SOIL, SEDIMENT AND SLUDGE: IN-SITU CHEMICAL/PHYSICAL TREATMENT
In-Situ Soil Vapour Extraction (SVE)
Typical In Situ Soil Vapour Extraction System
Process:
A vacuum is applied through extraction wells to create a pressure/concentration gradient - this induces gas-phase volatiles to be removed from soil through the extraction wells.
- Vertical extraction vents (to depth ~1.5-90m) are typically used.
- Geo-membrane covers are often placed over soil surface to increase the radius of influence of the wells (& prevent short circuiting).
- Ground water depression pumps may be used to reduce ground water up-welling (due to vacuum) or to increase the depth of the vadose zone.
- Air injection (air-sparging) enhances the process allowing extraction of deeper contamination, contamination in low permeability soils & contamination in the saturated zone.
- Typically medium-long-term duration (1-3years).
Uses:
- Removes/treats VOCs, some SVOCs & some fuels from unsaturated (vadose) zone.
- Will not remove heavy oils, metals, PCBs, or dioxins.
Limitations:
- Would require higher vacuums If soil has a high percentage of fines/high degree of saturation, - increasing cost.
- Reduced rate of treatment if soil has high organic content/high sorption capacity of VOCs.
- Uneven treatment.If highly variable soil (i.e. permeability/stratification)
- In-effective in the saturated zone (can lower W.T.by groundwater depression pump or enhance process by air injection).
- Off-gas requires further treatment (possibly hazardous).
- After SVE, other remedial measures (e.g. bioremediation) may be required to further reduce concentration levels.
Cost:
Highly variable - dependent on the depth/areal extent of contamination, the concentration of the contaminants, depth to water table, and soil type and properties - all of which govern the duration of treatment & no. of wells required.