Fact sheet: Aerobic composting

From: Public Services and Procurement Canada

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Description

Composting is an ex situ remediation technique to treat soil contaminated with organic contaminants. Composting is performed at elevated temperatures (under thermophilic conditions, between 54 °C to 65 °C), under aerobic conditions. This technique is efficient to treat contamination with fuel-range organic, explosives (Trinitrotoluene [TNT], Royal Demolition Explosive [RDX], High Melt Explosive [HMX]), volatile and semi-volatile organic compounds (VOC and SVOC), and other biodegradable organic compounds.

Composting consists of mixing contaminated soil with structuring agents (such as woodchips, hay and manure), organic fertilizers, and plant residues. Humidity, temperature, porosity, pH, oxygen concentrations, carbon concentrations and contaminant concentrations must be monitored to optimize contaminant biodegradation. Indigenous microorganisms are typically used; some practitioners add exogenous cultures (with mixed results).

There are three distinctive composting methods:

  • aerated static pile composting (composting is formed into piles and aerated by an injection or extraction system, in other word biopile). Refer to “Aerobic Biopile” Factsheet for more information regarding this technology;
  • mechanically agitated in-vessel composting (a reactor mixes the compost to enhance aeration); and,
  • windrow composting (long row of compost with periodic mechanical turnover). Windrow composting is considered to be the most cost-effective alternative, but it may also have the highest levels of fugitive emissions of VOC.

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Implementation of the technology

Excavated contaminated soils are stockpiled, required soil amendments are added and mixed in the soils. Microbial activity is stimulated with fertilizer, pH control, moisture control, temperature control, inoculation with seed material and/or soil texture improvements. The soils are mixed regularly to aerate and blend the nutrients into the soils. 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 pile/windrow for its biological treatment onsite. The 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, farming 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).

Compost pile construction may 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, wood chips, 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 either by turning or 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.

When the contaminated soil contains volatile or semi-volatile organic compounds, it may be necessary to install a vapour collection and treatment system. Depending on precipitation, contaminant characteristics and depth to groundwater, a membrane may be required under the compost pile in order to prevent the migration of contaminants into the aquifer. Leachate and runoff collection and treatment systems may also be required.

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 compost piles.
  • 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.
  • Natural biodegradation, particularly for hydrocarbons, may also produce off-gases other than contaminants. Such off-gasses may include carbon dioxide (CO2), ammonia (NH3), methane (CH4) and/or hydrogen sulfide (H2S).
  • Off-gassing of VOC and SVOC compounds is expected. If contaminants pose an unacceptable downwind risk, the treatment system will normally include vapour collection with treatment. 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.

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

  • Vapour survey
  • Evaluation of optimal mixing rates

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

  • Applies to organic compounds that are biodegraded under aerobic conditions.
  • Treatment is generally performed at high temperatures ranging from 54 °C to 65 °C.
  • The soil pH should range from 6 to 8 to support bacterial activity.
  • Suitable for certain recalcitrant contaminants (explosives, polycyclic aromatic hydrocarbons [PAH], etc.).
  • VOC and odours must be controlled.

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.
  • Compost piles 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
Applies
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
Applies
Metals
Does not apply
Monocyclic aromatic hydrocarbons
With restrictions
Non metalic inorganic compounds
Does not apply
Pesticides
With restrictions
Petroleum hydrocarbons
Applies
Phenolic compounds
With restrictions
Policyclic aromatic hydrocarbons
Applies
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:

Composting may take months to years to complete. 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)

There are 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

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.

Although, specific recalcitrant compounds, such as explosives, may generate toxic metabolites during composting.

Limitations and Undesirable Effects of the Technology

  • Requires a large area for the construction of a composting pile system.
  • Climatic conditions influence treatment parameters and can interfere with the control of the composting pile environment.
  • Volatilization of VOC is possible (and may be deliberate design features) and can affect the surrounding air quality. A vapour collection and treatment system may be required.
  • May require the control, collection and treatment of runoff water and/or leachate. 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.
  • There is an important cost associated with the handling of contaminated soils compared to in situ treatment.
  • High contaminant concentrations may be toxic for microorganisms.
  • Volume of treated soil is greater than the initial volume of contaminated soil due to the addition of structuring agents, plant residues and bulking.
  • Significant heavy metal concentrations may interfere with the biodegradation processes while heavy metal and recalcitrant organic compounds may remain in the treated soil.
  • 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.
  • With well-designed composting piles (in other words, designed in reference to contemporary guidance) and/or through the use of bench and pilot testing, metal mobilization is typically not a major issue.
  • 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, but fires are rare. Enhanced desorption or volatilization are possible (and may be deliberate design features).
  • 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.
  • Compost piles 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.

Complementary technologies that improve treatment effectiveness

Special techniques, pretreatment (with heat or oxidants), cometabolites (methane or propane 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.

Required secondary treatments

  • Collection and treatment of volatile compounds, if required

Application examples

An application example is available at this link:

Performance

Comparing all three composting methods, the windrow composting method (long row of compost with periodic mechanical turnover) is generally the most efficient and cost-effective process. Soils contaminated with explosives or PAH have been successfully treated by compositing.

Measures to improve sustainability or promote ecological remediation

  • Erosion and sediment transport control (such as 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 (such as 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.

References

Author and update

Composed by : Magalie Turgeon, National Research Council

Updated by : Jennifer Holdner, M.Sc., Public Works Government Services Canada

Updated Date : March 1, 2015

Latest update provided by : Daniel Charette, P.Eng., ing., Chris T. Kimmerly, M.Sc., P. Geo., exp Services Inc.

Updated Date : March 31, 2017

Version:
1.2.1