BioCycle September 2010, Vol. 51, No. 9, p. 38
Current and past research on the use of compost to both remove and remediate hydrocarbons documents its value in the Gulf oil spill clean-up.
THE Deepwater Horizon Oil Spill in the Gulf of Mexico is the largest environmental disaster in U.S. history. Steps toward remediation and recovery – a long-term process – are underway. One method that figures prominently in this process is bioremediation, utilizing microorganisms, plants or enzymes to return contaminated sites back to their natural condition.
If a concerted push is made to the right audiences, compost will present one of the best bioremediation tools across the Gulf region. Research, both recent and historical, has shown compost has the ability to remove oil from water and once removed, can be used to breakdown the hydrocarbons. While these studies are limited to land-based ecosystems, they provide evidence that compost can be an effective biologically based tool to separate oil from water, particularly as oil makes its way from the Gulf waters to its shores, and once ashore can be both filtered and effectively bioremediated in situ. (See “Oil And Compost Could Prove A Good Mix In The Gulf” and “Bioremediate Petroleum,” July 2010.)
A study conducted at the U.S. Department of Agriculture’s Agricultural Research Service and published in the Journal of Environmental Quality reported that compost filter socks (Filtrexx FilterSoxx) removed between 43 and 99 percent of three types of commonly spilled hydrocarbons – motor oil, diesel and gasoline. Addition of a natural additive to the compost increased removal efficiencies to between 55 and 99 percent. Concentrations for the three hydrocarbons ranged from 74 to 5400 mg/L (Faucette et al, 2009). Preliminary data from a follow up study funded by the U.S. EPA is showing that this same tool can remove 99 percent of motor oil through 25 consecutive runoff exposure events with concentrations near 40 mg/L.
The City of Chattanooga, Tennessee has used compost filter socks to reduce oil and grease in storm water originating from a 5-acre parking lot in order to comply with their National Pollutant Discharge Elimination System (NPDES) Stormwater Permit, part of the U.S. Clean Water Act. The compost system reduced oil/grease concentrations by 65 to 95 percent (Table 1) over the 2-year field evaluation program and continues to be used today (Faucette et al, 2009a).
While compost has shown promise to remove petroleum hydrocarbons from water in terrestrial environments, what is the eventual fate of this pollutant within the compost media? Compost provides a high diversity of microorganisms, including ones that degrade hydrocarbons, and an optimum environment – sufficient and preferably sustainable (slow release) source of nutrients (mostly N and P), water, air, mild ambient temperature and a moderate pH – for them to thrive.
At optimum levels, these environmental factors provide the energy and metabolic resources that create a widely diverse group of beneficial microorganisms that will suddenly reproduce on a very rapid scale. They also work rapidly and effectively to degrade organic compounds, including petroleum hydrocarbons, for food (from carbon) to sustain their growth pattern (Faucette et al, 2009a). Additionally, it is often the humus content of compost (six times higher in mature compost than typical soils) that catalyzes the degradation process of organic compounds/contaminants (Stevenson, 1994 and USEPA, 1998).
Bacteria and fungi are the primary biological actors responsible for the degradation of organic contaminants (Alexander, 1994), and increasing the diversity, population and community structure can accelerate the degradation of these contaminants (Cole et al, 1994). Microbial diversity and population density are greatly increased by the addition of compost compared to fertile, productive soils; therefore, bioremediation takes far less time with compost than under natural conditions (Cole et al, 1994 and USEPA, 1998).
For example, normal bacteria populations in fertile soils are approximately 26 million/gram of dry soil, while in compost, populations are approximately 417 million/gram of dry compost. Similarly, fungi populations in fertile soils are approximately 28,000/gram dry soil and 155,000/gram for dry compost (Cole, 1976 and Cole et al, 1994). Additionally, microbial activity in mature compost can be nearly 40 times greater than in soil ecosystems (USEPA, 1998). It is no surprise that hydrocarbon-degrading microorganisms are often isolated from compost and used for inoculating bioremediation projects (Civilini et al, 1996 and Castaldi et al, 1995).
In a study evaluating the degradation rates of petroleum in contaminated soils, those amended with compost exhibited degradation rates of 375 mg kg-1/day compared to only 40 mg kg-1/day without compost (Stegmann et al, 1991 and Hupe et al, 1996). At the rate exhibited by the compost amended soil, typical petroleum hydrocarbon contaminated soils (normal range is between 5,000 to 20,000 mg kg-1) would be completely degraded in 14 to 60 days (USEPA, 1998). Roling et al (2004) have reported that oil contaminated soils treated with slow release nutrient sources can produce higher bacteria community structures and higher degradation rates (in the form of CO2 evolution) compared to liquid nutrient sources.
An experiment conducted by Hupe et al (1996) to quantify the fate of bioremediated hydrocarbons reported that 59 percent was converted to CO2, 24 percent was bound to residue, 4 percent was volatilized, 4 percent was converted into the biomass of the microorganisms and 8 percent was extractable (in its original form). The fraction that bonds with the residue is often incorporated into the core structure of the humic materials, making it relatively non-bioavailable for decades and even centuries (Stevenson, 1994 and USEPA, 1998).
Clearly compost has been shown to remove and remediate hydrocarbons in terrestrial ecosystems. While the current oil spill in the Gulf of Mexico is surely an ecological catastrophe of historic proportions, it also represents a unique educational opportunity to showcase the under utilized bioremediation benefits of compost in the most challenging of situations.
Britt Faucette, Ph.D., LEED AP, CPESC, is Director of Research & Technical Services for Filtrexx International.
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USEPA, 1998. An Analysis of Composting As an Environmental Remediation Technology. US EPA Solid Waste and Emergency Response (5305W). EPA530-R-98-008, April 1998: 2-38.