IN-SITU BIOLOGICAL TREATMENT OF CONTAMINATED SOILS
Enhanced Bio-Venting/Air Sparging
Typical Enhanced Bio-Degradation System
Enhanced by adding nutrients or injecting hot air/more electron acceptors to optimise the growth of indigenous MOs. It may be either aerobic or anaerobic:
1) Aerobic Conditions:
With injected oxygen (air-sparged) and nutrients, the MOs degrade the contaminant (at a quicker rate) to form CO2, H20 and microbial cell mass.
-Typically a long-term technology (several years for cleanup of a plume).
Enhancement methods:
Addition of oxygen, water, nutrients and/or heat may be done using:
- Water (saturated with oxygen & nutrients) injection wells (if deeper contamination) or spray irrigation (if shallow contamination).
- sometimes hydrogen peroxide (H2O2) is added as an alternative - oxygen rich compound.
- heat blankets covering the soil surface to reduce heat escaping from the soil.
2) Anaerobic Conditions:
Electron acceptors (other than oxygen) &/or nutrients added the biodegrade the contaminants to methane (CH4), much less CO2 & trace amounts of hydrogen gas. Less common conditions, but is very effective at remediating contamination from munitions waste (more persistent).
- Under sulphate-reduction conditions, sulphate is converted to sulphide/elemental sulphur
- Under nitrate-reduction conditions, di-nitrogen gas is produced.
Uses:
Advantages:
Disadvantages/Limitations:
Cost:
Highly variable - dependent on the soil type & chemistry and type & extent of contamination.
- Typically £20-£80/cubic metre.
1) Aerobic Conditions:
With injected oxygen (air-sparged) and nutrients, the MOs degrade the contaminant (at a quicker rate) to form CO2, H20 and microbial cell mass.
-Typically a long-term technology (several years for cleanup of a plume).
Enhancement methods:
Addition of oxygen, water, nutrients and/or heat may be done using:
- Water (saturated with oxygen & nutrients) injection wells (if deeper contamination) or spray irrigation (if shallow contamination).
- sometimes hydrogen peroxide (H2O2) is added as an alternative - oxygen rich compound.
- heat blankets covering the soil surface to reduce heat escaping from the soil.
2) Anaerobic Conditions:
Electron acceptors (other than oxygen) &/or nutrients added the biodegrade the contaminants to methane (CH4), much less CO2 & trace amounts of hydrogen gas. Less common conditions, but is very effective at remediating contamination from munitions waste (more persistent).
- Under sulphate-reduction conditions, sulphate is converted to sulphide/elemental sulphur
- Under nitrate-reduction conditions, di-nitrogen gas is produced.
Uses:
- Most commonly treats PAHs, non-halogenated SVOCs & BTEX - e.g. contaminated by wood preserving processes & petroleum refining industries.
- Not for inorganics - however, again, change their valence state to contain them.
Advantages:
- Can treat contaminated ground water & soil simultaneously - reduced cost.
- Readily treats petroleum hydrocarbons.
- Enhancing nutrients are readily available & low cost.
- Generally no heating required.
- Rarely generates residuals - no need for additional treatment/disposal.
- Cheaper treatment of non-halogenated SVOCs - compared to many other techniques.
Disadvantages/Limitations:
- Generally longer-term process (years).
- Less certainty of uniform treatment (soil & aquifer variability).
- Difficult to monitor progress.
- Risk the contaminant becomes more mobile & leaches into the ground water.
- Does NOT degrade inorganics (can change their valence state - causing adsorption, immobilisation, uptake or accumulation - makes further treatment easier).
- Not effective if high concentration of heavy metals, highly chlorinated organics, long chain hydrocarbons or very low temperature.
- Not used if clay, highly layered or heterogeneous ground conditions.
- Treatability & feasibility tests are required to determine whether enhanced bioremediation is feasible in a given situation - dependent on contaminants & soil characteristics.
- **The contaminants may degrade to intermediate products that are in fact more hazardous than the original contaminant (would require further treatment/aerobic conditions to be created).
Cost:
Highly variable - dependent on the soil type & chemistry and type & extent of contamination.
- Typically £20-£80/cubic metre.