The following description of biopiles is an excerpt from Chapter IV of OUST's publication: How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites: A Guide for Corrective Action Plan Reviewers. (EPA 510-B-95-007). This publication also describes 9 additional alternative technologies for remediation of petroleum releases. You can download PDF files of every chapter of the document at: http://www.epa.gov/swerust1/pubs/tums.htm.
Biopiles, also known as biocells, bioheaps, biomounds, and compost piles, are used to reduce concentrations of petroleum constituents in excavated soils through the use of biodegradation. This technology involves heaping contaminated soils into piles (or "cells") and stimulating aerobic microbial activity within the soils through the aeration and/or addition of minerals, nutrients, and moisture. The enhanced microbial activity results in degradation of adsorbed petroleum-product constituents through microbial respiration. Biopiles are similar to landfarms in that they are both above-ground, engineered systems that use oxygen, generally from air, to stimulate the growth and reproduction of aerobic bacteria which, in turn, degrade the petroleum constituents adsorbed to soil. While landfarms are aerated by tilling or plowing, biopiles are aerated most often by forcing air to move by injection or extraction through slotted or perforated piping placed throughout the pile.
Biopiles, like landfarms, have been proven effective in reducing concentrations of nearly all the constituents of petroleum products typically found at underground storage tank (UST) sites. Lighter (more volatile) petroleum products (e.g., gasoline) tend to be removed by evaporation during aeration processes (i.e., air injection, air extraction, or pile turning) and, to a lesser extent, degraded by microbial respiration.
The mid-range hydrocarbon products (e.g., diesel fuel, kerosene) contain lower percentages of lighter (more volatile) constituents than does gasoline. Biodegradation of these petroleum products is more significant than evaporation. Heavier (non-volatile) petroleum products (e.g., heating oil, lubricating oils) do not evaporate during biopile aeration; the dominant mechanism that breaks down these petroleum products is biodegradation. However, higher molecular weight petroleum constituents such as those found in heating and lubricating oils, and, to a lesser extent, in diesel fuel and kerosene, require a longer period of time to degrade than do the constituents in gasoline.
Biopiles are designed to optimize the conditions for aerobic bacteria to biodegrade organic contaminants. The effectiveness of a biopile system depends on many parameters which can be grouped into three categories:
- soil characteristics,
- constituent characteristics, and
- climatic conditions.
Soil texture affects the permeability, moisture content, and bulk density of the soil. Fine-grained soils are less permeable than coarse-grained soils. Soils with lower permeability are more difficult aerate, but tend to retain moisture better than soil with higher permeability. However, lower permeability is usually associated with soils that clump together making it difficult to evenly distribute moisture, air, and nutrients. At certains times during the operational life of the biopile the soil may need to be turned (or tilled) to promote continued biodegradation.
Soil normally contains large numbers of diverse microorganisms including bacteria, algae, fungi, protozoa, and actinomycetes. In well-drained soils, which are most appropriate for biopiles, these organisms are generally aerobic. Of these organisms, bacteria are the most numerous and biochemically active group, particularly at low oxygen levels. Bacteria require a carbon source for cell growth and an energy source to sustain metabolic functions required for growth. Bacteria also require nitrogen and phosphorus for cell growth. Although sufficient types and quantities of microorganisms are usually present in the soil for landfarming, recent applications of ex situ soil treatment include blending the soil with cultured microorganisms or animal manure.
Soil microorganisms require moist soil conditions for proper growth. Excessive soil moisture, however, restricts the movement of air through the subsurface thereby reducing the availability of oxygen which is essential for aerobic bacterial metabolic processes. In general, soils should be moist but not wet or dripping wet.
Bacterial growth rate is a function of temperature. Soil microbial activity has been shown to significantly decrease at temperatures below 10 degrees C. The microbial activity of most bacteria important to petroleum hydrocarbon biodegradation also diminishes at temperatures greater than 45 degrees C. Within the range of 10 degrees C to 45 degrees C, the rate of microbial activity typically doubles for every 10 degrees C rise in temperature. Because soil temperature varies with ambient temperature, there will be certain periods during the year when bacterial growth and, therefore, constituent degradation will diminish. When ambient temperatures return to the growth range, bacterial activity will be gradually restored.
The presence of very high concentrations of petroleum organics or heavy metals in site soils can be toxic or inhibit the growth and reproduction of bacteria responsible for biodegradation in biopiles. Conversely, very low concentrations of organic material will result in diminished levels of microbial activity.
The typical height of biopiles varies between 3 and 10 feet. Additional land area around the biopile(s) will be required for sloping the sides of the pile, for containment berms, and for access. The length and width of biopiles is generally not restricted unless aeration is to occur by manually turning the soils. In general, biopiles which will be turned should not exceed 6 to 8 feet in width.
Biopiles are typically constructed in "lifts". Blended soil is mounded up to a depth of no more than a few feet and then aeration and moisturizing piping is laid prior to the addition of the next lift. This process is repeated until the pile is at the desired height.
Blending the soil may involve the addition of (a) manure, to both augment the microbial population and provide additional nutrients, (b) soil amendments (e.g., gypsum) and bulking materials (e.g., sawdust, or straw), to ensure that the biopile medium has a loose or divided texture, and (c) chemicals to adjust the soil pH, because to support bacterial growth, the soil pH should be within the 6 to 8 range, with a value of about 7 (neutral) being optimal.
Periodically, moisture must be added to the biopile because soils become dry as a result of evaporation, which is increased during aeration operations. Excessive accumulation of moisture can occur within biopiles in areas with high precipitation or poor drainage.
Because volatile constituents tend to evaporate from the biopile into the air during extraction or injection, rather than being biodegraded by bacteria, capture or containment of vapors may be required. This can be accomplished by covering the biopile and installation of collection piping beneath the cover. If air is added to the pile by applying a vacuum to the aeration piping, volatile constituent vapors will pass into the extracted air stream which can be treated, if necessary. In some cases (where allowed), it may be acceptable to reinject the extracted vapors back into the soil pile for additional degradation. In some cases the vapors may need to be treated (typically through carbon adsorption).
To prevent possible leaching of contaminants from the biopile into the underlying groundwater, biopiles may be required to be constructed on top of an impermeable liner. Leachate that drains from the biopile is then collected for treatment and disposal.
- Relatively simple to design and implement.
- Short treatment times: usually 6 months to 2 years under optimal conditions.
- Cost competitive: $30-90/ton of contaminated soil.
- Effective on organic constituents with slow biodegradation rates.
- Requires less land area than landfarms.
- Can be designed to be a closed system; vapor emissions can be controlled.
- Can be engineered to be potentially effective for any combination of site conditions and petroleum products.
- Concentration reductions > 95% and constituent concentrations < 0.1 ppm are very difficult to achieve.
- May not be effective for high constituent concentrations (>50,000 ppm total petroleum hydrocarbons).
- Presence of significant heavy metal concentrations (>2,500 ppm) may inhibit microbial growth.
- Volatile constituents tend to evaporate rather than biodegrade during treatment.
- Requires a large land area for treatment, although less than landfarming.
- Vapor generation during aeration may require treatment prior to discharge.
- May require bottom liner if leaching from the biopile is a concern.
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