Buddy Shaw
Amoco Production Co.
Farmington, N.M.Catherine S. Block
Environmental Protection Co.
Farmington, N.M.C. Hamilton Mills
Industrial Ecosystems Inc.
Pacifica, Calif.
Natural and augmented bioremediation provides a safe, environmental, fast, and effective solution for removing hydrocarbon stains from soil.
At Amoco Production Co. in northwest New Mexico, various processes have been proven at more than 600 sites for over 3 years. These sites provided a comprehensive testing ground for land farming, composting, and bioremediation for a variety of hydrocarbons, soil types, and extreme weather conditions.
In 1992, Amoco sponsored a study with six bioremediation companies, which evaluated 14 different techniques. From this study, Amoco continued using Environmental Protection Co.'s (EPC) microbes for bioremediating more than 145 sites near Farmington, N.M.
EPC's microbes proved effective on various types of hydrocarbon molecules found in petroleum stained soils from heavy crude and paraffin to volatiles such as BTEX (benzene, toluene, ethylbenzene, xylene) compounds.
Controlled laboratory tests have shown that these microbes can digest the hydrocarbon molecules with or without free oxygen present. It is believed that this adaptation gives these microbes their resilience.
BIOREMEDIATION
Bioremediation (phagocytosis) occurs naturally in soils between indigenous microflora and organic carbon-bearing molecules, such as hydrocarbons from petroleum. The carbon-bearing molecules are a nutrient source for the microflora.
When introduced to a hydrocarbon environment, the microorganisms follow the pattern of their food source. They create a biofilm around the hydrocarbon molecule and digest it, or break it down into simpler compounds of carbon and oxygen.
The microbes use the chemical energy liberated to drive the thermodynamically nonspontaneous process, such as the synthesis of cell components.
The process converts complex substances by utilizing carbon as a nutrient source. The complex hydrocarbon molecules are naturally biodegraded into simple compounds such as carbon dioxide and water. When the hydrocarbon or nutrient source is depleted, microbe activity ceases and the microbes die.
ENVIRONMENTAL ADVANTAGES
Bioremediation removes hydrocarbons from soil organically, while maintaining microbial populations necessary to keep soil healthy and viable. In some cases, the soil is much improved after the bioremediation process.
Some evidence also suggests that it is environmentally advantageous for soil to remain or be back filled to its point of origin. Soil has developed its own characteristics, in one place, for millions of years. Climate and geological forces have created these site-specific characteristics such as pH, granulation, and soil composition.
IN SITU
In situ bioremediation is the most cost-effective method when conditions are optimum. Good soil permeability is an important factor with in situ methods. But good soil permeability also causes the need for excavation because the same optimum conditions may have permitted the petroleum to saturate the soil deeply.
Like water, oil takes the path of least resistance. Permeable soils can allow hydrocarbons to flow to depths that cannot be treated by in situ methods.
EX SITU
The ex situ method removes soil to a biopile prepared on site or off site at a permitted location. The hydrocarbon-stained area is excavated until clean soil is exposed. Fig. 2 shows a trenched off site biopile.
Both land farming and biopiles were used in ex situ situations.
During conventional land farming, excavated material is deposited in a contained area to a depth of 8-10 in. No additional enhancement or augmentation of the natural process is used.
Periodic tilling of soil ensures oxygen, moisture, and natural microorganisms are distributed throughout the material. When land farming is applicable, Amoco implements it first because of its cost efficiency.
Some long-chain hydrocarbons resist simple land farming or aeration methods yet respond rapidly to microbial digestion processes. In these instances, the excavated soil was windrowed into biopiles. Microbes were applied to enhance the bioremediation process.
In the first weeks of treatment, bioremediation begins to reduce the long-chain hydrocarbons in soil. The material is usually turned one additional time, and a second application of microbes is applied.
REMEDIATION GOALS
In remediation, the goal is to reduce the action level of hydrocarbons to acceptable level standards set by local regulatory agencies. Each state has an authorized agency that sets these standards based on studies they have completed.
In New Mexico, the agency is the New Mexico Oil Conservation Commission (NMocc). Their guidelines, as in most states, are based on groundwater protection. Depending on the proximity of the groundwater table to the oil stain, NMocc acceptable action levels range from less than 100 ppm to not more than 5,000 ppm. For total petroleum hydrocarbons (TPH), benzene, toluene, ethylbenzene, and xylene (BTEX) compounds must not exceed a maximum of 50 ppm. These levels include the TPH and BTEX compounds.
TEMPERATURE
Temperature is an issue in the bioremediation process.
Generally, microbial growth rate, as with chemical reactions, is a function of temperature. But the fact is, some microbes grow well at extreme temperatures. It depends on the microbe.
Many of the bioremediation sites discussed in this article were started as late as November and bioremediated throughout the winter (Table 1) (5359 bytes) and (Table 2) (12915 bytes).
The following techniques improve winter bioremediation:
- Use of hardy microorganisms
- Performance of proper engineering and maintenance
- Maintainance of proper moisture levels.
Ambient temperatures may slow the microbial growth or even halt some growth in surface layers, but soil is a great insulator. The center of the biopile is literally a "hot bed" of microbial growth activity. Internal biopile temperatures during the acceleration phase can reach from 38 to 72 C. (100-160 F.).
TIME
A common criticism of bioremediation is that it takes more time than other methods. This, under most conditions, could be called a myth. Following are optimum conditions for the process:
- Plenty of hardy microbes
- Permeable soil conditions
- Abundant quantities of hydrocarbons
- Proper moisture levels
- Correct engineering (such as large enough biopiles to maintain internal biopile temperatures).
The right bioremediation system can save time, as well as money. On the average, this method takes 3-12 weeks to complete.
The ex situ method can also save loading and transporting time when done on site. Once the on site windrows or biopiles are in place, the time involved for actual treatment is minimal, and the results are far better for the client and the environment.
COST
Generally, the in situ method is the least labor intensive and the least expensive of the three bioremediation methods: in situ, ex situ on site, and ex situ off site.
Excavating and transporting materials are unnecessary with in situ bioremediation. The microbes are pumped or flowed directly into petroleum-stained pits (Fig. 3). Most often separators, storage tanks, etc., can remain in place and get a good cleaning along with the soils around them. In situ bioremediation can be done for 40-60%/ton less than thermal desorption methods.
Ex situ on site also saves cost on sites with adequate room for a biopile. This method can save expense and time, in regard to the transporting of soil.
Trucking stained earth off site is unnecessary, and the treated soil can be reused on site to back fill pits. Therefore, the cost of replacing and transporting excavated earth with purchased soil from elsewhere is also deleted.
To save time and money, ex situ soils transported to permitted sites away from the production sites can also be reused after bioremediation.
The ex situ on site cost savings are in the 30-40% range over thermal desorption. The average cost of bioremdiation at the Amoco sites ranged from $3-23/cu yd. The average cost of thermal desorption ranges from $30-50/cu yd and was not considered because of the cost difference.
EXAMPLE SITES
Two in situ and two ex situ sites illustrate the effectiveness of the microbes in different soil conditions, hydrocarbon levels, and treatment methods. These sites and types of hydrocarbons are typical to the region and petroleum industry in the San Juan area of New Mexico. These sites were chosen because of the high saturation ratio of petroleum hydrocarbons.
All sites selected are now, or were, discharge or blowdown pits on production well pads (Fig. 4) (19523 bytes). Until 1985, the oil and water mixture was discharged directly into earthen pits.
The in situ pit size varies depending on the gradient flow of the stain. Ex situ methods are usually chosen because of the hydrocarbon stain type, such as paraffin or light stain, or if the depth exceeds 18 ft.
PIT WITH TANK
The site at Florance C LS No. 3 is a good example of the convenience of the in situ method.
A 9.5-sq ft pit had a metal tank set into the soil (Fig. 5). The soil under and around the production tank was stained with paraffin. Even though the heavy hydrocarbons (paraffin) and the compacted clay soil did not provide the optimum soil conditions for the in situ method, treatment was successful, even without the tank being removed.
Soil samples taken from the pit surface at the beginning showed 51,000 ppm TPH, 163 ppm BTEX, and no detectable benzene. On Nov. 23, 1992, the pit was soaked three times with liquid microbe solution and the surface area around the tank was sprayed with solution. Then the site was left undisturbed for 3 weeks.
After 22 days, samples were taken again.
TPH levels had dropped to 5,980 ppm, 10 times less than the original levels, in the dead of winter. From Dec. 9, 1992, to Jan. 1, 1993, the temperatures ranged from - 3 to 6 C. (26-43 F.).
On Feb. 15, 41 days later, this in situ site was assessed at levels less than regulatory standards with 136 ppm TPH, and 0.055 ppm BTEX. This was achieved without excavation, transportation of any soil, or destruction of the topography and natural stability of the indigenous microbe spectrum of the immediate area.
BLOWDOWN PIT
The Elliott Annie G1 site had an earthen blowdown pit containing liquids from an oil and gas separation system. The site was treated in situ, with no physical disturbance except for assessment holes. Initial levels registered 10,800 ppm TPH, 36,320 ppm BTEX, and 80 ppm benzene.
The first treatment with liquid microbes solution was applied directly to the surface soil on Oct. 26, 1992. The site was left undisturbed for 23 days.
On Nov. 18, 1992, samples taken at 4 ft read: 365 ppm TPH, 7,430 ppm BTEX, and no detectable benzene. To close the pit for further use, on Nov. 23, 1992, the pit was excavated and back filled, mixing treated soil with untreated soil (Fig. 6).
On Dec. 11, 1992, the pit was excavated. Samples taken every 3 ft confirmed the success of the treatment (Table 3) (5461 bytes).
ON SITE BIOPILE
At the GCU 243E site on Sept. 29, 1994, excavated soil from a separator pit had initial TPH levels of 39,400 ppm. BTEX and benzene were not a problem at this site.
On Oct. 4, 1994, soil was windrowed and trenched into a biopile on site. The pile was. turned and treated with solution on Oct. 5. Samples taken on Oct. 21, revealed 6,100 ppm TPH. The pile was turned 3 days later and treated again with solution.
The most current assessment showed 3,100 ppm TPH on Nov. 4, 1994. The TPH was reduced to less than 10% of original levels in less than 30 days. This is bioremediation at its best - fast, efficient, and cost effective.
The site remains under remediation.
OFF SITE BIOPILE
The Elliott Annie L B4 pit is one of four pits that were excavated and the soil trucked to a central site. The site in this case is a permitted area near Amoco's wells.
Soil is transported there when engineering of the biopile requires more space than available on site.
The Elliott Annie L B4 is an example of a pit with high TPH levels in the initial assessment. The site pit information (Table 4) (7026 bytes) includes hydrocarbon levels and tonnage for all four pits which were transported to the central site and commingled in the same biopile.
The soils were deposited at the central site and windrowed. This windrow or biopile was treated with liquid microbe solution on Oct. 7, 1992. A sample taken 8 days later revealed 406 ppm TPH. The pit was left undisturbed until samples were taken Dec. 31, 1992. Final readings were 82 ppm TPH and no detectable BTEX or benzene.
Copyright 1995 Oil & Gas Journal. All Rights Reserved.
