Public Services and Procurement Canada
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:
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:
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.
Chlorobenzenes: suitable for chlorobenzene, dichlorobenzene and trichlorobenzene
Phenolic compounds: suitable for cresol, pentachlorophenol and tetrachlorophenol
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%).
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.
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.
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.
An application example is available at this link:
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.
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