IN-SITU BIOLOGICAL TREATMENT FOR CONTAMINATED SOILS
Bio-Venting
A Typical Bioventing system
It is a natural biological process in which aerobically degradable compounds bio-degrade by providing oxygen to existing soil microorganisms.
- Nutrients (e.g. Nitrogen and phosphorous) may be pumped into the soil through the injection wells.
Bio-venting requires:
Uses:
Limitations:
- highly variable dependent on soil surface area and soil type e.g.
- Air is slowly pumped into the contaminated area (in the unsaturated zone) through (vertical) injection wells.
- The number, location, and depth of the wells depend on many geological factors and engineering considerations.
- It is a medium-long-term technology (few months-several years).
- Enhanced by adding heat, water, nutrients and oxygen to increase the growth rate of MOs.
- Nutrients (e.g. Nitrogen and phosphorous) may be pumped into the soil through the injection wells.
Bio-venting requires:
- Sufficient concentrations of native (pre-existing) MOs.
- Air to be passed through the soil at the apt rate: Quickly enough to maintain aerobic conditions (for microbial activity) BUT slowly enough to minimise VOCs rising to the surface.
- Soil pH ~ 6-8 and warm temperatures.
Uses:
- Treats VOCs, petroleum hydrocarbons (adsorbed residuals from LNAPLs (above the W.T.)) non-chlorinated solvents and some pesticides & wood preservatives.
- Contaminants must be in the unsaturated zone of biologically active soil.
- Does not degrade inorganic contaminants (but can change their valence state causing adsorption, uptake, accumulation or stabilisation).
Limitations:
- Not effective if water-table is very close to ground surface.
- Not effective if extremely high moisture content (lowers the air permeability of the soil decreasing oxygen flow) or extremely low moisture content (too little inhibits microbial activity).
- Not when very low temperatures.
- highly variable dependent on soil surface area and soil type e.g.
- A greater surface area requires more injection/extraction wells - increased cost.
- Sand/gravelly strata (better air flow rate) requires less injection/extraction wells - reduced cost.