Patent Grant
7550085
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.
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).
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
These and other aspects of the present invention will be more apparent from the following description.
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.
The following examples illustrate various aspects of the invention, and are not intended to limit the scope of the invention.
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.
A butane injection system was then combined with the existing SVE system, as illustrated in
Table 1 and
A total of six butane/air sparge wells were installed in a treatment area as illustrated in
These wells were piped to a butane injector located in a treatment shed in the northern portion of the site, as shown in
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.
The effluent from the SVE system was piped back into the butane biotreatment zone, as shown in
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.
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.
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.
1,460
1,140
22,600
27,800
109,000
37,600
46,900
2,170
34,600
18,110
12,200
10,250
6,610
5,260
18,500
8,470
140,000
9,740
17,520
6,770
2,400
4,800
9,600
1,700
3,600
10,000
1,900
1,900
6,300
74,000
3,500
11,400
7,040
4,320
83,300
16,900
31,900
9,440
9,030
3,950
210,000
16,600
24,750
19,600
1,500
4,710
16,920
14,500
2,000
30,000
50,000
6,000
6,000
6,000
1,000
1,000
5,000
7,000
4,000
50,000
6,000
50,000
50,000
4,000
20,000
4,000
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.
This application is a divisional of U.S. patent application Ser. No. 10/641,736 filed Aug. 15, 2003, now U.S. Pat. No. 7,314,548, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/403,934 filed Aug. 16, 2002.
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|---|---|---|---|
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| Number | Date | Country | |
|---|---|---|---|
| Parent | 10641736 | Aug 2003 | US |
| Child | 11967623 | US |
