Fact sheet: Bioventing

From: Public Services and Procurement Canada

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Description

Bioventing consists of increasing soil aeration to stimulate in situ biological activity and enhance the aerobic biodegradation of residual contamination in the unsaturated (vadose) zone. This technique is suitable for all chemical compounds that are biodegradable under aerobic conditions.

The objective of a bioventing system is to enhance contaminant biodegradation by injecting air in the subsurface soils while minimizing contaminant volatilization. The rate of air injection or extraction is calculated and monitored in order to provide the required volume of air for optimal bacterial activity. There are two methods of bioventing: air injection and air extraction. The advantages of an air injection system over an air extraction system are: a reduction of contaminant volatilization; augmentation of the radius of influence; and, lower costs. However, an air injection system is not recommended when there are buildings and/or underground structures within or near the contaminated site.

Occasionally, bioventing systems may include the use of water irrigation (with or without the use of nutrients), gaseous nutrients (nitrous oxide [N2O], triethyl phosphate [TEP]), moisture addition or the use of gases other than air (methane [CH4], propane [C3H8], hydrogen [H]). Such applications are rare. Methane injection in order to stimulate the cometabolic destruction of chlorinated solvents (tetrachloroethene/perchloroethylene [PCE], trichloroethene [TCE], 1,1,1-trichloroethane [TCA] and 1,2-cis dichloroethylene [DCE] for example) has been demonstrated, but is unusual.

Bioventing using air extraction is not to be confused with soil vapour extraction (SVE) which uses relatively large amounts of energy to create relatively high subsurface vacuums and gas flow rates, stripping volatile contaminants out of the soil (refer to the SVE Technology Factsheet for more information). Bioventing uses lower energy inputs to refresh the subsurface supply of oxygen to stimulate biodegradation. Low energy “passive” bioventing systems and solar-powered bioventing systems have been successfully demonstrated. Passive bioventing utilizes the difference between gas pressure in unsaturated soil and in the atmosphere to move air into or out of vent wells.

Internet links:

Implementation of the technology

Bioventing induces the flow of air (sometimes mixed with another gas), in the subsurface soils, above the water table (vadose zone), to stimulate the biodegradation of contaminants. Bioventing systems may include the following:

  • Mobilization, site access and temporary facilities;
  • Air extraction or injection point installation;
  • Installation of gas conveyance pipelines, a vacuum system and (as required) air emission controls for an extraction system;
  • Installation of individual air injection blowers or a central blower and distribution pipe system for an injection system;
  • Air injection or extraction;
  • Equipment removal and extraction/injection point decommissioning.

The vast majority of bioventing systems use air as the working gas. However, cometabolic remediation of chlorinated solvents has been demonstrated using injected methane or propane gas.

Bioventing is based on adapted monitoring well and gas-handling technologies. Pre-assembled systems complete with air emissions controls are commonly available in trailers, shipping containers or on skids. Bioventing systems may require only electrical power and fresh air for operation.

Wells, trenches, permeable drains or other structures are used to extract or inject air. If air is injected, monitoring points should be used to verify that deleterious vapour migration is not occurring. If air is extracted, it is typically subjected to treatment and then, once treated, discharged into the atmosphere. Since extracted air is typically moist, it is directed through an air/water separator before treatment. Treatment systems are commonly comprised of combustion (thermal oxidation, catalytic oxidation) or filtration/sorption (activated carbon, biofiltration) units.

Materials and Storage

Bioventing typically require very little on-site storage. Storage could include auxiliary fuel (such as natural gas or propane for thermal oxidizers) or air treatment media (i.e. sorbent which is usually granular activated carbon). 

Waste and Discharges

  • If air extraction and treatment are used, spent air treatment sorbent (for example, activated carbon) could require periodic off-site transport and regeneration or disposal. This is also true for the drained contaminated water from the air/water separator.
  • Treatment of extracted vapours, before discharge to the atmosphere, using oxidation systems, if improperly configured, can discharge products of incomplete combustion.
  • Oxidation of chlorinated organic produces acid gases. These are typically managed with a caustic scrubber. In such a case, scrubber water would then require periodic neutralization and disposal.

Recommended analyses for detailed characterization

Biological analysis

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

Chemical analysis

  • pH
  • Organic carbon content
  • 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
  • Electron acceptor concentrations/reaction by-products including:
    • dissolved oxygen
    • nitrate
    • sulfate
    • ferric and ferrous iron
    • methane
    • dissolved manganese

Physical analysis

  • Vadose zone oxygen, nitrogen dioxide, and methane concentrations
  • Soil water content
  • Soil granulometry
  • Contaminant physical characteristics including:
    • viscosity
    • density
    • solubility
    • vapour pressure
  • Presence of non-aqueous phase liquids (NAPLs)
  • Evaluation of biological conditions and ecological factors

Recommended trials for detailed characterization

Biological trials

  • Microcosm mineralization trial
  • In situ respirometry trials
  • Biodegradation trial

Physical trials

  • Gas permeability trials
  • Vapour survey
  • Evaluation of the radius of influence
  • Airflow rate
  • Evaluation of operating pressure/vacuum

Hydrogeological trials

  • Pneumatic trials
  • Tracer tests

Notes:

Tests examining the effect of temperature change on hydraulic conductivity and establishing the zone of freezing with a pilot scale tubing system are recommended to properly design the full-scale containment system.

Other information recommended for detailed characterization

Phase II

  • Contaminant delineation (area and depth)
  • Presence of receptors:
    • presence of potential environmental receptors
    • presence of above and below ground infrastructure
    • the risk of off-site migration

Phase III

  • Soil stratigraphy
  • Identification of preferential pathways for contaminant migration
  • Volume of contaminated material to treat
  • Conceptual site model with hydrogeological and geochemical inputs
  • Characterization of the hydrogeological system including:
    • the direction and speed of the groundwater flow
    • the hydraulic conductivity
    • the seasonal fluctuations
    • the hydraulic gradient

Applications

The bioventing remediation technology is suitable when the following conditions are met:

  • Contaminants are present in the vadose zone;
  • Contaminants can be biodegraded or transformed under aerobic conditions (presence of oxygen);
  • Contaminants have an atmospheric vapour pressure of less than 0.5 mm Hg;
  • Soils have permeability greater than 0.1 Darcy (9.869233x10-14 m2) to permit air and gas movement. When the soil permeability is below 0.1 Darcy (9.869233x10-14 m2), field trials must be performed.

Applications to sites in northern regions

  • Passive bioventing and/or self-contained low-intensity bioventing systems may be viable in northern environments. 
  • Extreme cold can hamper biodegradation and volatilization in shallow soils, but deep soil temperatures are relatively constant over the course of the year.
  • Northern systems require climate-appropriate design, including consideration of deep frost, permafrost, seasonal changes in soil saturation and air permeability and long periods without operator intervention, fuel supply and sorbent removal.

Treatment type

Treatment typeApplies or Does not apply
In situ
Applies
Ex situ
Does not apply
Biological
Applies
Chemical
Applies
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 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;
  • Non-metallic inorganic compounds: suitable for ammoniacal nitrogen only.

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

Long-term considerations (following remediation work)

Few to none. 

Secondary by-products and/or metabolites

Biodegradation of organic contaminants does not typically generate any deleterious secondary by-products or metabolites, as it results in innocuous by-products including carbon dioxide and water.

Although, vinyl chloride and cis -1,2-dichloroethylene are produced in the biodegradation of trichloroethylene (TCE) and tetrachloroethylene/perchloroethylene (PCE).

Limitations and Undesirable Effects of the Technology

  • Vadose zone treatment only;
  • Soil permeability must be greater than 0.1 Darcy (9.869233x10-14 m2);
  • Water tables must be lower than 1 metre from the soil surface;
  • Preferential pathways and/or lower permeability soil horizons limits the uniformity of air distribution;
  • Only the air extraction method is suitable when there are buildings or underground structures within or near the contaminated site limits;
  • Low nutrient concentrations can limit contaminant biodegradation;
  • Low soil water content limits microbial activity, and high soil water content reduces soil gas permeability;
  • The soil pH should be between 6 and 8;
  • High contaminant concentrations may reduce contaminant biodegradation rates and extents;
  • Fluctuations in the water table limit the application of bioventing;
  • Extraction bioventing can cause groundwater upwelling (for example a groundwater mound) which can partially submerge extraction well screens and in turn reduce extraction capacity;
  • May increase microbial biomass, metabolites and waste products;
  • Injection bioventing has the potential to force gas through, for example, preferential pathways such as foundation drains, utility trenches backfill, etc.;
  • Uncontrolled vapour migration can impinge on off-site receptors;
  • Possible vapour migration is a concern with injection bioventing;
  • Mobilization of naturally occurring radon is theoretically possible (but is generally not an issue of significance).

Complementary technologies that improve treatment effectiveness

  • The addition of dissolved or gaseous nutrients (N2O, TEP) in the vadose zone through trenches or injection wells can, in certain cases, enhance the biodegradation process;
  • The addition of microbial strains (such as bioaugmentation) can also have an impact on mineralization processes or the transformation of contaminants. Laboratory trials and small-scale assays are recommended before the addition of such microbial strains. Regulatory agencies also need to be consulted before the addition of microbial strains can be performed;
  • Bioventing may be enhanced by the addition of heat, by fracturing subsurface soils to increase air flow (pneumatic or hydraulic fracturing), or by sealing the ground surface to prevent “short-circuiting” (with asphalt pavement for example);
  • Bioventing is effective only where gas flow in soil (within the non-saturated [vadose] zone) can be induced. To extend treatment below the water table, dewatering or air sparging may be used (refer to the Biosparging Technology Factsheet for more information). 

Required secondary treatments

  • The air extraction method of bioventing requires a treatment system for the extracted gas;
  • Although not recommended if an air injection bioventing system is used on a site where buildings and/or underground infrastructure is present, an air extraction system, such as a soil vapour extraction system, must be installed.

Application examples

Application examples are available at these links:

Performance

The treatment time to lower contaminant concentrations required by a bioventing system is highly variable and depends upon many parameters including: contaminant properties, the natural bacterial population, the physical and chemical properties of the soil and vadose zone and the design of the bioventing system. Bioventing systems are more effective when treating low molecular weight, less hydrophobic contaminants.

Measures to improve sustainability or promote ecological remediation

  • Pump size optimization;
  • Schedule optimization for resource sharing and fewer days of mobilization;
  • Use of renewable energy and energy-efficient machinery for extraction (i.e. geothermal or solar energy);
  • If the site is a landfill generating methane gas, consider use of microturbines for converting methane gas to heat and generate electricity for local establishments;
  • Passive or pulsed bioventing;
  • Promote air injection mode as opposed to air extraction mode to avoid extracted air treatment and in turn uses lower energy and eliminates generated wastes;
  • Use a biofilter for air treatment;
  • Allow longer treatment time to avoid operation in winter conditions (avoid having to winterize the system) and thus lowers energy requirements;
  • Evaluate pulse vs continuous air injection;        
  • On-site treatment of condensate and purged water from air conduits (extraction mode);
  • Minimizing site visits by the use of telemetry for remote monitoring of site conditions.

Potential impacts of the application of the technology on human health

Unavailable for this factsheet

References

Author and update

Composed by : Serge Delisle, Eng. M.Sc., National Research Council

Updated by : Karine Drouin, M.Sc., National Research Council

Updated Date : January 1, 2008

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: