Fact Sheet - Bioremediation
Bioremediation (also known as biological treatment or biotreatment) uses microorganisms (bacteria and fungi) to biologically degrade hydrocarbon-contaminated waste into nontoxic residues. The objective of biotreatment is to accelerate the natural decomposition process by controlling oxygen, temperature, moisture, and nutrient parameters. Land application is a form of bioremediation that is described in greater detail in a separate fact sheet. This fact sheet focuses on forms of bioremediation technology that take place in more intensively managed programs, such as composting, vermiculture, and bioreactors. McMillen et al. (2004) summarizes over ten years of experience in biotreating exploration and production wastes and offers ten lessons learned.
Bioremediation decisions can be facilitated through the use of risk-based decision making (RBDM), a process that uses risk considerations to develop cleanup levels that are environmentally acceptable for the given characteristics and anticipated land use of a specific site. The application of RBDM for cleaning up contaminated exploration and production sites is detailed in a book published by the U.S. Department of Energy and the Petroleum Environmental Research Forum, titled, Risk-Based Decision-Making for Assessing Petroleum Impacts at Exploration and Production Sites (McMillen et al. 2001).
Some advantages of biological treatment are: it is relatively environmentally benign; it generates few emissions; wastes are converted into products; and it requires minimal, if any, transportation. Sometimes, bioremediation is used as an interim treatment or disposal step, which reduces the overall level of hydrocarbon contamination prior to final disposal. Bioremediation can create a drier, more stable material for land filling, thereby reducing the potential to generate leachate. Depending on the composition of the hydrocarbon components, the bioremediation environment, and the type of treatment utilized, bioremediation may be a fairly slow process and require months or years to reach the desired result.
In composting, wastes are mixed with bulking agents such as wood chips, straw, rice hulls, or husks to increase porosity and aeration potential for biological degradation. The bulking agents provide adequate porosity to allow aeration even when moisture levels are high. To increase the water-holding capacity of the waste-media mixture, and to increase trace nutrients, manure or agricultural wastes may be added. Adding nitrogen- and phosphorus-based fertilizers and trace minerals can also enhance microbial activity and reduce the time required to achieve the desired level of biodegradation.
Composting is similar to land treatment, but it can be more efficient. Also, with composting systems, treated waste is contained within the composting facility where its properties can be readily monitored. With composting, mixtures of the waste, soil (to provide indigenous bacteria), and other additives may be placed in piles to be tilled for aeration, or placed in containers or on platforms to allow air to be forced through the composting mixture. To optimize moisture conditions for biodegradation, the compost mixture is maintained at 40 to 60% water by weight. Elevated temperatures (30 to 70 degrees C) in compost mixtures increase microbial metabolism. However, if temperatures exceed 70 degrees, cell death can occur. Tilling the soil pile or forced aeration can help control temperature and oxygen levels. Composting in closed containers can control volatile emissions. Composted wastes that meet health-based criteria can be used to condition soil, cover landfills, and supply clean fill. McMillen and Gray (1994) reported estimated costs for windrow composting of exploration and production wastes to range from $40 to $70 per cubic meter.
Bioreactors work according to the same aerobic biological reactions that occur in land treatment and composting, but the reactions occur in an open or closed vessel or impoundment. This environment accelerates the rate of biodegradation by allowing better control of the temperature and other conditions that affect the biodegradation rate. Bioreactor processes are typically operated as a batch or semi-continuous process. In a bioreactor, nutrients are added to a slurry of water and waste, and air sparging or intensive mechanical mixing of the reactor contents provides oxygen. This mechanical mixing results in significant contact between microorganisms and the waste components being degraded. To accelerate system start-up, introduction of microbes capable of degrading the organic constituents of the waste may be useful, although some companies have not had favorable experience with designer bugs. Many of the additives used for bioreactors are common agricultural products and plant or animal wastes.
After the desired treatment level has been reached, and depending on the constituents, liquids may be reused, transported to wastewater treatment facilities, injected, or discharged. Solids may be buried, applied to soils, used as fill, or treated further to stabilize components such as metals.
In tank-based bioreactors, operating conditions (temperature, nutrient concentration, pH, oxygen transport and mixing) can be monitored and controlled easily. Optimized biological processes ensure the best rate of biodegradation and allow for reduced space requirements relative to land-based biological treatment processes. However, capital and operation and maintenance costs for bioreactors are high relative to other forms of biological treatment. McMillen and Gray (1994) reported estimated costs for bioreactor treatment of oily cuttings wastes of approximately $500 per cubic meter.
Vermiculture is the process of using worms to decompose organic waste into a material capable of supplying necessary nutrients to help sustain plant growth. For several years, worms have been used to convert organic waste into organic fertilizer. Recently, the process has been tested and found successful in treating certain synthetic-based drilling wastes (Norman et al. 2002).
Researchers in New Zealand have conducted experiments to demonstrate that worms can facilitate the rapid degradation of hydrocarbon-based drilling fluids and subsequently process the minerals in the drill cuttings. Because worm cast (manure) has important fertilizer properties, the process may provide an alternative drill cutting disposal method. In the experiments, drill cuttings were mixed with sawdust to facilitate transport, shipped to the vermiculture site, blended with undigested grass, mixed with water, and applied to worm beds. The feeding consists of applying the mixture as feedstock to windrows, which are covered to exclude light from the worm bed and protect it from becoming waterlogged. Controlled irrigation systems correct the moisture content during periods of low rainfall. The feedstock was applied to the windrows, generally once per week, at an average depth of 15 to 30 mm. The worms "work" the top of each windrow, consuming the applied material over a 5- to 7-day period. The resulting worm cast organic fertilizer is harvested and packaged for distribution and use as a beneficial fertilizer and soil conditioner.
The experiments showed decreases in hydrocarbon concentration from 4,600 mg/kg to below 100 mg/kg in less than 28 days, with less than 200 mg/kg remaining after 10 days. The specific biological mechanism responsible for these decreases is not known. Hypotheses include microbial degradation within the worm beds, favorable aerobic conditions generated by the burrowing and mixing actions of the worms, and metabolic consumption of the hydrocarbons by the worms.
The results also indicated the complete degradation of the cuttings (originally 5-10 mm in diameter) and no detectable mortality among the worms. The occurrence of increased heavy metal concentrations and indications of bioaccumulation in the worm cast at higher application and feeding rates would require further study or the use of alternative weighting materials (Getliff et al. 2002). The apparent optimal portion of cuttings in the feedstock is 30 to 50%. An important factor for success is the use of drilling fluids designed for bioremediation and vermiculture technology. Linear, paraffin-type base fluids, combined with nitrate or acetate brine phases, enable the worms to add value to the cuttings that are already relatively clean due to the specific design of the fluids (Getliff et al. 2002).
Getliff, J., G. McEwen, S. Ross, R. Richards, and M. Norman, 2002, "Drilling Fluid Design and the Use of Vermiculture for the Remediation of Drill Cuttings," AADE-02-DFWM-HO-16, paper presented at the American Association of Drilling Engineers 2002 Technology Conference, Drilling and Completion Fluids and Waste Management, Houston TX April 2-3.
McMillen, S.J., and N.R. Gray, 1994, "Biotreatment of Exploration and Production Wastes," SPE 27135, presented at the Second International Conference on Health, Safety & Environment in Oil & Gas Exploration and Production, Jakarta, Indonesia, January 25-27.
McMillen, S.J., R. Smart, R. Bernier, and R.E. Hoffman, 2004, "Biotreating E&P Wastes: Lessons Learned from 1992-2003," SPE 86794, presented at the Seventh International Conference on Health, Safety, and Environment in Oil and Gas Exploration and Production, Calgary, Alberta, Canada, March 29-31.
McMillen, S.J., R. I. Magaw, and R.L. Carovillano, 2001, "Risk-Based Decision-Making for Assessing Petroleum Impacts at Exploration and Production Sites," U.S. Department of Energy and Petroleum Environmental Research Forum, October.
Norman, M., S. Ross, G. McEwen, and J. Getliff, 2002, "Minimizing Environmental Impacts and Maximizing Hole Stability - the Significance of Drilling with Synthetic Fluids in New Zealand," New Zealand Petroleum Conference Proceedings, February 24-27.