Privacy | Legal

Table of Contents Back | Next | Home

Missoula Technology & Development Center

Treatment of Petroleum-Contaminated Soils

Treatment Options (continued)

Composting

Petroleum products are readily biodegradable under the proper conditions. Composting takes advantage of this potential. The most common form of composting consists of spreading the contaminated soil out in rows, commonly called windrows. If required, nutrients are mixed in with the soil. Oxygen required for the aerobic biodegradation is supplied by frequently mixing and turning the windrows. Even when the windrows are not being turned, some oxygen is provided to the organisms through diffusion from the air into the soil matrix. Bulking agents are often mixed with the soil to enhance the oxygen transfer. Soil type and condition control the rate of diffusion.

Sorption of contaminants onto soil surfaces decreases biodegradability. The biodegradation of some PAHs will be hindered because of their high propensity to partition to soil surfaces (table 5). High molecular weight compounds, such as some of the PAH compounds, may be slow to degrade because of their structural complexity. Readily biodegradable compounds include low molecular weight aromatics (BTEX), a key factor when considering the toxicity of these compounds. These compounds are relatively soluble compared to other petroleum hydrocarbons, making them available to microorganisms (table 3). For petroleum hydrocarbons, the ranking of biodegradability from most biodegradable to least is generally: straight chain alkanes, branched chain alkanes, low molecular weight aromatics, cycloalkanes (Leahy and Colwell 1990).

The rate of biodegradation of petroleum hydrocarbons is a function of temperature and soil moisture. Conventional wisdom is that the temperature must be higher than 10 °C for microorganisms to reduce the mass of petroleum hydrocarbons in the soil. Biodegradation may occur at temperatures below the optimum. However, the rate of biodegradation may be low. Further discussion of the effect that temperature has on the rate of biodegradation follows. Optimum soil moisture content for composting is about 15 percent by weight. For petroleum-contaminated soils, moisture content should not be less than about 50 percent field capacity.

Historically, composting has been per-formed on organic wastes that have an abundant carbon source. Sewage sludge is an example of waste that can be composted easily. In some situations, petroleum-contaminated soil may not have enough carbon to support an acceptable rate of biodegradation. In these cases, a carbon source, such as sewage sludge, animal or vegetable wastes, and wood chips, can be mixed with the soil to enhance biodegradation.

Microorganisms have three basic requirements to biodegrade petroleum hydrocarbons: an adequate carbon source, oxygen (an electron acceptor), and an adequate supply of nutrients. Carbon and oxygen requirements have been addressed. Some soils do not have an adequate supply of nutrients (specifically nitrogen and phosphorous) to support biological growth. The solution to this problem is to add the proper amount of nutrients to the soil on a routine basis. Nitrogen has been added as urea or as ammonia salts (Cookson 1995). Typical sources of phosphorous include orthophosphoric and polyphosphate salts (Cookson 1995). The simplest method of adding nutrients is by mixing garden or lawn fertilizer into the soil (Cookson 1995). The amount of nutrients required can be estimated by knowing the amount of carbon in the material to be degraded. Estimation methods are well documented in Alexander (1999) and Cookson (1995).

Some believe that adding cultured microorganisms known to biodegrade petroleum hydrocarbons will enhance the biodegradation. This technique is often called "seeding." Several studies have examined the possibility of increasing the rate or extent (or both) of biodegradation in soils by seeding. Leahy and Colwell (1990) reviewed literature pertaining to seeding. The results of that review indicate that seeding is not required for soil contaminated by petroleum products. The main reasons Leahy and Colwell (1990) cite for this conclusion is the large number of hydrocarbon-degrading microorganisms that are found naturally in unsaturated soil. Hydrocarbon degraders naturally present in the soil have adapted to their environment. The additional seeding of organisms that have not adapted to such an environment will have a minimal effect on the rate of biodegradation. Braddock and others (2000) performed a comprehensive study examining the biodegradation of diesel-contaminated Arctic soil in biopiles. Results from this study showed that seeding of petroleum-contaminated soil had little influence on the rate of biodegradation.

Variations on the classic windrow design include aerated static piles and mechanically agitated vessels. Composting soil in aerated static piles is advantageous because the soil does not require mechanical mixing. Slotted pipes attached to blowers or wind-actuated exhaust fans are used to pull (or push) air through the excavated soil that has been formed in a pile (Fahnestock and others 1998). Mechanically mixing soil requires a constructed vessel to contain the soil during treatment. Soil is loaded into the vessel where it is mixed periodically to reaerate it.

In its simplest form, composting requires minimal infrastructure. Berms may be required to control runoff. The main requirement for windrow composting is frequent turning of the windrows to introduce oxygen (unless wind-actuated exhaust fans are used). The more advanced forms of composting require additional equipment.

Use in Cold, Wet, Remote Regions

Temperature is the driver for biological treatment of petroleum-contaminated soils. Microorganisms can degrade hydrocarbons in contaminated soil over a temperature range of 10 to 60 °C (Cookson 1995). For composting, maximum microbial activity has been measured at temperatures between 50 and 55 °C (Cookson 1995). Because biological reactions are exothermic (generate heat), these temperatures can be achieved even in cold regions with properly engineered controls. At some point air temperatures may drop to a level where composting will be ineffective, even with engineered controls. This limitation makes composting a seasonal activity.

A moderate level of moisture is required for biodegradation of petroleum in contaminated soils. However, moisture content exceeding 70 percent of field capacity will hinder gas transfer (Cookson 1995). Protection against water runoff during storms should be provided when contaminated soils are composted in wet regions. Composting alternatives that include covering the soil may be more applicable in wet regions.

The need for frequent mixing of the windrows makes traditional composting difficult in remote regions. Alternatives to traditional composting—static piles and mechanical agitation—may be more applicable.

Experience with composting in cold regions includes the use of static piles and a variation of a static pile, a biocell (information provided by Shannon and Wilson, Inc., appendix C). A biocell is essentially a contained static pile. The soil is loaded in a constructed reactor that includes a form of air introduction. To extend the available treatment time in cold regions, static piles have been covered and the inflowing air has been heated. Engineers have successfully treated petroleum-contaminated soil in the Arctic with this configuration. Thermally enhanced biocells have also successfully treated petroleum-contaminated soil in the Arctic.

Cost Estimate

The items to be considered in developing a cost estimate for composting are shown in table 13. Assumptions include:

• After composting, the treated soil can be used as fill material.
• The Forest Service owns the backhoe.
• The backhoe used for soil excavation can be used to place the soil into the area or the reactor used for composting and for removing treated soil.
• No hazardous waste is produced.

E-mail: wo_mtdc_webmaster@fs.fed.us

Back | Next Table of Contents

Missoula Technology & Development Center

---

This page last modified September 23, 2002
Visitor since September 23, 2002