- Email: [email protected]
Bubble dynamics and oxygen transfer in a hypolimnetic aerator
~
Pergamon
Wal. Sci. Tech. Vol. 37, No.2, pp. 293-300, 1998.
C 1998 IAWQ. Published by El5evier Science Ud Printed in Oreal Britain.
PU: 50273-1223(98)00036-5
0273-1223198 $ 19'00 + 0-00
BUBBLE DYNAMICS AND OXYGEN TRANSFER IN A HYPOLIMNETIC AERATOR Vickie L. Burris and John C. Little Department ofCivil Engineering, Virginia Polytechnic InstitUle and State University, Black.sburg, Virginia 24061·0105, USA
ABSTRACT A hypolimnelic aetalOr operaling in one of the Cily of Norfolk's waler supply reservoilS was lesled. Dissolved oxygen (DO) profiles, water flow rale, and gas-phase holdup were measured over a wide range of applied air flow rales. A model thaI was developed 10 predict oxygen transfer in a Speece Cone was modified 10 conform 10 the conditions of the hypolimnetic aeralor. By varying a single parameler (the initial bubble size) the model was found 10 provide a close fillo the experimental DO profiles as well as Ihe observed gas• phase holdup. 1be model was used 10 show thaI a doubling in oxygen transfer may be achieved if initial bubble size is reduced from 5 mm 10 2.5 mm. Knowing the initial bubble size, il should be possible 10 predicI waler velocity by incorporating the efrect of momenlum. Further work is now underway 10 leslthis approach and 10 examine the possibility of extending this generalized model 10 cover the range of hypolimnetic aeration and oxygenation devices. @ 1998 IAWQ. Published by Elsevier Science Ltd
KEYWORDS Aeration; bubble dynamics; gas holdup; hypolimnion; lake; model; oxygen transfer; reservoir. INTRODUCTION Lake Prince and Lake Western Branch are two water supply reservoirs located in Suffolk County, Virginia. The lakes are owned and maintained by the City of Norfolk. which provides potable water to a significant portion of the Tidewater Virginia area. The capacity of Norfolk's entire water supply system is approximately IS billion gallons; the capacity of Lakes Prince and Western Branch is about 13 billion gallons. The lakes therefore comprise the majority of Norfolk's raw water supply. The lakes have historically had relatively low levels of dissolved oxygen (DO) within the hypolimnion during stratification. This resulted in releases of iron and manganese from the sediments to the hypolimnetic waters and, eventually, to the entire water column during lake tum-over in the fall. Hypolimnetic aerators were installed in Lake Prince and Lake Western Branch in an effort to increase DO levels and, hence, to improve raw water quality. Aerators were first installed in Lake Prince in 1991 and in Lake Western Branch in 1993. During this time period, improvements in hypolimnetic water quality, particularly the upper hypolimnion, have been observed in each lake including lowered concentrations of iron. manganese, sulfide, and phosphorus. Consequently, chlorine demand at the treatment plant has decreased. In many respects, the aeration system can now be considered the first step in the water treatment process. 293
294
V. L. BURRIS and J. C. LITI1..E
Although water quality improvements have been observed, steady dissolved oxygen loss from the hypolimnetic region has occurred during aerated periods. This condition leads to the development of anoxic conditions throughout most of the hypolimnion of each lake by late July even though the aerators are apparently functioning under normal operating conditions (Schafran and Scully, 1995). These conditions lead to increased releases of iron and manganese from the lake sediments and a deterioration in water quality. A more thorough understanding of hypolimnetic aerator dynamics is required to avoid this continuing problem. The project described in this paper has three principle objectives. The first is to fully characterize the perfonnance of the aerators with respect to water flow rate and oxygen transfer. These data will be used to validate a fundamental process model that can accurately predict aerator performance based on applied air flow rate and intake water conditions. The second objective is to couple the process model with a cost model and hence to optimize aerator performance. The final objective is to identify ways to improve upon the efficiency of the aerators. In this paper, progress towards meeting these three objectives will be described.
EQUIPMENT AND METHODS The specific aerator tested was Lake Prince Aerator 1 (LPA1). This aerator was selected because of its newer design and relatively well defined flow pattern. As shown in Figure 1, LPAI is a full-lift hypolimnetic aerator. Table I gives a summary of the relevant aerator dimensions. Compressed air is supplied from a set of compressors located on shore. The air is fed through a weighted polyethylene pipe with an internal diameter of 2.0 in. Once the air reaches the aerator, it is released from a diffuser located at the bottom of the riser tube. The diffuser unit consists of two pieces of 1.5 in. schedule 80 PVC pipe mounted vertically on the ends of a tee. The length of each diffuser pipe is approximately 34 in., with orifices located towards the top. The orifice diameter is 0.125 in. Diffuser orifices are placed in 5 vertical rows with 12 holes per row and a spacing of 1.0 in. When air is introduced to the water column in the riser, it creates an air/water mixture that is less dense than the surrounding water. This causes the mixture to travel up the riser tube. Once the mixture reaches the top of the riser, the majority of the bubbles escape into the atmosphere. The remaining bubbles are entrained in the flow of water that travels down either of the two downcomers. As the water leaves the downcomers, it is deflected away from the sediments by a circular baffle that is suspended from the outlet In addition to the diffuser unit located in the riser, there are also two auxiliary diffusers located in each of the downcomers. Experimental data using the auxiliary diffusers are not presented here. Since the only variable that can be adjusted on LPAI is the air flow rate, a series of experiments was conducted over a range of air flow rates (40 - 140 sefm). Tests were perfonned in duplicate. The data recorded for each run included water temperature, overall gas-phase holdup, water velocity, and DO profiles in the riser and both downcomers. In addition, a background DO profile was taken beside the aerator on each day of testing. Two main pieces of experimental equipment were used to collect data from the aerators. The first was a YSI Model 58 dissolved oxygen meter. The DO meter was calibrated at the beginning of a testing day. and the calibration was checked at the end of the day for any drift. The second piece of equipment used was a Swoffer Model 2100 stream current propeller meter. This meter was modified to obtain readings in both vertical directions in the riser and downcomers. Prior to testing, the perfonnance of the meter was checked in a laboratory using a flume, and good agreement between readings was obtained. The propeller meter was also calibrated daily in the field.
Bubble dynamics and oxygen transfer
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diffusers Figure I. Schematic representation of hypolimnetic aerator LPAI.
Table J. LPAI dimensions Parameter
Value(m) 10.0 5.0 0.46 1.07 1.07 9.0
Water flow rate was calculated using the velocity-area method. Velocity measurements were obtained at a depth of approximately 2.75 m to minimize entrance and exit effects. Data were collected at five different points within the riser lUbe cross-section to obtain a good representation of the flow. At each point, four consecutive readings were obtained, with each reading being a thirty second average of the actual velocity at that point. The four readings at each point were then averaged. After analysis of the data, it was determined that there was no statistical difference between the values at the different points. Therefore, all twenty readings were averaged to calculate the mean water velocity produced at a given air flow rate. Overall gas• phase holdup was determined by the volume expansion technique. The height of the gas-liquid dispersion dUring aeration was found using a tape measure. The volume expansion technique is a very simple and reliable method and has been used by a number of researchers examining air-lift bioreactors (Chisti, 1989). Jl/ST 31'2"
296
V. L. BURRIS and 1. C. LITIl..E
RESULTS AND DISCUSSION In closely related work.• a model has been developed (McGinnis and Little. 1998) that describes bubble dynamics and oxygen transfer in a Speece Cone. another type of submerged aeration device. The model is based on the known functional dependence of the bubble rise velocity and mass transfer coefficient on bubble size. It is assumed that the bubbles are spherical and of uniform initial size. that no bubble coalescence or breakup occurs. and that both water and gas are in plug flow. Mass balances for water and gas lead to a system of equations that incorporate gas transfer between the phases, the change in gas partial pressure with depth, and the influence of gas holdup. The model can be easily modified to apply to hypolimnetic aerators, which have structural differences and opposite bubble/water flow patterns. For greater detail about the model and an explanation of supporting equations. the reader is referred to McGinnis and Little (1998). Using the water flow rate measured in the hypolimnetic aerator, the oxygen transfer model was fitted to the DO profiles in the riser. This was accomplished by varying the initial bubble size created by the diffuser, the only unknown for the system studied. The measured profiles and best-fit model predictions are given in Figures 2-7. The model follows the oxygen concentration profiles well over the entire range of experimental air flow rates. The data show an initial rapid transfer of oxygen to the water in the bottom portion of the riser tube. This is caused by a high driving force due to the increased hydrostatic pressure coupled with the relatively low dissolved oxygen concentration in the water. As the water travels up the riser, the hydrostatic pressure decreases while the dissolved oxygen concentration increases. Both of these occurrences act to lower the driving force, hence decreasing the rate of oxygen transfer.
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Fi,UTe 2. DO profile at 40 sefm.
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Figure 3. DO profile at 60 sefm.
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Bubble dynamics and oxygen transfer
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Figure 7. DO profile at 140 sefm
298
V. L. BURRlS andJ. C. UTILE
Table 2 gives the initial bubble diameters that provided the best fit of the model to the experimental data. Ignoring the low value of 2.0 mm, the range of initial bubble diameters is 4.0-6.5 rom. Photographs taken of bubbles at the top of the riser tube appear to support this (after allowing for expansion). Ashley et al. (1992) reported similar results (7.2 mm) for the same orifice diameter tested in a bench-scale study of coarse bubble diffusers. The values calculated by the model seem to be relatively independent of applied air flow rate, an effect also observed by Ashley et al. (1992). Table 2. Initial bubble size predicted by model Air Flow Rate. (scfm) 40.5 40.4 60.0 60.2 79.8 80.0 100 99.9 122
119 133 136
Water Flow Rate Initial Bubble Diameter 3 (rom) (m ts) 0.18 5.0 0.20 4.5 0.27 6.0 0.29 5.0 0.38 5.0 0.41 5.0 0.53 5.0 0.50 5.0 0.72 2.0 0.49 6.0 0.68 4.0 0.51 6.5 .m3ts - scfm x 4.72 x 10""
Overall gas-phase holdup was also calculated by the model. The predicted results are compared to experimental values in Figure 8. Good agreement is seen between measured and calculated gas holdup, with most of the predicted values within ±30%.
o
2
3
4
5
6
7
Measured Gas Holdup (%) Figure 8. Comparison of measured and predicted gas holdup using model.
Figure 9 illustrates the effect of initial bubble size created by the diffuser on the change in the dissolved oxygen concentration as the water travels up the riser tube. This analysis was performed using an applied air flow rate of 140 scfm, approximately equal to the normal operating condition of LPAI. It can be seen that there is a significant increase in the amount of oxygen added to the water as the bubble size decreases. This is caused by an increase in specific surface area per unit volume of gas. A smaller bubble size decreases the tenninal rise velocity of the bubble, decreasing the mass transfer coefficient. However, the model shows that
Bubble dynamics and oxygen transfer
299
an increase in the interfacial surface area more than compensates for the decrease in the mass transfer coefficient Similar results were obtained by Ashley et al. (1992) in experimental work on coarse bubble diffusers. Figure 9 suggests that there can be considerable gains in oxygen transfer from an aerator if the initial bubble size is decreased by using fine pore diffusers, provided that the induced water flow rate remains relatively constant. Assuming that the average initial bubble size is approximately 5 mm and the water flow rate produced is independent of bubble size, the overall oxygen transfer can be increased by about I 00% if the bubble size is decreased to 2.5 mm. Comparable results were obtained by Ashley et al.
(1990).
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SUMMARY AND CONCLUSIONS Data from a full-lift hypolimnetic aerator have been collected over a wide range of applied air flow rates. Dissolved oxygen concentration profiles, water flow rates, and gas-phase holdup were measured. A model that was developed to describe bubble dynamics and oxygen transfer in a Speece Cone was modified to confono to the conditions of a hypolimnetic aerator. By varying a single parameter (the initial bubble size) the model was found to provide a close fit to the experimental DO profiles as well as the observed gas holdup. The model was used to show that a doubling in oxygen transfer may be achieved if initial bubble size is reduced from 5 mm to 2.5 mm. Knowing the initial bubble size, it should be possible to predict water velocity by simultaneous integration of an additional equation based on the conservation of momentum. Further work is now under way to test this approach and to examine the possibility of extending this generalized model to cover the entire range of hypolimnetic aeration and oxygenation devices including the Speece Cone, full- and partial-lift hypolimnetic aerators, and bubble-plume diffusers (McGinnis et al., 1997). ACKNOWLEDGMENTS We thank the City of Norfolk, Virginia for financial support. We also thank Dr. Gary Schafran and Tim Hare of Old Dominion University, and Dan McGinnis of Virginia Tech for help with the experimental work.
REFERENCES Ashley. K. I.• Mavinic. D. S. and Hall. K.1. (1992). Bench-s<:ale Sludy of oxygen transfer in coum bubble diffUS<:d aeration. Wal. Res.• 26, 1289-1295. Ashley, K. I., Mavinic. D. S. and Hall, K. 1. (1990). Effects of orifice size and surface condilions on oxygen transfer in a bench scale diffused aeration system. Environ. Tech., JJ, 609-618.
300
V. L. BURRIS andJ. C. LITn.E
Chisti. M. Y. (1989). Air Lift Biort/U/ors. Elsevier Applied Science. London. England. McGinnis. D. F. and Little. J. C. (1997). Bubble dynamics and o~ygen transfer in a Speece Cone. War. ScL Tech.. 37(2) (this issue). McGinnis. D. F.• Little. J. C. and Cumbie. W. (1997). Nutrient control in Standley Lake: Evaluation of three o~ygen transfer devices. Wa/. Sci. Tech.. 37(2) (this issue). Schafran. G. C. and Scully. F. E. (1995). Water Quality Momtoring of the Western Reservoirs for 1994. Final Report to the City of Norfolk.
Pergamon
Wal. Sci. Tech. Vol. 37, No.2, pp. 293-300, 1998.
C 1998 IAWQ. Published by El5evier Science Ud Printed in Oreal Britain.
PU: 50273-1223(98)00036-5
0273-1223198 $ 19'00 + 0-00
BUBBLE DYNAMICS AND OXYGEN TRANSFER IN A HYPOLIMNETIC AERATOR Vickie L. Burris and John C. Little Department ofCivil Engineering, Virginia Polytechnic InstitUle and State University, Black.sburg, Virginia 24061·0105, USA
ABSTRACT A hypolimnelic aetalOr operaling in one of the Cily of Norfolk's waler supply reservoilS was lesled. Dissolved oxygen (DO) profiles, water flow rale, and gas-phase holdup were measured over a wide range of applied air flow rales. A model thaI was developed 10 predict oxygen transfer in a Speece Cone was modified 10 conform 10 the conditions of the hypolimnetic aeralor. By varying a single parameler (the initial bubble size) the model was found 10 provide a close fillo the experimental DO profiles as well as Ihe observed gas• phase holdup. 1be model was used 10 show thaI a doubling in oxygen transfer may be achieved if initial bubble size is reduced from 5 mm 10 2.5 mm. Knowing the initial bubble size, il should be possible 10 predicI waler velocity by incorporating the efrect of momenlum. Further work is now underway 10 leslthis approach and 10 examine the possibility of extending this generalized model 10 cover the range of hypolimnetic aeration and oxygenation devices. @ 1998 IAWQ. Published by Elsevier Science Ltd
KEYWORDS Aeration; bubble dynamics; gas holdup; hypolimnion; lake; model; oxygen transfer; reservoir. INTRODUCTION Lake Prince and Lake Western Branch are two water supply reservoirs located in Suffolk County, Virginia. The lakes are owned and maintained by the City of Norfolk. which provides potable water to a significant portion of the Tidewater Virginia area. The capacity of Norfolk's entire water supply system is approximately IS billion gallons; the capacity of Lakes Prince and Western Branch is about 13 billion gallons. The lakes therefore comprise the majority of Norfolk's raw water supply. The lakes have historically had relatively low levels of dissolved oxygen (DO) within the hypolimnion during stratification. This resulted in releases of iron and manganese from the sediments to the hypolimnetic waters and, eventually, to the entire water column during lake tum-over in the fall. Hypolimnetic aerators were installed in Lake Prince and Lake Western Branch in an effort to increase DO levels and, hence, to improve raw water quality. Aerators were first installed in Lake Prince in 1991 and in Lake Western Branch in 1993. During this time period, improvements in hypolimnetic water quality, particularly the upper hypolimnion, have been observed in each lake including lowered concentrations of iron. manganese, sulfide, and phosphorus. Consequently, chlorine demand at the treatment plant has decreased. In many respects, the aeration system can now be considered the first step in the water treatment process. 293
294
V. L. BURRIS and J. C. LITI1..E
Although water quality improvements have been observed, steady dissolved oxygen loss from the hypolimnetic region has occurred during aerated periods. This condition leads to the development of anoxic conditions throughout most of the hypolimnion of each lake by late July even though the aerators are apparently functioning under normal operating conditions (Schafran and Scully, 1995). These conditions lead to increased releases of iron and manganese from the lake sediments and a deterioration in water quality. A more thorough understanding of hypolimnetic aerator dynamics is required to avoid this continuing problem. The project described in this paper has three principle objectives. The first is to fully characterize the perfonnance of the aerators with respect to water flow rate and oxygen transfer. These data will be used to validate a fundamental process model that can accurately predict aerator performance based on applied air flow rate and intake water conditions. The second objective is to couple the process model with a cost model and hence to optimize aerator performance. The final objective is to identify ways to improve upon the efficiency of the aerators. In this paper, progress towards meeting these three objectives will be described.
EQUIPMENT AND METHODS The specific aerator tested was Lake Prince Aerator 1 (LPA1). This aerator was selected because of its newer design and relatively well defined flow pattern. As shown in Figure 1, LPAI is a full-lift hypolimnetic aerator. Table I gives a summary of the relevant aerator dimensions. Compressed air is supplied from a set of compressors located on shore. The air is fed through a weighted polyethylene pipe with an internal diameter of 2.0 in. Once the air reaches the aerator, it is released from a diffuser located at the bottom of the riser tube. The diffuser unit consists of two pieces of 1.5 in. schedule 80 PVC pipe mounted vertically on the ends of a tee. The length of each diffuser pipe is approximately 34 in., with orifices located towards the top. The orifice diameter is 0.125 in. Diffuser orifices are placed in 5 vertical rows with 12 holes per row and a spacing of 1.0 in. When air is introduced to the water column in the riser, it creates an air/water mixture that is less dense than the surrounding water. This causes the mixture to travel up the riser tube. Once the mixture reaches the top of the riser, the majority of the bubbles escape into the atmosphere. The remaining bubbles are entrained in the flow of water that travels down either of the two downcomers. As the water leaves the downcomers, it is deflected away from the sediments by a circular baffle that is suspended from the outlet In addition to the diffuser unit located in the riser, there are also two auxiliary diffusers located in each of the downcomers. Experimental data using the auxiliary diffusers are not presented here. Since the only variable that can be adjusted on LPAI is the air flow rate, a series of experiments was conducted over a range of air flow rates (40 - 140 sefm). Tests were perfonned in duplicate. The data recorded for each run included water temperature, overall gas-phase holdup, water velocity, and DO profiles in the riser and both downcomers. In addition, a background DO profile was taken beside the aerator on each day of testing. Two main pieces of experimental equipment were used to collect data from the aerators. The first was a YSI Model 58 dissolved oxygen meter. The DO meter was calibrated at the beginning of a testing day. and the calibration was checked at the end of the day for any drift. The second piece of equipment used was a Swoffer Model 2100 stream current propeller meter. This meter was modified to obtain readings in both vertical directions in the riser and downcomers. Prior to testing, the perfonnance of the meter was checked in a laboratory using a flume, and good agreement between readings was obtained. The propeller meter was also calibrated daily in the field.
Bubble dynamics and oxygen transfer
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diffusers Figure I. Schematic representation of hypolimnetic aerator LPAI.
Table J. LPAI dimensions Parameter
Value(m) 10.0 5.0 0.46 1.07 1.07 9.0
Water flow rate was calculated using the velocity-area method. Velocity measurements were obtained at a depth of approximately 2.75 m to minimize entrance and exit effects. Data were collected at five different points within the riser lUbe cross-section to obtain a good representation of the flow. At each point, four consecutive readings were obtained, with each reading being a thirty second average of the actual velocity at that point. The four readings at each point were then averaged. After analysis of the data, it was determined that there was no statistical difference between the values at the different points. Therefore, all twenty readings were averaged to calculate the mean water velocity produced at a given air flow rate. Overall gas• phase holdup was determined by the volume expansion technique. The height of the gas-liquid dispersion dUring aeration was found using a tape measure. The volume expansion technique is a very simple and reliable method and has been used by a number of researchers examining air-lift bioreactors (Chisti, 1989). Jl/ST 31'2"
296
V. L. BURRIS and 1. C. LITIl..E
RESULTS AND DISCUSSION In closely related work.• a model has been developed (McGinnis and Little. 1998) that describes bubble dynamics and oxygen transfer in a Speece Cone. another type of submerged aeration device. The model is based on the known functional dependence of the bubble rise velocity and mass transfer coefficient on bubble size. It is assumed that the bubbles are spherical and of uniform initial size. that no bubble coalescence or breakup occurs. and that both water and gas are in plug flow. Mass balances for water and gas lead to a system of equations that incorporate gas transfer between the phases, the change in gas partial pressure with depth, and the influence of gas holdup. The model can be easily modified to apply to hypolimnetic aerators, which have structural differences and opposite bubble/water flow patterns. For greater detail about the model and an explanation of supporting equations. the reader is referred to McGinnis and Little (1998). Using the water flow rate measured in the hypolimnetic aerator, the oxygen transfer model was fitted to the DO profiles in the riser. This was accomplished by varying the initial bubble size created by the diffuser, the only unknown for the system studied. The measured profiles and best-fit model predictions are given in Figures 2-7. The model follows the oxygen concentration profiles well over the entire range of experimental air flow rates. The data show an initial rapid transfer of oxygen to the water in the bottom portion of the riser tube. This is caused by a high driving force due to the increased hydrostatic pressure coupled with the relatively low dissolved oxygen concentration in the water. As the water travels up the riser, the hydrostatic pressure decreases while the dissolved oxygen concentration increases. Both of these occurrences act to lower the driving force, hence decreasing the rate of oxygen transfer.
o -2
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"-'
-=g-6
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-8
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4 DO (mgIL)
6
8
Fi,UTe 2. DO profile at 40 sefm.
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Figure 3. DO profile at 60 sefm.
6
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Bubble dynamics and oxygen transfer
297
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Figure 4. DO profile 81 80 serm.
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Figure 6. DO profile al 120 sefm.
---1-------.......- ---2
468 DO (mgIL)
Figure 7. DO profile at 140 sefm
298
V. L. BURRlS andJ. C. UTILE
Table 2 gives the initial bubble diameters that provided the best fit of the model to the experimental data. Ignoring the low value of 2.0 mm, the range of initial bubble diameters is 4.0-6.5 rom. Photographs taken of bubbles at the top of the riser tube appear to support this (after allowing for expansion). Ashley et al. (1992) reported similar results (7.2 mm) for the same orifice diameter tested in a bench-scale study of coarse bubble diffusers. The values calculated by the model seem to be relatively independent of applied air flow rate, an effect also observed by Ashley et al. (1992). Table 2. Initial bubble size predicted by model Air Flow Rate. (scfm) 40.5 40.4 60.0 60.2 79.8 80.0 100 99.9 122
119 133 136
Water Flow Rate Initial Bubble Diameter 3 (rom) (m ts) 0.18 5.0 0.20 4.5 0.27 6.0 0.29 5.0 0.38 5.0 0.41 5.0 0.53 5.0 0.50 5.0 0.72 2.0 0.49 6.0 0.68 4.0 0.51 6.5 .m3ts - scfm x 4.72 x 10""
Overall gas-phase holdup was also calculated by the model. The predicted results are compared to experimental values in Figure 8. Good agreement is seen between measured and calculated gas holdup, with most of the predicted values within ±30%.
o
2
3
4
5
6
7
Measured Gas Holdup (%) Figure 8. Comparison of measured and predicted gas holdup using model.
Figure 9 illustrates the effect of initial bubble size created by the diffuser on the change in the dissolved oxygen concentration as the water travels up the riser tube. This analysis was performed using an applied air flow rate of 140 scfm, approximately equal to the normal operating condition of LPAI. It can be seen that there is a significant increase in the amount of oxygen added to the water as the bubble size decreases. This is caused by an increase in specific surface area per unit volume of gas. A smaller bubble size decreases the tenninal rise velocity of the bubble, decreasing the mass transfer coefficient. However, the model shows that
Bubble dynamics and oxygen transfer
299
an increase in the interfacial surface area more than compensates for the decrease in the mass transfer coefficient Similar results were obtained by Ashley et al. (1992) in experimental work on coarse bubble diffusers. Figure 9 suggests that there can be considerable gains in oxygen transfer from an aerator if the initial bubble size is decreased by using fine pore diffusers, provided that the induced water flow rate remains relatively constant. Assuming that the average initial bubble size is approximately 5 mm and the water flow rate produced is independent of bubble size, the overall oxygen transfer can be increased by about I 00% if the bubble size is decreased to 2.5 mm. Comparable results were obtained by Ashley et al.
(1990).
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Figure 9. Effect of bubble diameter on DO profile al 140 s<:fm.
SUMMARY AND CONCLUSIONS Data from a full-lift hypolimnetic aerator have been collected over a wide range of applied air flow rates. Dissolved oxygen concentration profiles, water flow rates, and gas-phase holdup were measured. A model that was developed to describe bubble dynamics and oxygen transfer in a Speece Cone was modified to confono to the conditions of a hypolimnetic aerator. By varying a single parameter (the initial bubble size) the model was found to provide a close fit to the experimental DO profiles as well as the observed gas holdup. The model was used to show that a doubling in oxygen transfer may be achieved if initial bubble size is reduced from 5 mm to 2.5 mm. Knowing the initial bubble size, it should be possible to predict water velocity by simultaneous integration of an additional equation based on the conservation of momentum. Further work is now under way to test this approach and to examine the possibility of extending this generalized model to cover the entire range of hypolimnetic aeration and oxygenation devices including the Speece Cone, full- and partial-lift hypolimnetic aerators, and bubble-plume diffusers (McGinnis et al., 1997). ACKNOWLEDGMENTS We thank the City of Norfolk, Virginia for financial support. We also thank Dr. Gary Schafran and Tim Hare of Old Dominion University, and Dan McGinnis of Virginia Tech for help with the experimental work.
REFERENCES Ashley. K. I.• Mavinic. D. S. and Hall. K.1. (1992). Bench-s<:ale Sludy of oxygen transfer in coum bubble diffUS<:d aeration. Wal. Res.• 26, 1289-1295. Ashley, K. I., Mavinic. D. S. and Hall, K. 1. (1990). Effects of orifice size and surface condilions on oxygen transfer in a bench scale diffused aeration system. Environ. Tech., JJ, 609-618.
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Chisti. M. Y. (1989). Air Lift Biort/U/ors. Elsevier Applied Science. London. England. McGinnis. D. F. and Little. J. C. (1997). Bubble dynamics and o~ygen transfer in a Speece Cone. War. ScL Tech.. 37(2) (this issue). McGinnis. D. F.• Little. J. C. and Cumbie. W. (1997). Nutrient control in Standley Lake: Evaluation of three o~ygen transfer devices. Wa/. Sci. Tech.. 37(2) (this issue). Schafran. G. C. and Scully. F. E. (1995). Water Quality Momtoring of the Western Reservoirs for 1994. Final Report to the City of Norfolk.