Principle
The following techniques can be used:
Figure: Diagram of compressed air injection
In-situ injection of compressed air is a technique which involves injecting air under pressure into the soil. Compressed air injection takes place in the saturated zone, whereby volatile pollutants from the water phase are transfered to the compressed air. If air is added, a turbulent zone is created in the soil whereby the contact between the ground-water and soil is increased and the transfer of pollutants is stimulated. The air rises via the pores into the unsaturated zone, where it is collected via soil air extraction. In order to avoid uncontrolled lateral dispersion of pollutants via the ground-water, compressed air injection is normally combined with ground-water extraction.
Implementation area and implementation conditions
To date, the injection of air has only been used in well-permeable heterogeneous soils (permeability > 10-3 m/sec). For layers with a permeability of less than 10-3 m/sec, pollutants can only be removed or broken down via slow diffuse transfer, which means that more time is needed for clean up. The clean up efficiency can be increased by injecting intermittently or by injecting smaller volumes.
If polluted substances are volatile and can be broken down, then it is difficult to make a distinction between compressed-air injection and bio-sparging, considering that a part of the polluted substance is stripped from the saturated zone, after which it is broken down in the unsaturated zone.
Compressed-air injection, with the purpose of stripping the polluted substances in the saturated zone, is almost always performed in combination with soil air extraction in the unsaturated zone. The soil air extraction system collects the stripped polluted substances. In some cases soil air extraction is omitted and the unsaturated zone functions as a bio-reactor
The technique is being more frequently implemented as a supplement to the extraction of ground-water. This supplementary step can lead to an improvement in the clean up yield because a vertical transport component is introduced via air injection to the already existing horizontal component through which the extraction of ground-water is realised.
Costs
Investment costs for implementing compressed-air injection (installation including plc-control and compressors) are greatly determined by the size of the location. The installation costs for filters amount to ca. € 50-100/m. Exploitation costs for basic implementation, for 50 m³/hour and 2 bar over-pressure, amount to € 750-1500 per month.
Environmental burden and measures to be implemented
When air is injected into the ground, a release of this air through the soil surface is possible. This can be prevented by installing a soil air extraction system. The extracted air must be purified.
For the injection of air into soil under high pressure, a relatively large quantity of energy is required. In general the energy use is higher then that of soil air extraction.