Bedessem, James M. "In Situ Air Sparging" In Situ Treatment Technology Boca Raton: CRC Press LLC,2001 ©2001 CRC Press LLC CHAPTER 5 In Situ Air Sparging James M. Bedessem CONTENTS Introduction Governing Phenomena In Situ Air Stripping Direct Volatilization Biodegradation Applicability Examples of Contaminant Applicability Geological Considerations Description of the Process Air Injection into Water-Saturated Soils Mounding of Water Table Distribution of Airflow Pathways Groundwater Mixing System Design Parameters Air Distribution (Zone of Influence) Depth of Air Injection Air Injection Pressure and Flow Rate Injection Mode (Pulsing and Continuous) Injection Well Construction Contaminant Type and Distribution Pilot Testing Monitoring Considerations Process Equipment Air Compressor or Air Blower Other Equipment ©2001 CRC Press LLC Modifications to Conventional Air Sparging Application Horizontal Trench Sparging In Well Air Sparging Bio-Sparging Vapor Recovery via Trenches Pneumatic Fracturing for Vapor Recovery Cleanup Rates Limitations Design Example Problem Solution Pilot Test Planning Conducting the Pilot Test Evaluating the Data References INTRODUCTION In situ air sparging is a remediation technique which has been used since about 1985, with varying success, for the remediation of volatile organic compounds (VOCs) dissolved in the groundwater, sorbed to the saturated zone soils, and trapped in soil pores of the saturated zone. This technology is often used in conjunction with vacuum extraction systems (Figure 1) to remove the stripped contaminants, and has broad appeal due to its projected low costs relative to conventional approaches. Figure 1 Air sparging process schematic. ©2001 CRC Press LLC The difficulties encountered in modeling and monitoring the multiphase air sparging process (i.e., air injection into water saturated conditions) have contributed to the current uncertainties regarding the processes responsible for removing the contaminants from the saturated zone. Engineering design of these systems, even today, is largely dependent on empirical knowledge. At this point, the air sparging process should be treated as a rapidly evolving technology with a need for contin- uous refinement of optimal system design and mass transfer efficiencies. The mass transfer mechanisms during in situ air sparging relies on the interactions between complex physical, chemical, and microbial processes, many of which are not well understood. A typical air sparging system has one or more subsurface points through which air is injected into the saturated zone. When this technology was first emerging, it was commonly perceived that the injected air traveled up through the saturated zone in the form of air bubbles (Angell 1992, Brown 1992a, Brown 1992b, and Sellers and Schreiber 1992); however, it is more realistic that the air travels in the form of continuous air channels (Johnson et al. 1993, Wei et al. 1993, and Ardito and Billings 1990). While the airflow path will be influenced by the pressure and flow rate of injected air and depth of injection, the structuring and stratification of the saturated zone soils appear to be the predominant factors (Johnson et al. 1993, Wei et al. 1993, and Ardito and Billings 1990). Significant channeling may result from relatively subtle permeability changes, and the degree of channeling will increase as the size of the pore throats get smaller. Research (Wei et al. 1993) shows that even minor differences in permeability due to stratification can impact the sparging effectiveness. In addition to conventional air sparging where air is injected as shown in Figure 1, many modifications of the technique to overcome geologic/hydrogeologic limi- tations will also be discussed in this chapter. GOVERNING PHENOMENA In situ air sparging is potentially applicable when volatile and/or easily aerobi- cally biodegradable organic contaminants are present in water-saturated zones, under relatively permeable conditions. The in situ air sparging process can be defined as injection of compressed air at controlled pressures and volumes into water-saturated soils. The three primary contaminant mass removal mechanisms that occur during the operation of air sparging systems include, (1) in situ stripping of dissolved VOCs; (2) volatilization of trapped and adsorbed phase contamination present below the water table and in the capillary fringe; and (3) aerobic biodegradation of both dissolved and adsorbed phase contaminants resulting from delivery of oxygen. It was determined during in situ air sparging of petroleum hydrocarbon sites that stripping and volatilization account for much more of the hydrocarbon mass removal in the initial weeks/months of operation than does biodegradation. Bio- degradation becomes more significant for mass removal only during long-term system operation. ©2001 CRC Press LLC In Situ Air Stripping Among the three contaminant mass removal mechanisms mentioned above, in situ air stripping may be the dominant process for some dissolved contaminants. The ability of a dissolved contaminant to be removed by air sparging through a stripping mechanism is a function of its Henry’s Law constant (vapor pressure/sol- ubility). Compounds such as benzene, toluene, xylene, ethylbenzene, trichloroeth- ylene, and tetrachloroethylene are considered to be very easily strippable (see Chap- ter 3 for a discussion of Henry’s Law constants). However, a basic assumption made in analyzing the air stripping phenomenon during air sparging is that Henry’s Law applies to the volatile contaminants, and that all the contaminated water is in close communication with the injected air. In depth evaluation of these assumptions exposes the shortcomings and complexities of interphase mass transfer during air sparging. First of all, Henry’s Law is valid only when partitioning of dissolved contaminant mass has reached equilibrium at the air/water interface. However, the residence time of air, traveling in discrete channels, may be too short to achieve the equilibrium due to the high air velocities and short travel paths. Another issue is the validity of the assumption that the contaminant concentration at the air/water interface is the same as in the bulk water mass. Due to the removal of contaminants in the immediate vicinity of the air channels, it is safer to assume that contaminant concentrations are going to be lower immediately around the channels than away from the channels. To replenish the mass lost from the water around the air channels, mass transfer by diffusion and convection must occur from water away from the air channels. There- fore, it is likely that the density of air channels plays a significant role in mass removal and that mass transfer efficiencies increase as the distance between air channels decreases. In addition, the density of air channels will also influence the interfacial surface area available for mass transfer. This mass transfer limitation may also prevent this technology from reaching final cleanup criteria as discussed in Chapters 1 and 2. The literature suggests that the air channels formed during air sparging mimic a viscous fingering effect, and that two types of air channels are formed: large-scale channels and pore-scale channels (Clayton, Brown, and Bass 1995). The formation of both types of channels enhances the channel density and the available interfacial surface area. It has been proposed that in situ air sparging also helps to increase the rate of dissolution of the sorbed phase contamination and eventual stripping below the water table. This is due to the enhanced dissolution caused by increased mixing, and the higher concentration gradient between the sorbed and dissolved phases under sparg- ing conditions. Direct Volatilization The primary mass removal mechanism for VOCs present in the saturated zone during pump and treat operations is resolubilization into the aqueous phase and the ©2001 CRC Press LLC eventual removal with the extracted groundwater. During in situ air sparging, direct volatilization of the sorbed and trapped contaminants is enhanced in the zones where airflow takes place. The volatile compounds do not have to transfer through the water to reach the air. If an air channel intersects pure compound, direct volatilization can occur. Direct volatilization of any compound is governed by its vapor pressure. Most volatile organic compounds are easily removed through volatilization. The schematic presented in Figure 1 includes air channels or bubbles moving through an aquifer containing sorbed or trapped NAPL contamination. In the regions where the soil is predominantly air saturated or the air channel is next to the zone of trapped contamination, the process is similar to soil vapor extraction or bioventing, albeit on a microscopic scale. Where significant levels of residual contamination of VOCs or NAPLs are present in the saturated zone, direct volatilization into the vapor phase may become the dominant mechanism for mass removal where air is flowing. The high level of mass that the air can carry, combined with the rapid exchange of pore volumes, results in a process that can remove significant pounds of contaminants in a relatively short period of time. This may explain the significant increase in VOC concentrations typically observed in the soil vapor extraction effluents at many sites (Geraghty & Miller 1995). Biodegradation In most natural situations, aerobic biodegradation of biodegradable compounds in the saturated zone is rate limited by the availability of oxygen. Biodegradability of any compound under aerobic conditions is dependent on its chemical structure and environmental parameters such as pH and temperature. Some VOCs are considered to be easily biodegradable under aerobic conditions (e.g., benzene, toluene, acetone, etc.) and some of them are not (e.g., trichloroethylene and tetrachloroethylene). Typical dissolved oxygen (DO) concentrations in uncontaminated groundwater are less than 4.0 mg/l, and under anaerobic conditions induced by the natural degradation of the contaminants, are often less than 0.5 mg/l. DO levels can be raised by air sparging up to 6 to 10 mg/l under equilibrium conditions (Brown 1992a, Geraghty & Miller 1995, and Brown, Herman, and Henry 1991). An increase in the DO level will contribute to enhanced rates of aerobic biodegradation in the saturated zone. This method of introducing oxygen to increase the DO level is one of the inherent advantages of in situ air sparging. However, the oxygen transfer into the bulk water is a diffusion limited process. The diffusion path lengths for transport of oxygen through the groundwater are defined by the distances between air channels. Where channel spacing is large, diffusion alone is not sufficient to transport oxygen into all areas of the aquifer for enhanced biodegradation. The pore-scale channels formed and the induced mixing during air sparging enhance the rate of oxygen transfer (Clayton, Brown, and Bass 1995). The specific costs and methodology of enhanced biodegradation will be discussed in Chapters 7 and 8. ©2001 CRC Press LLC APPLICABILITY Examples of Contaminant Applicability Based on the discussion in the previous section, Table 1 describes the applica- bility of air sparging for a few selected contaminants in terms of the contaminant properties of strippability, volatility, and aerobic biodegradability. For air sparging to be effective, the VOCs must transfer from the groundwater or from the saturated zone into the injected air, and oxygen present in the injected air must transfer into the groundwater to stimulate biodegradation. In practice, the criterion for defining strippability is based on the Henry’s Law constant being greater than 1x10 -5 atm-m 3 /mole. In general, compounds with a vapor pressure greater than 0.5 to 1.0 mm Hg can be volatilized easily; however, the degree of volatilization is limited by the flow rate of air. The half lives presented in Table 1 are estimates in groundwater under natural conditions without any enhancements to improve the rate of degradation (enhancements are discussed in Chapter 8, Reac- tive Zone Remediation). Many constituents present in heavier petroleum products such as No. 6 fuel oil will not be amenable to either stripping or volatilization (Figure 2). Hence, the primary mode of remediation, if successful, will be due to aerobic biodegradation. Required air injection rates under such conditions will be influenced only by the requirement to introduce sufficient oxygen into the saturated zone. Figure 2 qualitatively describes different mass removal phenomena in a simpli- fied version under optimum field conditions. The amounts of mass removed by stripping and volatilization have been grouped together, due to the difficulty in separating them in a meaningful manner. However, the emphasis should be placed on total mass removal, particularly of mobile volatile constituents, and closure of the site regardless of the mass transfer mechanisms. Table 1 A Few Examples of Contaminant Applicability for In Situ Air Sparging Contaminant Strippability Volatility Aerobic* Biodegradability Benzene High (H = 5.5 x 10 -3 ) High (V P = 95.2) High (t 1/2 = 240) Toluene High (H = 6.6 x 10 -3 ) High (V P = 28.4) High (t 1/2 = 168) Xylenes High (H = 5.1 x 10 -3 ) High (V P = 6.6) High (t 1/2 = 336) Ethylbenzene High (H = 8.7 x 10 -3 ) High (V P = 9.5) High (t 1/2 = 144) TCE High (H = 10.0 x 10 -3 ) High (V P = 60) Very low (t 1/2 = 7,704) PCE High (H = 8.3 x 10 -3 ) High (V P = 14.3) Very low (t 1/2 = 8,640) Gasoline Constituents High High High Fuel Oil Constituents Low Very low Moderate where H = Henry's Law constant (atm-m 3 /mol); V P = Vapor pressure (mm Hg) at 20°C; t 1/2 = Half life during aerobic biodegradation, hours; and * = It should be noted that the half lives can be very dependent on the site specific subsurface environmental conditions. ©2001 CRC Press LLC Geological Considerations Successful implementation of in situ air sparging is greatly influenced by the ability to achieve significant air distribution within the target zone. Good vertical pneumatic conductivity is essential to avoid bypassing or channeling of injected air horizontally, away from the sparge point. It is not an easy task to evaluate the pneumatic conductivities in the horizontal and vertical direction for every site con- sidered for in situ air sparging. Geologic characteristics of a site are important when considering the applica- bility of in situ air sparging. The most important geologic characteristic is stratio- graphic homogeneity or heterogeneity. The presence of lower permeability layers under stratified geologic conditions will impede the vertical passage of injected air. Laboratory-scale studies have illustrated the impact of geologic characteristics on air channel distribution (Wei et al. 1993). Under laboratory conditions, injected air was shown to accumulate below the lower permeability layers and travel in a horizontal direction. In field application, this condition may have the potential to enlarge the contaminant plume (Figure 3). High permeability layers may also cause the air to preferentially travel laterally, again potentially causing an enlargement of the plume (Figure 3). Horizontal migration of injected air limits the volume of soils that can be treated by direct volatilization due to the inability to capture the stripped contaminants. Horizontal migration can also cause safety hazards if hydro- carbon vapors migrate into confined spaces such as basements and utilities. Hence, homogeneous geologic conditions are essential for the success and safety of in situ air sparging. Figure 2 Qualitative presentation of potential air sparging mass removal for petroleum compounds. ©2001 CRC Press LLC Both vertical pneumatic conductivity and the ratio of vertical to horizontal permeability decrease with decreasing average particle size of the sediments in the saturated zone. The reduction of vertical permeability is directly proportional to the effective porosity and average grain size of the sediments (Bohler, Hotzl, and Nahold 1990). Hence, based on the empirical information available, it is recommended that application of in situ air sparging be limited to saturated zone conditions where the hydraulic conductivities are greater than 10 -3 cm/sec (Johnson et al. 1993, and USEPA 1993). It is unlikely that homogeneous geologic conditions across the entire cross section will be encountered at most sites. The optimum geologic conditions for air sparging may be where the permeability increases with increasing elevation above the point of air injection. Decreasing permeabilities with elevation above the point of air injection will have the potential to enlarge the plume due to lateral movement of injected air. DESCRIPTION OF THE PROCESS Air Injection into Water-Saturated Soils The ability to predict the performance of air sparging systems is limited by the current understanding of airflow in the water-saturated zone and limited performance data. There are two schools of thought in the literature describing this phenomenon. The first, and the widely accepted one, describes that the injected air travels in the vertical direction in the form of discrete air channels. The second describes that the injected air travels in the form of air bubbles. Airflow mechanisms cannot be directly Figure 3 Potential situations for the enlargement of a containment plume during air sparging. ©2001 CRC Press LLC observed in the field; however, conclusions can be reached by circumstantial evi- dence collected at various sites and from laboratory-scale visualization studies. Sandbox model studies performed (Wei et al. 1993 and Johnson 1995) tend to favor the air channels concept over the air bubbles concept. In laboratory studies simulating sandy aquifers (grain sizes of 0.075 to 2 mm) stable air channels were established in the medium at low injection rates, whereas, under conditions simulating coarse gravel (grain sizes of 2 mm or larger), the injected air rose in the form of bubbles. At high air injection rates in sandy, shallow, water table aquifers, the possibility for fluidization (loss of soil cohesion) around the point of injection exists (Johnson et al. 1993, and Johnson 1995), and thus the loss of control of the injected air may occur. Mounding of Water Table When air is injected into the saturated zone, groundwater must necessarily be displaced. The displacement of groundwater will have both a vertical and lateral component. The vertical component will cause a local rise in the water table, sometimes called water table mounding. Mounding has been used by some as an indicator of the radius of influence of the sparge well during the early stages of development of this technology (Brown 1992a, Brown 1992b, Brown, Herman, and Henry 1991, Kresge and Dacey 1991, and Boersma, Diontek, and Newman 1995). Mounding is also considered to be a design concern because it represents a driving force for lateral movement of groundwater and dissolved contaminants and can therefore lead to spreading of the plume. The magnitude of mounding depends on the site conditions and the location of the observation wells relative to the sparge well. Mounding can vary from a negligible amount to several feet in magnitude. Simulations of the flow of air and water around an air sparging well were performed with a multiphase, multicomponent simulator (TETRAD) originally developed for the study of problems encountered during exploration of petroleum and geothermal resources (Lundegard and Anderson 1993, and Lundegard 1995). The simulations were performed by defining two primary phases of transient behav- ior that lead to a steady state flow pattern (Figures 4 and 5). The first phase is characterized by an expansion in the region of airflow (Figure 4). During this phase, the rate of air injection into the saturated zone exceeds the rate of airflow through the saturated zone into the vadose zone. It is during this transient expansion phase that groundwater mounding first develops and reaches its highest level. The ground- water mound during this phase extends from near the injection well to beyond the region of airflow in the saturated zone. When injected air breaks through to the vadose zone, the region of airflow in the saturated zone begins to collapse or shrink (Figure 5). During this second transient phase of behavior, the preferred pathways of higher air permeability from the point of injection to the vadose zone are estab- lished. The air distribution zone shrinks until the rate of air leakage to the vadose zone equals the rate of air injection. During this collapse phase, mounding near the sparge well dissipates. When steady state conditions are reached, little or no mound- ing exists. This behavioral pattern has also been observed in the field (Johnson et al. 1993, Boersma, Diontek, and Newman 1995, and Lundegard 1995). The time- frame over which these phases occur is dependent on geology. At Port Hueneme in [...]... Advantages ©2001 CRC Press LLC Piston air-cooled Piston water-cooled Diaphragm 1/2 - 50 0 10 - 50 0 10 - 200 10 - 250 10 - 250 10 - 250 Efficient, light-weight Efficient, heavy-duty No seal, contamination-free Rotary Nonpositive displacement compressors Reciprocating Sliding vane Screw (helix) Lobe, low-pressure Lobe, high-pressure 10 - 50 0 10 - 50 0 15 - 200 7 1/ 2- 200 10 10 5 20 Compact, Pulseless Compact, Compact,... peripheral blower 50 - 50 0 1,000 - 10,000 1/4 - 20 - 150 150 40 250 40 - 250 400 - 50 0 1 -5 high-speed delivery oil-free high-speed Compact, oil-free, high-speed High volume, high speed Compact, oil-free, high volume Figure 13 Air compressor characteristics Horizontal Trench Sparging Trench sparging was developed to apply air sparging under less permeable geologic conditions when the depth of contamination is... difficulties of installing trenches, described in the previous section Figure 15 In well air sparging The injection of air into the inner casing (Figure 15) induces an air lifting effect that is limited only to the inner casing The water column inside the inner casing will be lifted upwards (in other words water present inside the inner casing will be pumped) and will flow over the top of the inner casing as... any, regarding the implementation of this technology Since vapor extraction is a complimentary technology to in situ air sparging, pilot testing of the integrated system is highly recommended Short-term pilot tests play a key role in the selection and design of in situ air sparging systems Most conventional pilot tests are less than 24 to 48 hours in duration and consist of monitoring changes in: ©2001... PARAMETERS In the absence of any reliable models for the in situ air sparging process, empirical approaches are used in the system design process The parameters that are of significant importance in designing an in situ air sparging system are listed below: • • • • • • Air distribution (zone of in uence) Depth of air injection Air injection pressure and flow rate Injection mode (pulsing or continuous) Injection... observed increase in DO levels in monitoring wells is due to the changes in the bulk water Direct introduction of air into the monitoring wells due to an air channel being intercepted could also be a reason for increased DO levels in monitoring wells Chapters 7 and 8, In Situ Bioremediation and Reactive Zone Remediation, provide more details of the designs, operations, and associated costs for biosparging... to demonstrate the mass removal efficiency of the in situ air sparging process This can be determined by measuring the net increase in contaminant levels in the vapor extraction system after the initiation of the air sparging system To evaluate the net increase in contaminant levels in the effluent, the field test should be conducted as a sequential test in two phases The first phase should be to perform... air channels into the water surrounding the channels It is estimated that only 0 .5 percent of the oxygen present in the injected air will be transferred into the dissolved phase during air sparging (Johnson 19 95, Geraghty & Miller 19 95, and Boersma, Diontek, and Newman 19 95) Therefore, caution must be exercised in terms of evaluating the changes in DO levels after the initiation of biosparging It is common... levels under pseudo-steady state conditions, which can generally be accomplished after removing 1 .5 to 2 pore volumes from the unsaturated zone The air sparging is initiated during the second phase with continued monitoring of the contaminant levels in the vapor extraction system air stream An increase in the contaminant level and the duration of increase would indicate the short-term mass removal... casing as shown in Figure 15 As a result, contaminated water will be drawn into the lower screen from the surrounding formation and will be continuously air lifted in the inner tube Due to the mixing of air and contaminated water, as the air/water mixture rises inside the inner tube, strippable VOCs will be air stripped and captured for treatment ©2001 CRC Press LLC as shown in Figure 15 Treated, clean . " ;In Situ Air Sparging" In Situ Treatment Technology Boca Raton: CRC Press LLC,2001 ©2001 CRC Press LLC CHAPTER 5 In Situ Air Sparging James M. Bedessem CONTENTS Introduction Governing. Test Planning Conducting the Pilot Test Evaluating the Data References INTRODUCTION In situ air sparging is a remediation technique which has been used since about 19 85, with varying success,. discussed in this chapter. GOVERNING PHENOMENA In situ air sparging is potentially applicable when volatile and/or easily aerobi- cally biodegradable organic contaminants are present in water-saturated