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Application of Oxygen Microbubbles for in Situ Biodegradation of p-Xylene-Contaminated Groundwater in a Soil Column Kristen B. Jenkins' Texaco R&D Department, Port Arthur, Texas 77640
Donald L. Michelsen and John T. Novak Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
In situ biodegradation of p-xylene was studied in a 7-cm (2.754~1.)soil column using oxygen microbubbles to supply the electron acceptor. Pseudomonas pu tida continuously degraded p-xylene below detectable limits until the oxygen supply was exhausted. Retention time in the biodegradation zone was approximately 45 min. Vent losses claimed 5-10% of the injected oxygen, with 71-82 76 being utilized. The pressure drops resulting from increased biomass showed a slight increase over the first few days followed by a gradual decline, indicating that the biomass will not plug the soil matrix under the conditions of this test. The addition of ferrous iron in the feed and its subsequent oxidation to ferric hydroxide did not affect biodegradation, nor did it cause appreciable soil plugging. In order to precipitate the ferrous iron before it reached the microbubbles, an air-sparging section was added. This resulted in volatilization of p-xylene with very little ferrous oxidation until the groundwater reached the oxygen microbubbles.
Introduction and Background Various treatment technologies can be applied to contaminated soil and groundwater, including air-sparging, soil vapor extraction, and pump and treat. A recent EPA study of 19 Superfund sites in which contaminated groundwater was pumped and treated found that this method wastes groundwater, is costly, and is slow in reducing pollutant concentrations to acceptable levels (Cartwright, 1991). With in situ biodegradation, no excavation is required, the operating costs are low, and it is often cheaper overall. In situ biodegradation is applicable to a large volume of water containing relatively low concentrations of biodegradable contaminants and is being encouraged by the regulatory agencies as a treatment technology (Thayer, 1991). In order for biodegradation to occur, the degrading microorganisms, nutrients, and an electron acceptor must be present. The rate of aerobic degradation is faster than that of anaerobic, but aerobic degradation often requires the addition of oxygen to serve as the electron acceptor, and methods for the delivery of oxygen to the saturated zone are frequently ineffective. Research has been conducted on the injection and retention of oxygen microbubbles in the saturated zone of a pilot-scale vertical slice test cell (VSTC, 2.1 m X 2.1 m X 12.7 cm, front to back), as well as the transfer of oxygen to flowing groundwater (Michelsen andLotfi, 1991). These tests have led to a proposed field scenariofor in situ aerobic biodegradation of contaminated ground water. By using alternate layering of coarse concrete sand and clay, the groundwater flow is directed through the coarse sandlayers which contain degrading microorganisms, nutrients, and oxygen microbubbles. The clay layers help to retain oxygen in the saturated zone, which increases microbubble retention and oxygen transfer. Microbubbles are a dispersion of spherical gas bubbles in an aqueous surfactant solution with 95 5% of the bubbles 8756-7938/93/3009-0394$04.00/0
not exceeding 100pm in diameter and an average diameter of 50.7 f 22.7 pm (Longe, 1989). Microbubbles are a wet foam and show colloidal properties for bubble diameters less than 50 pm (Longe, 1989). One colloidal property exhibited by the foam is that the bubbles rarely coalesce as long as they are completely immersed in water. Microbubbles can be made using a bench-scale spinningdisk generator (Sebba, 1987) or a pilot-scale generator. Through several studies, oxygen microbubbles have proven to be more efficient in transferring oxygen to the liquid phase than air-sparging, hydrogen peroxide addition, or oxygenated water injection (Lotfi, 1990). The cost of microbubble generation is more than that of air-sparging but less than that of hydrogen peroxide injection (Michelsen and Lotfi, 1991). However, introduction of the surfactant contributes to the consumption of dissolved oxygen as the surfactant biodegradation consumes approximately 14% of the injected oxygen. The use of pure oxygen also requires careful handling since it causes ignitions easily. Previously, a soil column study of in situ biodegradation of glucose was performed to determine the extent of plugging caused by biodegradation and oxidation and precipitation of reduced iron (Achanta, 1991). Much of the groundwater in the U.S. contains reduced iron which will precipitate as ferric hydroxide in the presence of oxygen. The highest concentration of iron in groundwater in the U.S.is 16 ppm with most groundwater containing betweenOandEippm(Pettyjohnetal.,1979). Thisresearch indicated that biodegradation would cause some plugging and that, when the feed contained ferrous iron, iron oxidation took precedence over biodegradation. Both biomass accumulation and ferric hydroxide precipitation contributed to plugging of the soil matrix, perhaps due to a high flow rate of 20 fttday. These results suggest that removal of the ferrous iron prior to the biodegradation zone and a means of flushing out the ferric hydroxide would be desirable in order to facilitate in situ biodegradation.
0 1993 American Chemical Society and American InstCute of Chemical Engineers
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MICRON F I L T E R S NITROGEN F E E D
NITROGEN CATCH V E S S E L
-PURE
P-XYLENE
P-XYLENE SATURATED WATER
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co
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Ne SPARGED
NITROGEN SPARGED GROUNDWATER FEED
pH = 9.5 2 4 PPn
1.5 m l / m i n
L
I
Figure 1. Feed system for vertical column test cell. The mass-transfer coefficients were determined using the following formula for a packed bed (Treybal, 1980): where Kla is the mass-transfer coefficient (h-9, L is the flow velocity (cm/h), XZ is the effluent DO (dissolved oxygen) (mg/L), XIis the influent DO (mg/L), 2 is the mass-transfer zone (cm), and X A- XALis the log mean driving force (mg/L) [= [(C,- XI)- (C,- Xa)l/ln[(C,X1)/(C, - Xz)]where C,is the saturation concentration of 02 in HzO (mg/L)l.
Experimental Methods In this study, in situ biodegradation of a petroleum hydrocarbon frequently found in contaminated groundwater, p-xylene, was evaluated. The first objective was to show that p-xylene could be biodegraded in a soil column and to follow the degradation and pressure drop as a function of time. Oxygen microbubbles were intermittently injected into the column to provide an electron acceptor for biodegradation. Microbubbles were periodically reinjected when the oxygen level declined. The second objective was to demonstrate the potential for biodegradation of p-xylene in the presence of ferrous iron and to follow bioremediation and anticipated pressure drops as a function of time. The third objective, to determine whether the ferrous iron could be removed inexpensively, was studied by adding an air-sparging section prior to the biodegradation section. For the fourth objective,air-sparging followed by back-washing was used in an attempt to loosen and remove ferric hydroxide particles. Equipment Design and Operations. The experimental system was designed to contain p-xylene so that none was lost to volatilization or adsorption. As shown in Figure 1,p-xylene-saturated water with a pure p-xylene layer was contained in a glass bottle. Nitrogen gas entered
through the top of the bottle and induced solution flow. The saturated solution a t approximately 200 ppm was diluted to 7 ppm. PTFE tubing was used in order to reduce the amount of p-xylene adsorbed in the tubing. The water feed was sparged with nitrogen to keep the dissohd oxygen concentration below 1 ppm. The ferrous iron solution was made using ferrous ammonium sulfate hexahydrate and was stored in a separate system to minimize iron oxidation. In all testa, the total flow to the column shown in Figure 2 was 3 mL/min or 9 ft/day. Samples were obtained using a syringe with a female luer lock end and placed in an 8-mL sample vial with no head space to minimize volatilization. The dissolved oxygen was measured on-line as shown in Figure 2. Gas that accumulated at the top of the column due to microbubble breakthroughs was removed and recorded as vent losses. Operating Procedure. A 7 ppm solution of p-xylene was fed to the 7 cm diameter vertical column at 3 mL/min. Once the steady state was reached, as evidenced by the effluentp-xylene concentration being equal to the influent concentration, a 7-day-old Pseudomonas putida culture was injected into the column followed by 192 mL of microbubbles (125 mg of oxygen) through the injector port shown in Figure 2. The bacteria culture had been obtained from a spill site and acclimated to p-xylene. Two biodegradable surfactants, NaDBS and Tergitol 15-5-12 (Swisher, 19791, a t a concentration of 150 ppm were used to make the microbubbles in a spinning-disk generator. This was the last microorganisminjection into the column for the 3-month experiment. During the experiment, measurements were taken 4 times a day, which included influent and effluent dissolved oxygen, pressure drop using manometers, flow rate, and sampling for p-xylene analysis. p-xylene analysis was performed on a Hewlett-Packard 5710A direct injection gas chromatograph. Microbubbles were injected once the
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L
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v3
M
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Figure 2. Vertical column test cell.
oxygen supply from the previous injection was exhausted. The criterion for exhaustion was either an effluent xylene concentration above 1ppm or an effluent DO concentration less than 0.1 ppm. To fulfill the second objective, to study iron oxidation, water containing 11ppm ferrous iron and 6 ppm p-xylene was fed to the column at a total flow rate of 3 mL/min. Ferrous iron analysis was included in the daily measurements. Ferric hydroxide precipitate was observed in the column only where oxygen microbubbles were located. This length was approximately 9 cm and was assumed to be the length of the biodegradation zone. The residence time in this zone is approximately 45 min. To fulfill the third objective, an evaluation of iron flushing, the air-bubbling section was added in the pea gravel zone shown in Figure 2. Ferrous iron and xylene concentrations were monitored before (SP1) and after (SP2) the air-sparging zone and leaving the column (SP3). To flush the ferric hydroxide precipitate from the column, the water flow was stopped, and approximately 4 L of back-washing water was introduced through manometer 3. The back-washing water exited through valve V6 at the top of the column, as shown in Figure 2. Air was also bubbled through prior to back-washing to determine its effectiveness in loosening the ferric hydroxide. Pressure drops across the column were recorded after each back-wash.
Results Run 1. Throughout the first run, which evaluated p-xylene degradation, the xylene concentration dropped to below detectable limits immediately following a microbubble injection and remained low until the oxygen supply was depleted as shown in Figure 3. This trend continued throughout the experiment.
Figure 4 shows how the intermittently injected oxygen microbubbles were utilized in run 1 by using a material balance around the column. The difference in the influent DO (SP1)and effluent DO (SP3) was the amount of oxygen dissolved into the flowing groundwater. The difference between the xylene concentrations entering and leaving the column was assumed to have been the result of biodegradation. Stoichiometrically, oxidation of 1ppm of xylene requires 3.2 ppm of oxygen. This factor was used in determining the amount of oxygen contributing to biodegradation. If some of the xylene contributes to the formation of new cell matter, then less oxygen is required. Overall, 69.9% of the injected oxygen was used for xylene biodegradation. An additional 14% of the injected oxygen was depleted, assuming the surfactant used for generating the microbubbles was biodegraded. Vent losses were 4.6% of the injected oxygen. The pressure drop over time in the biodegradation zone (M3-M5) is plotted in Figure 5. These readings were taken prior to a microbubble injection when the microbubbles were depleted, so that the pressure drop represents only the amount caused by biomass accumulation. Figure 5 shows that the pressure increased up to the 6th day, stayed relatively constant for about 4 days, and then declined. As the biomass increased, the velocity and pressure drop also increased. It could be that the biofilm could only build up so thick until the high velocity caused the biofilm to shear off (Cunningham et al., 1991). Similar results were obtained by Cunningham et al. (1991). If biomass accumulation during in situ biodegradation were to cause plugging,then this approach would not be feasible because groundwater flow through the biodegradation area would be restricted and would prevent contaminants from reaching the microorganisms.
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m
8
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0 INFLUENT + Figure 3. Xylene concentration as a function of time, run 1.
EFFLUENT
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1 100%
UNACCOUNTED LOSSES 93.6% FREE 02 VENTED 8 9% 02 UTILIZED TO BIODEGRADE SURFACTANT
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ua
2t s e 02 UTILIZED TO
BIODEGRADE DISSOLVED XYLENE
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-.
....... __
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. . . . . . . . . . . . . . . . .,
90 25
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247.50
161.40
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Figure 4. Oxygen material balance, run 1. The oxygen mass-transfer coefficients for this test ranged from 0.01 to 0.43h-1 with an average value of 0.24 h-1. The coefficient was highest immediately after a microbubble injection, and then it decreased steadily. The effluentp-xylene concentrationwas below detectable limits until the effluent DO declined and remained at 0.1 ppm for 1-2 h. It is speculated that when dissolved oxygen is readily available, both dissolved p-xylene in the groundwater and adsorbedp-xylene on the soil matrix and biofilm are biodegraded. However, once the dissolved oxygen is depleted, adsorption may become a temporary means for xylene removal and may delay breakthrough of dissolved xylene in the column effluent. These oxygen-transfer coefficientscorrespond to values obtained in previous work using a 9.5cm (3.75in.) diameter column, which ranged from 0.07 to 0.40 h-l (Michelsen et
0
INTRODUCED AS MICROBUBBLES
al., 1991). Mass-transfer coefficients for horizontal groundwater flow through the vertical slice test cell were considerably lower, ranging from 0.01 to 0.09 h-l. This could indicate that, in the field, oxygen transfer may not be as good aa in laboratory columns. In subsequent testing, oxygen microbubbles may need to be injected more frequently. However,the contaminated groundwater will have a residence time in the biodegradation zone which is much greater than the 45 min in the vertical column, so that the microorganismsmay be able to degrade a similar or greater concentration of xylene. Run 2. The xylene and ferrous iron concentrations are plotted against time in Figure 6. A portion of the run is plotted here to illustrate the concentration profile. Once the oxygen was depleted, the iron concentration started to increase before xylene appeared in the effluent. The
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READINGS TAKEN PRIOR TO MICROBUBBLE I N J E C T I O N 1
1
90 25
I
233 25
157 91 TIME (HOURS)
Figure 5. Pressure drop over biozone as a function of time, run 1.
W 0 0 U
z
17 83
41.83
60.83
81 25
103.83
TIME (HOURS) +
EFFLUENT XYLENE
INFLUENT XYLENE
INF FERROUS IRON
EFF FERROUS IRON
MICROBUBBLE INJECTION
Figure 6. Xylene and ferrous iron concentrations, run 2. rest of the experimental run (hours 100-400)shows the same trend. These resulta indicate that this concentration of iron does not inhibit p-xylene biodegradation as previously observed in glucose biodegradation testa (Achanta, 1991). The oxygen material balance is illustrated in Figure 7. The ferrous iron oxidation was calculated by assuming that the oxidation of 7 ppm ferrous iron requires 1 ppm oxygen. In this test, more microbubbles were breaking through the fine sand layer above the biodegradation zone and rising to the top of the column. This volume of oxygen was 8.79% of the injected oxygen. In this run,13.3% of the oxygen was an unaccounted loss. The pressure drops, reported in Figure 8, were again measured just prior to microbubble injection. The head loss showed no sustained increase,which is promising since
a large increase in pressure drop was expected with ferrous iron in the feed. The mass-transfer coefficients for the test ranged from 0.12 to 0.49 h-’ with an average of 0.29 h-l. These values are similar to those calculated for run 1. Run 3. The third objective in the research was to oxidize the ferrous iron before it entered the biodegradation zone in an attempt to remove the ferric precipitate from the sand matrix. An air-sparging zone was added to the top part of the column. In this test, only 59 9% of the oxygen was accounted for. Vent losses could not be determined since there was head space at the top of the column due to the air-sparging recycle system. The oxygen mass-transfer coefficientsfor this run ranged from 0.006 to 0.3 h-l with an average of
Bbtechnol. Prog.., 1993, Vol. Q, No. 4 100% UNACCOUNTED LOSSES
88 7% 2
FREE 02 VENTED 80% 02 U T I L I Z E D TO BIODEGRADE SURFACTANT 6 6%
02 U T I L I Z E D FOR BIODEGRADING XYLENE AND OXIDIZING FERROUS TO FERRIC IRON (FERROUS IRON OXIDATION CONSUMES 38% OF THE TOTAL INJECTED OXYGEN)
8 6% 02 DISSOLVED INTO FLOWING GROUNDWATER
0
71 33
0.00
165.83
263 50
390.80
TIME (HOURS)
0
02 U T I L I Z E D
+
02 DISSOLVED
0
INTRODUCED AS MICROBUBBLES
Figure 7. Oxygen material balance, run 2. 84
,
I
82 80 78 76 n
74
0
I 111 E "
72 70
0 a
68 W
CL 3 v)
W CL Q
66 64 62 60 58 56 54
52 10.33
80.83
152.83
228.83
318.83
TIME (HOURS)
Figure 8. Pressure drop over biozone as a function of time, run 2.
0.17 h-1. These coefficients are lower than the values calculated in runs 1 and 2. The main difference in this run was that the microorganisms were degrading an average of 3.4 ppm xylene due to stripping losses, whereas in runs 1 and 2 the microorganisms were degrading 6-7 ppm. The lower oxygen transfer could be due, in part, to an error in the DO meter. It could also be that a portion of the xylene is contributing to new cell matter, so that less oxygen would be required for biodegradation. The head loss over the biodegradation zone (M3-M5) for this run shows variations from 2.54 to 18.7cm (1-7in.) of water, which would indicate that the ferric precipitate is not causing a steady increase in pressure drop as was observed in previous research (Achanta, 1991). The variable pressure drop may have resulted from ferric
precipitate particles partially plugging the soil matrix and then later dislodging. The air-sparging zone was stripping as much as 4 ppm p-xylene out of the influent stream, which had an average concentration of 5.7 ppm, and oxidizing approximately 1 ppm ferrous iron. These results indicate that the airsparging zone succeeded in stripping xylene and not oxidizing iron. According to S t u " and Lee (1961),the oxidation of iron could take an hour at a pH of 7. In these experiments, the ferrous iron was not in contact with oxygen for an hour. Therefore, a better air delivery system would improve iron precipitation. Smaller bubbles have a lower rise velocity, which would increase the contact time between the air bubbles and groundwater which should increase oxygen transfer and the amount of iron oxidation. However, the increased contact time may also increase
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xylene volatilization, which could require treatment of the off-gases. In back-washingthe air-spargingzone, it was determined that a flow rate a t approximately the fluidization velocity (32.5 cm/min) and a large volume of water (4 L)or alternate air and water flow would be needed to reduce the pressure drop, which may not be practical in the field.
Conclusions The results of this study show that microbubble injection is a promising technology for in situ biodegradation. In these experiments, the bacteria degraded xylene to below detectable limita as long as sufficient oxygen was available. Pressure drops showed a slight increase over the first few days and then a gradual decline, which suggests that in situ biodegradation would not cause media plugging under the conditions of this test. Oxygen utilization was very efficientcompared to the findings of earlier studies. There was no interference in xylene biodegradation in the presence of ferrous iron, and the ferric precipitate did not cause an appreciable amount of plugging. The introduction of an air-sparging zone resulted in volatilization of the xylene and very little ferrous hydroxide precipitation. However,oxygen microbubblesprecipitated the ferrous iron better than air-sparging, without volatilizing the xylene. Oxygen microbubbles have proven to be an efficient and very effective mode of oxygen transfer. They precipitated ferrous iron while still supplying the microorganisms with oxygen.
Literature Cited Achanta, S. G. Iron Oxidation Coupled with Biodegradation of Organic Contaminants in a Simulated Ground Water System. M.S. Thesis, Virginia Polytechnic Institute and State University, Blacksburg, VA, 1991.
Cartwright, G. C. Limitations of Pump and Treat Technology. Pollut. Eng. 1991, 23, (12). Cunningham, A.; Characklis, W.; Abedeem, F.; Crawford, D. Influence of Biofilm Accumulation on Porous Media Hydrodynamics. Environ. Sci. Technol. 1991, 25, (7). Longe, T. A. Colloidal Gas Aphrons: Generation, Flow Characterization and Application in Soil and Groundwater Decontamination. Ph.D. Dissertation, Virginia Polytechnic Institute and State University, Blacksburg, VA, 1989. Lotfi, M. Application of Oxygen Microbubbles in Groundwater Oxygenation to Enhance Biodegradation of Hydrocarbons in Soil Systems. M.S. Thesis, Virginia Polytechnic Institute and State University, Blacksburg, VA, 1990. Michelsen, D. L.; Lotfi, M. Oxygen Microbubble Injection for In Situ Bioremediation: Possible Field Scenario. In Innovative Hazardous Waste Treatment Technology Series; Freeman, J., Sferra, P., Eds.; Technomic: Lancaster, PA, 1991; pp 131142. Michelsen, D. L.; Lotfi, M.; Velander, W. H.; Mann, J. W.; Khalichi, P. Oxygen Mass Transfer to Flowing Ground Water Using Oxygen Microbubbles. Presented at the 2nd International Symposium on Gas Transfer at Water Surfaces, ASCE, 1991. Pettyjohn, W. A.; Studlick, J.; Bain, R.; L e k , J. A. GroundWater Quality Atlas of the United States; National Water Well Association for National Demonstration Water Project, National Water Well Association: Dublin, OH, 1979. Sebba, F. Foams and Bilipuid Foam-Aphrons; John Wiley and Sons: New York, 1987; pp 64-65. Stumm, W.; Lee, G. Oxygenation of Ferrous Iron. Ind. Eng. Chem. 1961,53,143-146. Swisher, R. D. Surfactant Biodegradation, 2nd ed.; Marcel Dekker: New York, 1987. Thayer, A. Bioremediation: Innovative Technology for Cleaning Up Hazardous Waste. Chem. Eng. News 1991, Aug. 26. Treybal, R. Mass Transfer Operations, 3rd ed.; McGraw-Hill: New York, 1980; pp 305-310. Accepted February 18, 1993.
