is lower than that in the upper part.
667
When the aeration rate is 2.08×10–5 m3/s, the bubble plumes in the lower part rise steadily with a slight swing, the plume columns in the upper part begin to concentrate to the plume flow center, and the distribution of the bubble plume velocity is uniform, being about 0.3475 m/s. If the aeration rate is 2.78× 10–5 m3/s, the amplitude of the swing becomes large at the bottom, the bubble plume begins to spread around in the upper part, and the gas-phase velocity increases up to about 0.3614 m/s. In the aeration process, the aeration rate directly affects the bubble size and number, the gas-phase velocity, the liquid turbulence, and then the oxygen transfer in the aeration containers. Figures 6 and 7 show the relationships between Q and K La and E A , respectively.
velocity will be improved continually. To a certain extent, the movement distribution and the stay time of the bubble plume flow change, the distribution in the flow field tends to be dispersed, and the gas-phase velocity distribution becomes asymmetrical. These are detrimental for the bubbles to form a stable movement distribution in the liquid phase, so as to further improve the oxygen transfer. Therefore, with the further increase of aeration rate, K La does not increase in a simple linear relationship. These phenomena mean that the aeration rates play an important role in rising aeration[6]. The bubble plume flow distribution affects the oxygen transfer. The velocity distribution of the bubble plume varies with the change of the aeration rate, when the aeration rate is 0.0000139 m3/s, the attractive structure caused by the single bubble plume is beneficial to the formation of the steady liquid circulation and the turbulent fluctuation. The velocity distribution of the gas-phase is uniform. 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 K La and E A . Thus the appropriate aeration rate is important to improve the operating efficiency of the MBBR.
Fig.6 Relationship between aeration rate and K La
References [1] [2]
[3]
[4] Fig.7 Relationship between aspect ratio and E A
It could be noticed from Figs.6 and 7 that the oxygen transfer coefficient K La increases, while the oxygen transfer efficiency E A decreases with the increase of the aeration rate. The variation of either K La or E A is nonlinear with the aeration rate. With the increase of the oxygen transfer, the gas-phase velocity in the flow field increases, so that the number of bubbles grows in the liquid flow field, and the contact areas increase between the liquid and the gas phases, and then the oxygen transfer rate speeds up[5]. However, when the aeration rate increases, the gas-phase
[5]
[6]
LI Shi-rong, CHENG Wen and WANG Meng et al. The flow patterns of bubble plume in an MBBR[J]. Journal of Hydrodynamics, 2011, 23(4): 510-515. 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. DEEN N. G., WILLEMS P. and Van SINTANNALAND M. On image pre-processing for PIV of single- and two-phase flows over reflecting object[J]. Experiments in Fluids, 2010, 49(2): 525-530. ZHANG Kai, BRANDANI S. CFD simulation of particle-fluid two-phase flow in fluidized beds[J]. Journal of Chemical Industry and Engineering, 2010, 61(9): 729-733(in Chinese). XIA Guo-dong, CUI Zhen-zhen and LIU Qing et al. A model for liquid slug length distribution in vertical gasliquid slug flow[J]. Journal of Hydrodynamics, 2009, 21(4): 491-498. LIU Wen-hong, WAN Tian and CHENG Wen-juan et al. Analysis on steady structure of bubble plume in the basis of image binarization[J]. Journal of Hydraulic Engineering, 2008, 40(11): 1369-1372(in Chinese).