Abstract:
Bioventing methods create a bacterial treatment zone at a contaminated site by supplying a hydrocarbon food source to the treatment zone, and recirculating the hydrocarbon to the treatment zone. The bioventing methods may inject, circulate, extract and reinject hydrocarbons such as butane or other alkanes to the subsurface at a contaminated site to create a bacterial treatment zone. Contaminated vapors extracted from the soil and/or groundwater may be reintroduced into the site. Hydrocarbons that are not consumed by the bacteria in the treatment zone may be extracted and recovered for recirculation into the treatment zone.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a divisional of U.S. patent application Ser. No. 10/641,736 filed Aug. 15, 2003, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/403,934 filed Aug. 16, 2002. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to remediation of sites contaminated with pollutants such as petroleum pollutants, chlorinated solvents and the like. More particularly, the invention relates to bioventing methods for remediating such sites.  
       BACKGROUND INFORMATION  
       [0003]     Gasoline and other volatile organic compounds such as chlorinated aliphatic hydrocarbons released into the subsurface may become distributed into different phases such as an adsorbed phase (on soil surface), a vapor/volatilization phase (in soil gas), a dissolved phase (in groundwater) and a free phase (e.g., a pure petroleum or chemical product floating on the groundwater table as a light non-aqueous phase liquid (LNAPL) or sinking below the water table as dense non-aqueous phase liquid (DNAPL).  
         [0004]     Soil vapor extraction (SVE) is a physical means of removing or reducing concentrations of volatile organic compounds (VOCs) that partition into the vapor phase. SVE technology was developed to remove volatiles from the subsurface. This technology targets the adsorbed, vapor and NAPL phases of the VOCs present in the unsaturated (vadose) portion of the subsurface. Dissolved-phase VOCs found beneath the groundwater table is not directly addressed by using an SVE system.  
         [0005]     Remediation by SVE involves applying a vacuum to soils in the unsaturated zone above the water table in order to induce airflow. Contaminated mass removal is achieved by drawing contaminant-free air into the soil void spaces. The contaminant-free air creates a concentration gradient and the compounds diffuse into the air stream. This VOC-laden air is continuously extracted and replaced with contaminant-free air. An additional benefit of SVE is the continuous flow of oxygen into the area where hydrocarbons are adsorbed on the soil. This continuous oxygen supply enhances the biodegradation of the hydrocarbons within the soil matrix.  
         [0006]     A typical SVE system consists of one or more vapor extraction wells strategically located. The SVE wells can be placed vertically or horizontally, depending on depth to groundwater and other site-specific characteristics. The piping system is commonly placed underground, primarily to provide extra protection from accidental damage. The piping system usually ends at a common header pipe, which is connected to a blower or a pump depending on the flow and vacuum desired. An air/water separator and or filter is required prior to the vacuum pump in order to protect equipment from moisture and particulates drawn into the system. Discharge from the blower or vacuum pump is either vented to the atmosphere or connected to an off-gas treatment system, depending upon emissions requirements and the nature and extent of VOCs.  
         [0007]     SVE alone is not effective for removing heavier material such as diesel fuel, jet fuel or fuel oils, because of the nonvolatile high-molecular weight fractions they contain. Venting techniques have been developed which utilize SVE hardware and vertical piping as a means of introducing or injecting and reinjecting air (oxygen) into the treatment zone. Such venting techniques may be appropriate when the water table is deep and the contaminant has not reached the groundwater.  
         [0008]     In-situ air sparging, also known as in-situ air stripping or in-situ volatilization, is a technology utilized to remove VOCs from the subsurface saturated zone. In-situ air sparging, when utilized with an SVE system, may greatly extend the utility of SVE to the saturated zone. Air sparging is a process in which contaminant-free air is injected under pressure (sparged) below the water table of an impacted aquifer system. In air sparging applications, the air injection pressure is the sum of the hydrostatic pressure (also known as breakout pressure), which is a function of submersion depth of the air sparging point, and the air entry pressure of the geologic formation, a function of capillary resistance to pore water displacement.  
         [0009]     Volatile compounds exposed to the injected air are transferred to the vapor phase, similar to air stripping. Once captured by an SVE system, the VOC-laden air is transferred to a subsequent emissions treatment system. Air sparging systems must operate in tandem with SVE systems intended to capture this VOC-laden air stream. Implementing an air sparging system without an SVE system can potentially create a net positive pressure in the subsurface, inducing groundwater migration into areas previously less affected by dissolved-phase VOCs. Air sparging systems may also add oxygen to the groundwater, thus accelerating the natural biological decay process.  
         [0010]     The primary mechanisms responsible for VOCs removal during operation of air sparging systems are believed to be in-situ stripping of dissolved-phase VOCs, volatilization of dissolved-phase and adsorbed-phase VOCs beneath the water table and in the capillary fringe, and aerobic biodegradation of both dissolved-phase and adsorbed-phase VOCs as a consequence of additional oxygen supplied by the injected air. When an air sparging system is optimized for stimulating biodegradation, it is sometimes referred to as biosparging. Typically biosparging systems are initially operated for volatilization and stripping. The system is then fine-tuned for enhancement of biodegradation.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention provides bioventing methods which create a bacterial treatment zone at a contaminated site by supplying a hydrocarbon food source to the treatment zone, and recirculating the hydrocarbon to the treatment zone. The bioventing methods may inject, circulate, extract and reinject hydrocarbons such as butane to the subsurface at a contaminated site to create a bacterial treatment zone. In one embodiment, contaminated vapors extracted from the soil and/or groundwater are reintroduced into the site. Hydrocarbons that are not consumed by the bacteria in the treatment zone may be extracted and recovered for recirculation into the treatment zone. Butane is a particularly preferred hydrocarbon food source which stimulates the growth of butane-utilizing bacteria. However, other hydrocarbons, such as other alkanes and the like, may be used as a bacterial food source in addition to butane or in place of butane.  
         [0012]     The bioventing method may be used to recirculate butane and/or other hydrocarbons at various locations such as in the unsaturated zone (above the water table), below the water table and/or at the capillary fringe (water table interface). The bioventing method may be used to remediate and restore contaminated soil and/or groundwater. When used to remediate soil alone, the butane may be reinjected above the water table. For arid or dry soils, it may be desirable to inject water in sufficient amounts to provide moisture to promote bacterial growth.  
         [0013]     An aspect of the present invention is to provide a method of remediating a contaminated site comprising introducing a hydrocarbon bacterial food source comprising at least 50 weight percent butane to a treatment zone of the site, extracting at least a portion of the hydrocarbon food source from the treatment zone, and recirculating at least a portion of the hydrocarbon food source to the treatment zone.  
         [0014]     These and other aspects of the present invention will be more apparent from the following description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is partially schematic elevation view of a bioventing system in accordance with an embodiment of the present invention.  
         [0016]      FIGS. 2 and 3  are graphs illustrating petroleum contaminant levels at a site treated with a bioventing system as shown in  FIG. 1 , before and after treatment with the bioventing system.  
         [0017]      FIG. 4  is a partially schematic plan view of a bioventing system in accordance with another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]      FIG. 1  schematically illustrates a bioventing system  10  in accordance with an embodiment of the present invention. The bioventing system  10  includes a supply of butane  12  or other hydrocarbon bacterial food source and an air intake  14  connected to a compressor  16 . Butane and air are fed to a mixing panel  18  which controls the flow of the butane and air to injection wells  20  in the treatment zone. Extraction wells  22  recover a portion of the butane as well as other liquids or gases from the treatment zone. A blower  24  is used to recirculate the recovered materials back to the treatment zone via recirculation injectors  26 .  
         [0019]     In one embodiment, the butane injection system may operate concurrently with an extraction system, such as a SVE system which is used to maintain a vacuum within vadose zone soils. Effluent from the SVE system may be piped back into the biobutane treatment zone, thus allowing vapor control while reducing operation costs by eliminating the need for carbon replacement or regeneration and by recycling the butane gas. The SVE system may further oxygenate the soil, resulting in enhanced microbial degradation of pollutants such as petroleum compounds. Butane injection may also be applied to traditional bioventing systems that are simply recirculating air into a treatment zone.  
         [0020]     The following examples illustrate various aspects of the invention, and are not intended to limit the scope of the invention.  
       EXAMPLE 1  
       [0021]     A butane injection panel was installed at a petroleum contaminated service station in Massachusetts. Three years prior to the installation, the site soil and groundwater were impacted by a release from a leak in a product line associated with a UST. The release also produced gasoline vapors that were detected in a building adjacent to the property. Drilling activities revealed the presence of VOCs in soil and contaminants on the water table. Initially, attempts were made to remediate the site by removing material via an ORS product recovery well and product recovery tank, as well as removal of 500 yards of contaminated soil, followed by installation of a conventional soil vapor extraction (SVE) system. The SVE system operated for 18 months and was shut down without successful remediation of the site. Subsequently, a biofeasibility study was conducted using site-specific groundwater. The data obtained from the study confirmed that butane-utilizing bacteria capable of effectively degrading the target pollutants existed at the site.  
         [0022]     A butane injection system was then combined with the existing SVE system, as illustrated in  FIG. 1 . The butane injection system may be similar to those described in U.S. Pat. Nos. 5,888,396, 6,051,130, 6110,372, 6,156,203, 6,210,579, 6,244,346 and 6,245,235, which are incorporated herein by reference. The butane injection system is used to stimulate the growth of butane-utilizing bacteria which degrade pollutants in the treatment zone. Examples of butane-utilizing bacteria are described in the aforementioned patents. The butane injection system may operate concurrently with the SVE system, which is used to maintain a vacuum within vadose zone soils to control potential migration of VOCs from the treatment area into adjacent buildings. The SVE system was converted to operate as a bioventing system to further oxygenate soils resulting in enhanced microbial degradation of petroleum compounds in the capillary fringe and vadose zone. The off-gas from the system is piped back into the biobutane treatment zone, thus allowing vapor control while reducing the overall operation and maintenance costs by eliminating the need for carbon replacement or regeneration, and by recycling the butane or other hydrocarbon gas.  
                                                                           TABLE 1                           Groundwater Test Data                C5-C8   C9-C12   C9-C10   C9-C18   C19-C36   C11-C22           Aliphatic   Aliphatic   Aromatic   Aliphatic   Aliphatic   Aromatic                        Initial Levels   54   830   1200   19000   2400   30000       Final Levels   0   0   583   0   0   0                  
 
       EXAMPLE 2  
       [0023]     A total of six butane/air sparge wells were installed in a treatment area as illustrated in  FIG. 4 . Each butane/air sparge well may consist of 1-inch or 1¼-inch inside diameter, black iron pipe or Schedule 80 PVC fitted with a 2-foot slotted screen, advanced from the ground surface to a clay deposit identified across the site at a depth of 9 feet below grade.  FIG. 4  illustrates the location of the injection wells labeled BAI- 1  through BAI- 6 .  
         [0024]     These wells were piped to a butane injector located in a treatment shed in the northern portion of the site, as shown in  FIG. 4 . An objective of the injection system is to oxygenate the groundwater without the customary effects and contaminant dispersal associated with aggressive air-sparging programs. The butane injector pulses butane gas at a selected volume into the flow stream of an air sparging well supplied with air by a compressor. The airflow was controlled to each well using valves. The airflow rate in each air sparge well is anticipated to vary between 3 and 5 cubic feet per minute. In addition, it is anticipated that in order to generate a dissolved butane concentration in groundwater of 10 to 20 ppm in the treatment zone, approximately 2.0 lbs (site total) of liquid butane was injected (as a gas) into the site aquifer daily (approximately 12.8 ft3).  
         [0025]     The SVE system previously installed at the site was operated concurrently with the butane biostimulation treatment system and used to maintain a vacuum within vadose zone soils. The SVE system served as a control for potential migration of volatile organic compounds (VOCs) from the treatment area into adjacent buildings. In addition, the SVE system further oxygenated the soils thus resulting in enhanced microbial degradation of petroleum compounds in the capillary fringe and in the vadose zone. VOCs were monitored in each vapor extraction well using a photoionization detector (PID) during site monitoring visits.  
         [0026]     The effluent from the SVE system was piped back into the butane biotreatment zone, as shown in  FIG. 4 , thus allowing for vapor control while reducing the overall O&amp;M costs by eliminating the need for carbon replacement or regeneration and by recycling the butane gas.  
         [0027]     The recirculated effluent from the SVE system was monitored using a PID meter during site visits. The level of VOCs concentration measured by the meter is a very good indication of the effectiveness of the butane biotreatment system with particular emphasis on the soil contamination located in the vadose or unsaturated zone. The results of the effluent screening are summarized in Table 2 below.  
                                           TABLE 2                           VOCs Content of SVE System Effluent                Date of Soil Gas Screening   PID Results (ppm)                            Before Butane Bioventing   690           4 Months After Initiation of   95           Butane Bioventing           7 Months After Initiation of   93           Butane Bioventing                      
 
         [0028]     The results clearly indicate that the VOCs concentrations in the soil gas were significantly decreased by continuous reinjection into the butane biotreatment zone established in the vadose zone soils. The soil gas represents the SVE system blower effluent prior to reinjection into the subsurface. This is the soil gas effluent normally piped into granular activated carbon canisters for treatment and eventual disposal.  
         [0029]     Table 3 is a summary of the groundwater quality data from the site. Butane bioventing was initiated in Month No. 7. Significant improvements in groundwater quality were achieved. It is noted that the GP-3 monitoring well was believed to be located upgradient of the treatment zone of influence.  
                                                                                                                                                             TABLE 3                           Summary of Groundwater Quality Data (System Startup Month No. 7)                Sample   Analytical       Ethyl-       Naph-           C5-C8   C9-C12   C9-C10           Location   Method   Benzene   benzene   MTBE   thalene   Toluene   Xylenes   Aliphatics   Aliphatics   Aromatics                        Month   GBI-1   MADEP VPH   ND   ND   10.1   ND   ND   ND   ND   ND   ND       No. 1   GBI-2   MADEP VPH   ND   ND   ND   ND   ND   ND   ND   ND   ND           GBI-3   MADEP VPH   ND   ND   196   ND   ND   ND   ND   ND   ND           GBI-4   MADEP VPH   ND   ND   289   ND   ND   122     1,460       1,140     ND           VW-1   MADEP VPH   ND   ND   15,700   ND   120   2,404     22,600       27,800     2.440           VW-2   MADEP VPH   ND   1,510     109,000     ND   504   4,090     37,600       46,900     ND           VW-3   MADEP VPH     2,170     3,130   12,700   ND     34,600       18,110     ND     12,200     ND           VW-4   MADEP VPH   247   1,540   2,500   517   799     10,250       6,610     ND     5,260             VW-6   MADEP VPH   942   1,260   7.800   332   85.8   3,740     18,500       8,470     3.800           VW-7   MADEP VPH   ND   ND   6.13   ND   ND   ND   ND   ND   ND           TRIP   MADEP VPH   ND   ND   ND   ND   ND   ND   ND   ND   ND           GP-3   MADEP VPH   1,380   689     140,000     473     9,740       17,520     ND   ND     6,770         Month   GBI-1   MADEP VPH   ND   ND   ND   ND   ND   ND   ND   ND   ND       No. 5   GBI-2   MADEP VPH   ND   ND   ND   ND   ND   ND   ND   ND   ND           GBI-3   MADEP VPH   ND   ND   41   ND   ND   ND   ND   ND   ND           GBI-4   MADEP VPH   ND   ND   200   ND   ND   ND   220   19   160           VW-1   MADEP VPH   ND   ND   16,000   ND   ND   ND   ND   930   ND           VW-2   MADEP VPH   400   ND   24,000   ND   ND   580   620     2,400     960           VW-3   MADEP VPH   130   300   72   130   3,200   1,590     4,800     310   1,900           VW-4   MADEP VPH   360   1,600   7,100   340   440     9,600       1,700       3,600       10,000             VW-6   MADEP VPH   690   1,500   4,500   160   110   3,250     1,900       1,900       6,300             VW-7   MADEP VPH   ND   ND   ND   ND   ND   ND   ND   ND   ND           TRIP   MADEP VPH   ND   ND   ND   ND   ND   ND   ND   ND   ND           GP-3   MADEP VPH   ND   ND     74,000     ND   ND   1,500   ND     3,500     3,400       Month   GBI-1   MADEP VPH   ND   ND   ND   ND   ND   ND   ND   ND   ND       No. 8   GBI-2   MADEP VPH   ND   ND   ND   ND   ND   ND   ND   ND   ND           GBI-3   MADEP VPH   ND   ND   497   ND   ND   ND   ND   ND   ND           GBI-4   MADEP VPH   26.8   ND   923   24.9   5.7   14.4   ND   81.6   301           VW-1   MADEP VPH   717   ND   10,600   ND   14.8   321   ND   ND   155           VW-2   MADEP VPH   1.472   292   41,900   337   713   4,012   ND   ND   3,680           VW-3   MADEP VPH   527   820   770   271     11,400       7,040     ND   ND   3,610           VW-6   MADEP VPH   126   1,810   8,700   283   467   3,719   ND   ND   3,810           VW-7   MADEP VPH   38.2   ND   11.5   ND   ND   10   ND   ND   ND           TRIP   MADEP VPH   ND   ND   ND   ND   ND   ND   ND   ND   ND           GP-3   MADEP VPH     4,320     1,000     83,300     947     16,900       31,900     ND   ND     9,440         Month   GBI-1   MADEP VPH   ND   ND   ND   ND   ND   ND   ND   ND   ND       No. 11   GBI-2   MADEP VPH   ND   ND   ND   ND   ND   ND   ND   ND   ND           GBI-3   MADEP VPH   ND   ND   1,530   ND   ND   ND   ND   ND   ND           GBI-4   MADEP VPH   ND   ND   922   ND   ND   62.6   908   ND   ND           VW-1   MADEP VPH   ND   ND   5,254   ND   ND   ND   ND   ND   ND           VW-2   MADEP VPH   216   ND   35,200   ND   675   1,500   ND   ND   3,690           VW-3   MADEP VPH   ND   771   587   ND   182   753   ND   ND   3,220           VW-6   MADEP VPH   1,250   1,930   10,600   ND   ND   4,068   ND   ND     9,030             VW-7   MADEP VPH   ND   ND   56.4   ND   ND   ND   ND   ND   ND           TRIP   MADEP VPH   ND   ND   ND   ND   ND   ND   ND   ND   ND           GP-3   MADEP VPH     3,950     1,610     210,000     ND     16,600       24,750     ND   ND     19,600         Month   GBI-1   MADEP VPH   ND   ND   ND   ND   ND   ND   ND   ND   ND       No. 14   GBI-2   MADEP VPH   ND   ND   ND   ND   ND   ND   ND   ND   ND           GBI-3   MADEP VPH   ND   ND   6,790   ND   ND   ND   ND   15.4   ND           GBI-4   MADEP VPH   32   28.7   2,510   ND   58   48.7   ND   33.6   352           VW-1   MADEP VPH   5.2   5.4   103   29.3   ND   40.9   ND   ND   303           VW-2   MADEP VPH   551   271   13,100   435   44.3   1,331   ND     1,500       4,710             VW-6   MADEP VPH   513   799   5,170   418   116   1,073   ND   888   3,740           VW-7   MADEP VPH   ND   ND   385.0   ND   ND   ND   ND   39.4   ND           TRIP   MADEP VPH   ND   ND   ND   ND   ND   ND   ND   ND   ND           GP-3   MADEP VPH   740   958   41,400   ND   243     16,920     ND   ND     14,500              GW-2 STANDARD     2,000       30,000       50,000       6,000       6,000       6,000       1,000       1,000       5,000         GW-3 STANDARD     7,000       4,000       50,000       6,000       50,000       50,000       4,000       20,000       4,000                   All concentrations expressed in μg/l (ppb)            Bold values exceed GW-2 and/or GW-3 Standards            VW-4 was dry on Jan. 24, 2002, Apr. 24, 2002 and Jul. 31, 2002            VW-3 was dry on Jul. 31, 2002             
 
         [0030]     Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention.