SOIL, SEDIMENT AND SLUDGE: IN-SITU THERMAL TREATMENT
Thermally Enhanced SVE
Hot Air/Steam Injection
Process:
- A full-scale technology that uses hot-air/steam injection to increase the volatilisation rate of semi-volatiles and facilitate extraction.
- It can also use electrical resistance/electromagnetic/fiber optic/radio frequency heating.
- The process requires heat resistant extraction wells, but is otherwise similar to standard SVE
- Normally a short- to medium-term technology.
Hot Air/Steam Injection Process:
- Hot air/steam is injected below the contaminated zone to heat up contaminated soil & enhance the release of contaminants.
- The VOCs are stripped from contaminated zone and brought to the surface through soil vapour extraction.
- All contaminants are treated in situ & under a vacuum during operation - reduces possible contaminant release.
- In thermally enhanced SVE, the heating evaporates the water & improves the air flow - thus effective even if soil has high moisture content - advantage over standard SVE.
The target contaminant treatment groups: SVOCs & some pesticides and fuels (dependent on temperature).
Limitations:
- Uneven treatment if variable permeability of soil.
- Residual liquids and spent activated carbon may require further treatment.
- Not effective in the saturated zone.
- Highly dependent upon the specific soil and chemical properties of the contaminated media.
- The technology is self limiting; as the soil heats and dries, current will stop flowing. The extracted vapour then requires other treatment (e.g. granular activated carbon or incineration).
Cost:
-Heavily dependent on the soil type and the depth to the contaminated area:
- Silts/silty clays - generally less permeable - less gas flow required - cheaper treatment
- A deeper and thicker region of contaminated soil has higher remedial cost.
Alternative methods of heating:
:1) Electrical Resistance Heating
An electrical current is used to heat less permeable soils (clays/fine-grained sediment)
- Water and contaminants are trapped & vapourised for vacuum extraction.
- Electrodes are placed directly into the less permeable soil and activated so the electrical current passes through the soil to heat it.
- The heat dries out the soil causing it to fracture - makes the soil more permeable, hence allowing the contaminants to be removed.
- The heating also forces the trapped liquids to vapourise and move to the steam zone - stripping the contaminants from the soil.
Increasing the temperature of the soil and contaminant, increases the soils permeability & the contaminant's vapour pressure, hence increasing the removal rate.
2) Radio Frequency/Electromagnetic Heating
Rows of vertical electrodes embedded in the soil are used to heat a discrete volume of soil and enhance soil vapour extraction (SVE).
- Heated soil volumes are bounded by two rows of ground electrodes & energy is applied to a third row midway between the ground rows.
- The three rows act as a buried triplate capacitor.
- When energy is applied, heating proceeds vertically downward and laterally outward from the centre.
- The technique can heat soils to >300°C.