Fact sheet: Aerobic biopile

From: Public Services and Procurement Canada

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Description

The aerobic biopile is an ex situ remediation technology used for the treatment of soils contaminated with organic compounds that are degraded under aerobic conditions. Biopiles are common remedial approaches for fuel-range organic compounds and non-halogenated volatile organic compounds. The method has been successfully applied to some polycyclic aromatic hydrocarbons (PAH). Indigenous microorganisms are typically used though some practitioners add exogenous cultures (with mixed results).

This technique consists of arranging excavated contaminated soil in piles (biopiles) of approximately one to three metres high. Biodegradation of contaminants within the biopiles is enhanced by controlling parameters such as pH, water content, nutrients and aeration. The addition of a structural agent, such as woodchips, is sometimes required to improve aeration of the biopile. If passive aeration of the biopile is not adequate (using the difference between gas pressure in the biopile and in the atmosphere to move air into or out of biopile), the use of an air injection or extraction system may be required.

Biopiles are generally covered to minimize volatilization of contaminants into the atmosphere and to ensure greater control over the water content of the biopile. When the contaminated soil contains volatile or semi-volatile organic compounds, it is necessary to install a vapour collection and treatment system. A membrane must be installed under the biopile in order to prevent the migration of contaminants into the aquifer. Leachate and runoff collection and treatment systems may be required.

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Implementation of the technology

Excavated contaminated soils are stockpiled, required soil amendments are added and mixed in the soils and the stockpile is enclosed for treatment. A typical biopile system includes a treatment bed, an aeration system, an irrigation/nutrient distribution system and a leachate collection system. In order to enhance the biodegradation process, moisture, heat, nutrients, oxygen and pH are controlled. An irrigation/nutrient distribution system is buried within the soils to allow for air, water and nutrients to flow through the soils. A biopile may be covered with a geomembrane cover to control runoff, evaporation and volatilization, as well as to promote solar heating of the soils. If the soils are contaminated with volatile organic compounds (VOC), the air extracted from the biopile may require to be treated in order to remove or destroy the VOC before it’s discharged into the atmosphere. Depending on precipitations, contaminant characteristics and depth of groundwater, leachate collection and treatment might also be required.

Microbial activity (and less frequently, fungal activity) is stimulated with fertilizer, pH control, moisture control, temperature control, inoculation with seed material and/or soil texture improvements. Biological activity degrades contaminants. After a period of weeks to years, the soil is remediated and can be spread out or used as fill.

Conventional excavation equipment is used to physically excavate the contaminated soil and build a biopile for its biological treatment onsite. Biopile construction and operation activities may include:

  • Mobilization, site access and temporary facilities;
  • Clearing/grubbing/demolition;
  • Typical excavation operations and procedures, including topsoil stripping/temporary stockpiling, slope stability controls, excavation dewatering, protection of structures to be kept, etc.;
  • Transport by truck or loader;
  • Mixing by speciality equipment or excavator;
  • Addition of fertilizer, pH adjustment and/or soil texture additives;
  • Control of moisture, aeration and temperature;
  • Re-use of treated soil offsite, by spreading onsite or by backfilling excavations; and,
  • Surface restoration (grading, paving, hydro seeding or planting).

Biopile construction generally include imported aggregate, geomembrane liners, geomembrane covers, slotted plastic (typically polyvinyl chloride [PVC] or high-density polyethylene [HDPE]) ventilation or drainage pipe, etc.

Soils are amended with fertilizers (typically agricultural products specified to achieve target carbon-nitrogen-phosphorous-potassium levels [C:N:P:K]), texture/bulking agents (such as alfalfa, woodchips, shredded cardboard or rice hulls) and/or pH controllers (typically lime; sometimes sulphur). Other nutrients may be added depending on soil analysis (iron, cobalt, copper, manganese, zinc, etc.). Soils are aerated by a ventilation system and are kept moist (for example, using drip irrigation).

A variety of proprietary slow-release fertilizers, admixtures (including emulsifiers and surfactants, such as lecithin) and microbial cultures are available. Indigenous microorganisms are typically used; sometimes exogenous cultures (with mixed results) are used.

Materials and Storage

  • Onsite storage is typically limited to small amounts of fuel and lubricant (daily fuelling is typically from a mobile tank) as well as miscellaneous construction site supplies during construction of biopiles.
  • Typically, a starting load of fertilizer and/or bulking texture additive is mixed or screened into soils.
  • Relatively modest quantities of fertilizer may be kept onsite, for example, to add to irrigation water over time.

Waste and Discharges

  • Ideally, at the end of the treatment process, there are few to no wastes. Non-degradable materials, such as liners and covers, are removed during decommissioning.
  • Site garbage is typical of a construction site. Ideally, soils are completely treated and then put to beneficial re-use.
  • Windblown dust, from the excavation face, stockpiles or track-out from heavy machinery for example, may deposit directly on downwind surfaces.
  • Diverted and collected/treated stormwater are typically passed into the local stormwater system.
  • Contaminated water is frequently recycled into the treatment process to maintain moisture levels and to treat (degrade) contaminants.
  • Vapour phase discharges during construction are typically limited to equipment exhaust and volatilization of contaminants from fresh excavation faces or soil stockpiles.
  • Soils under treatment are deliberately ventilated (with the ventilation system). They may warm up as a function of exothermic reactions or may be intentionally heated. As a consequence, off-gassing is expected. If contaminants pose an unacceptable downwind risk, the treatment system will normally include vapour collection (for example a low-vacuum collection system operating under a low permeability cover) with treatment (frequently with granular activated carbon, although biofilters are also used).
  • Alternatively, treatment may take place in a temporary structure, such as a pre-engineered steel-framed building with fabric cover, which may have a negative pressure collection and treatment system.
  • Natural biodegradation, particularly for hydrocarbons, may also produce off-gases other than contaminants. Such off-gasses may include carbon dioxide (CO2), ammonia (NH3), CH4 and/or hydrogen sulfide (H2S).

Recommended analyses for detailed characterization

Biological analysis

  • Total heterotrophic and specific bacterial counts (according to the contaminants of interest)

Chemical analysis

  • pH
  • Organic matter content
  • Metals concentrations
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
    • free
  • Nutrient concentrations including:
    • ammonia nitrogen
    • total Kjeldahl nitrogen
    • nitrates
    • nitrites
    • total phosphorus

Physical analysis

  • Vadose zone oxygen, nitrogen dioxide, and methane concentrations
  • Temperature
  • Soil water content
  • Soil granulometry
  • Evaluation of biological conditions and ecological factors

Recommended trials for detailed characterization

Biological trials

  • Microcosm mineralization trial
  • Biodegradation trial

Physical trials

  • Gas permeability trials
  • Vapour survey
  • Airflow rate
  • Evaluation of optimal mixing rates
  • Evaluation of operating pressure/vacuum

Other information recommended for detailed characterization

Phase II

  • Detailed topography
  • Regional climatic conditions (precipitation, temperature, etc.)

Phase III

  • Volume of contaminated material to treat

Applications

  • Suitable for residual contamination treatment.
  • Applies to organic compounds that are biodegraded under aerobic conditions.
  • Number of microorganisms must be greater than 103 Colony Forming Units per gram of soil.
  • Water content must be maintained between 50% to 80% of the total water holding capacity of the biopile soil.
  • Soil pH should range from 6 to 8 to support bacterial activity.
  • Biopile treatment is generally performed at temperatures ranging from 20 °C to 35 °C.

Applications to sites in northern regions

  • Remote sites are prone to high mobilization and onsite monitoring costs, limited equipment availability and short work windows.
  • Difficulties in procuring timely analytical results may necessitate reliance on field screening, staged interventions and/or risk management.
  • In less densely populated areas, risk management might require less intense monitoring and control measures than those generally used for more developed areas.
  • Low temperatures significantly slow biodegradation; research suggests that pile insulation and/or low-level heating may be effective in some situations.
  • Soils may be heated and/or insulated, for example, using electrical heat trace and expanded polystyrene (EPS) panels.
  • Biopiles are popular options in remote areas but must be designed to operate without operator intervention for long periods.

Treatment type

Treatment type
Treatment typeApplies or Does not apply
In situ
Does not apply
Ex situ
Applies
Biological
Applies
Chemical
Does not exist
Control
Does not exist
Dissolved contamination
Does not exist
Free Phase
Does not exist
Physical
Applies
Residual contamination
Applies
Resorption
Applies
Thermal
Does not exist

State of technology

State of technology
State of technologyExist or Does not exist
Testing
Does not exist
Commercialization
Exist

Target contaminants

Target contaminantsApplies, Does not apply or With restrictions
Aliphatic chlorinated hydrocarbons
With restrictions
Chlorobenzenes
With restrictions
Explosives
Does not apply
Metals
Does not apply
Monocyclic aromatic hydrocarbons
Applies
Non metalic inorganic compounds
Does not apply
Pesticides
With restrictions
Petroleum hydrocarbons
Applies
Phenolic compounds
With restrictions
Policyclic aromatic hydrocarbons
With restrictions
Polychlorinated biphenyls
Does not apply

Notes:

Chlorobenzenes: suitable for chlorobenzene, dichlorobenzene and trichlorobenzene;
Phenolic compounds: suitable for cresol, pentachlorophenol and tetrachlorophenol.

Treatment time

Treatment time
Treatment timeApplies or Does not apply
Less than 1 year
Applies
1 to 3 years
Applies
3 to 5 years
Does not apply
More than 5 years
Does not apply

Notes:

Less volatile and more recalcitrant compounds may require up to two years in treatment; readily degradable compounds may be remediated in weeks. High levels of treatment (contaminant reductions of 99% or more) typically require significantly more time than moderate contaminant reductions (70% to 90%).

Long-term considerations (following remediation work)

Few to no long-term considerations exist at sites where treatment has met appropriate criteria, equipment/facilities have been decommissioned and the site has been cleaned up. Poor backfilling practices (backfill placed frozen, not compacted, etc.) can create geotechnical stability problems or differential settlement.

Secondary by-products and/or metabolites

Few to none. The technique is predominantly used for fuel-range organic, which typically degrade to CO2 and water (or form biomass), thus doesn’t typically generate deleterious secondary by-products or metabolites.

Cometabolic systems or systems under anaerobic conditions designed for the degradation of chlorinated compounds might generate toxic by-products/metabolites, although these systems are usually rare.

Limitations and Undesirable Effects of the Technology

  • Requires a large area for the construction of a biopile system.
  • The physical disruption of excavation and pile construction is significant. Biopiles themselves are relatively limited in area and have limited physical effects during operation.
  • Large treatment compounds may opt to line, pave or condition (for example, with lime treatment) broad areas, creating a need for runoff management and changing local infiltration patterns.
  • Climatic conditions influence treatment parameters and can interfere with the control of the biopile environment.
  • There is an important cost associated with the handling of contaminated soils compared to in situ treatment.
  • It is difficult to reduce petroleum hydrocarbon concentrations more than 95% and to obtain residual concentrations less than 1 part per million (ppm) after treatment.
  • High contaminant concentrations may be toxic for microorganisms.
  • Significant heavy metal concentrations may interfere with the biodegradation processes.
  • Volatilization of volatile organic compounds (VOC) must be addressed.
  • Possible dust problems might occur during the excavation, handling and construction of the biopile.
  • Requires the control, collection and treatment of runoff water and/or leachate.
  • Localized changes to pH and ionic strength are possible.
  • Changes in oxidation-reduction potential (ORP), pH, ionic strength, and/or soil organic content can affect transport of metals such as cadmium, copper, zinc or chromium.
  • Whether as a consequence of fertilizer addition, changes in pH, changes in ORP, ionic strength, soil organic content, etc., the solubility of metals may change during treatment, for example through the formation or destruction of sorbent material, sulfides (S2-), phosphates (PO43-), carbonates (CO32-) and/or hydroxides (OH-).
  • With well-designed biopiles (i.e. designed in reference to contemporary guidance) and/or through the use of bench and pilot testing, metal mobilization is typically not a major issue. In some cases (for example, the use of high-phosphate fertilizer on soils with lead contamination), soil treatment may actually reduce the mobility or bioavailability of metals.
  • The presence of heavy machinery used for onsite work might create temporary nuisances for the population. The preoccupations of the neighbours and workers are often related to dust, noise, odours, light (during night time) and traffic problems. Dust control can be an important problem with the presence of fine particles in dry and windy conditions. Usually, spraying water will eliminate the dust problem. A few replacement approaches (more expensive) were used for specific reasons on certain sites, for instance the use of polymers, calcium chloride solutions, lignosulfonates, foaming agents, etc.
  • Incidents are unlikely to affect offsite receptors. Typical scenarios include slope failures, equipment collisions, small spills of fuel or hydraulic fluid, damage to utilities, damage to adjacent structures or breathing air quality issues in low-lying areas, such as the excavation pit itself.
  • Externally applied heat and/or high carbon utilization rates may lead to elevated temperatures in biopiles. Because piles are typically well irrigated, have temperature monitoring and have a low overall fuel value, fires are rare. Enhanced desorption or volatilization are possible (and may be deliberate design features).
  • Biopiles are active processes and may fail if constructed and then neglected, as it sometimes happens at remote sites. Poor management can result in extended treatment times or inadequate treatment. Weather damage can create problems with runoff or leachate. Regular inspection and maintenance are a part of normal system operation, and required to prevent incident/upset conditions.

Complementary technologies that improve treatment effectiveness

  • Special techniques, pretreatment (with heat or oxidants), cometabolites (methane [CH4] or propane [C3H8] ventilation) and/or inoculants (microbes and/or fungi) can extend this approach to the treatment of some chlorinated organic compounds, PAH mixtures, perchlorate, wood preservatives (pentachlorophenol), pesticides and/or explosives (Trinitrotoluene [TNT], Royal Demolition Explosive [RDX], High Melt Explosive [HMX]).
  • Attempts to degrade pentachlorophenol may require inoculation with white rot fungi (such as Phanerochaete chrysosporium or Trametes versicolor).

Required secondary treatments

  • Collection and treatment of volatile compounds;
  • Collection and treatment of runoff water and/or leachate.

Performance

Under optimal biopile conditions, petroleum hydrocarbon contaminated soil, for example, is treated over a time period of six months to two years. The cost of the biopile treatment varies from $30 to $90 per metric ton of contaminated soil.

Measures to improve sustainability or promote ecological remediation

  • Erosion and sediment transport control (i.e. topsoil stockpiling, straw-bale barrier installation, prevention of soil compaction by heavy machinery).
  • Water conservation measures.
  • Waste minimization.
  • Schedule optimization for resource sharing and fewer days of mobilization.
  • Use of renewable energy and energy-efficient machinery (i.e. geothermal, wind or solar energy for extraction).
  • Use of heat produced by blowers for maintaining temperature in the soil piles.
  • Re-use of leachate to promote nutrient addition to soil pile.
  • Rainfall collection for use as irrigation water.
  • Passive venting technologies to supply oxygen to soil piles.

Potential impacts of the application of the technology on human health

Unavailable for this factsheet

References

Author and update

Composed by : Magalie Turgeon, National Research Council

Latest update provided by : Karine Drouin, M.Sc., National Research Council

Updated Date : April 1, 2008

Version:
1.2.1