Fact sheet: Land farming

From: Public Services and Procurement Canada

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Description

Landfarming is an ex situ remediation technique used for the biological treatment of soil contaminated with petroleum hydrocarbons and/or non-volatile organic compounds.

This technique consists of spreading excavated contaminated soil on the ground, in an approximately 30 cm to 45 cm thick layer. This layer of soil may be covered when it is important to control soil water content and/or runoff water. Depending on the characteristics of the underlying soils (soil particle size and permeability) and the depth to the water table, the contaminated soil may be spread directly on the ground surface or on a membrane with an upper-protective layer.

Mixing of the contaminated soil is normally required to maintain aerobic conditions (presence of oxygen), to blend nutrients and amendments, to distribute moisture and to enhance the bioavailability of contaminants. The soil is often tilled using equipment such as a harrow pulled by a farm tractor (hence the name landfarming). During the mixing, volatilization of contaminants into the atmosphere will occur but should be kept to a minimum. Fertilizers that will stimulate bacterial growth and activity, as well as structuring agents that will improve soil porosity and allow control over soil aeration and water content may be added during the soil mixing process.

During treatment, nitrogen compound concentrations, pH and water content must be periodically monitored to ensure optimal conditions for contaminant biodegradation. Contaminated soils must be well drained while maintaining a water content ranging between 50% to 80% of the total water holding capacity of the soil. Irrigation systems, runoff control and leachate collection and treatment systems may be required. The collected runoff water and/or leachate may be either re-used in the treatment process or disposed of. The collected runoff water and/or leachate might have to be treated prior to its disposal. 

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Implementation of the technology

A cleared area (trees and/or other structures are removed) is excavated to predetermined or field-determined limits (or, less frequently, tilled in situ). A lift of contaminated soils is spread over in the landfarm area (usually 30 cm to 45 cm thick layer). 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. The soils are tilled/mixed regularly to aerate and blend the nutrients into the soils. Biological activity degrades contaminants (some fraction of volatile contamination is discharged to the atmosphere). 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 remove or mix shallow contaminated soil for biological treatment. Landfarming 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 specialty 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).

If soil contaminants include volatile compounds, vapour/off-gas controls might be required. Depending on precipitations, contaminant characteristics and depth to groundwater, a leachate collection and treatment system may also be required.

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 landfarm.
  • Typically, a starting load of fertilizer and/or bulking texture additive is tilled, mixed or screened into soils.
  • Relatively modest quantities of fertilizer may be kept onsite, for example, to add to irrigation water over time.
  • The landfarming method relies on traditional/commonly available civil/earthworks/aggregate equipment and methods (including excavators, bar screens, trommel screens), as well as speciality equipment and/or farming implements.
  • Treatment cells may use imported aggregate, geomembrane liners, geomembrane covers, slotted plastic (typically polyvinyl chloride [PVC] or high-density polyethylene [HDPE]) 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.

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, soil treatment areas or track-out for example, may deposit directly on downwind surfaces.
  • Diverted and collected/treated stormwaters 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 soils under treatment;
  • Soils under treatment are deliberately ventilated (by tilling/mixing). As a consequence, off-gassing is expected. 
  • Alternatively, excavation and/or 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), methane (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
  • 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

  • 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

  • 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

  • Suitable for residual contamination treatment.
  • Not suitable for light petroleum hydrocarbons and volatile organic compounds (VOC) because of the high potential for volatilization instead of degradation.
  • Chlorinated or nitrous compounds are difficult to biodegrade.
  • Soil water content should be in the range of 50% to 80% of the total water holding capacity of the soil.
  • Number of microorganisms should remain well above 103 colony forming units per gram of soil.
  • Soil pH should range from 6 to 8 to support bacterial activity.
  • Landfarming requires a large treatment area.

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.
  • Soils may be heated and/or insulated, for example, using electrical heat trace and expanded polystyrene (EPS) panels.

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 technologyExist or Does not exist
Testing
Does not exist
Commercialization
Exist

Target contaminants

Target contaminantsApplies, Does not apply or With restrictions
Aliphatic chlorinated hydrocarbons
Does not apply
Chlorobenzenes
With restrictions
Explosives
Does not apply
Metals
Does not apply
Monocyclic aromatic hydrocarbons
With restrictions
Non metalic inorganic compounds
Does not apply
Pesticides
With restrictions
Petroleum hydrocarbons
With restrictions
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 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 of 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 organics, which typically degrade to CO2 and water (or form biomass), thus doesn’t typically generate deleterious secondary by-products or metabolites.

Limitations and Undesirable Effects of the Technology

  • Requires a large treatment area.
  • The depth of treatment is limited to the depth of achievable tilling (approximately 30 cm to 45 cm).
  • Treatment conditions are highly dependent upon climatic conditions.
  • Soils contaminated with VOC must be pretreated such that volatilization into the atmosphere is minimized.
  • Dust liberated during mixing may cause problems.
  • Depending on the compounds and on initial concentrations, it may be difficult to reduce petroleum hydrocarbons concentrations more than 95%.
  • There is a significant soil handling cost compared to in situ treatment.
  • High initial contaminant concentrations can be toxic for the microorganisms.
  • Significant heavy metal concentrations may interfere with biodegradation.
  • Requires an underlying membrane when the water table is less than 3 m from the ground surface and/or when there is less than 5 m of impermeable soil (permeability of less than 10-6 cm/s) above the groundwater.
  • A ground surface slope is normally required for drainage. A slope exceeding 5% might require berms around the contaminated soil.
  • Whether as a consequence of fertilizer addition, changes in pH, changes in oxidation-reduction potential (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 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.
  • Landfarming, where a shallow lift of soil is spread out over a large area or is tilled in situ, has the potential to generate leachate impacts to groundwater. Landfarming designs should balance infiltration, leaching and attenuation to degrade contaminants before they reach groundwater.  
  • 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.
  • Landfarms are often bermed (surrounded by a low soil wall). If not, storm events can cause the transportation of contaminated soil in runoff.
  • Landfarms 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.
  • 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 nighttime) 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.  

Complementary technologies that improve treatment effectiveness

  • Soil may be vented in situ or in a biopile with an air treatment system prior to landfarming to avoid VOC emissions into the atmosphere. Coarse materials can be separated prior to landfarming of soil and may be washed if necessary.
  • Special techniques, pretreatment (with heat or oxidants), inoculants (microbes and/or fungi) can extend this approach to the treatment of some chlorinated organic compounds, polycyclic aromatic hydrocarbons (PAH) mixtures, perchlorate, wood preservatives (pentachlorophenol), pesticides and/or explosives (Trinitrotoluene [TNT], Royal Demolition Explosive [RDX], High Melt Explosive [HMX])(Where TNT: 2,4,6-trinitrotoluene; RDX: 1,3,5-Trinitroperhydro-1,3,5-triazine; HMX:  Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine).
  • Attempts to degrade pentachlorophenol may require inoculation with white rot fungi (such as Phanerochaete chrysosporium or Trametes versicolor).

Required secondary treatments

  • Runoff collection and treatment system.
  • Leachate collection and treatment system.
  • Gas emission collection and treatment system, if required.
  • Irrigation system.

Application examples

An application example is available at this link:

Performance

Under optimal landfarming conditions, petroleum hydrocarbon contaminated soil, for example, is treated over a time period of 3 months to 2 years.

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.
  • Re-use of leachate and/or runoff to promote moisture content of soils and to also allow nutrient addition to soils.
  • Rainfall collection for use as irrigation water.

Potential impacts of the application of the technology on human health

Unavailable for this fact sheet.

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., eng., Jan McNicoll, M.Sc., P. Geo., exp Services Inc.

Updated Date : March 31, 2017

Date Modified: