The following techniques can be used:
Biological soil extraction is a technique which allows one to clean degradable pollutants in the unsaturated zone. By applying sub-normal pressure or excessive pressure in the soil, the soil air is refreshed by the open air. By way of this refreshment, soil-containing air is inserted into the soil whereby the aerobic biological activity of micro-organisms is stimulated. Aeration of the soil for the stimulation of biological activity, is also referred to as bio-venting.
If volatile pollutants are present, evaporation** may occur whereby polluted air is extracted, which must be purified. If the concentrations in the soil are low enough to allow air to be injected without the risk of dispersion, this is the preference.
The quantity of air to be injected or extracted is determined by the quantity of present degradable product. The related sub-normal pressure or over-pressure is determined by the permeability of the under-soil.
If the pollutants are partly present in the saturated zone, then the unsaturated zone can be increased in size by extracting ground-water. This can be carried out via a separate ground-water and soil air extraction system or in a combined ground-water and soil air extraction system (bio-slurping), where air and water is simultaneously extracted through a filter.
Soil air can be extracted or infiltrated via vertical filters or horizontal drains. If there are buildings present, horizontal drains could possibly be placed under the buildings via controlled rotary drilling or persboringen**, in order to achieve a targeted extraction of air.
Implementation area and implementation conditions
Pollutant type
Pollutants that are biologically degradable can be cleansed via this technique. Oil pollutants with a chain length in excess of C30 should not be considered for in-situ cleansing via air extraction/bio-venting.
Compounds with a vapour tension of at least 100 N/m², or a Henri Coefficient of 0.01 (dimensionless), are not only stimulated for break down, but are also volatilised.
Potential floating or sinking layers must be removed prior to clean up because they prevent clean up from functioning.
Soil type
Soil must have a minimum permeability of 10-6 m/s. For permeability in excess of 10-5 m/s, soil air extraction can be well implemented. A heterogeneous soil structure can have a negative effect on the clean up time as well as the clean up result, because less permeable layers have lower amounts of air flowing through them.
Heterogeneity can also greatly change the radius of influence. Greater certainty can be attained on this front by carrying out an on-site pilot test. The permeability of a homogeneous soil that is difficult to permeate, can possibly be increased by implementing fracturing.
For optimal biological degradation, the soil must contain a dry-matter content of circa 95 A high organic matter content (> 1 %) and the presence of iron, can lead to a part of the oxygen causing oxidation in the components, and is thus unavailable for the micro-organisms.
Volatile pollutants
In well permeable soils (> 3x10-5 m/s) with a low organic matter content (< 0,3 %), the backround value is attainable. If the soil is less permeable (< 3x10-5 m/s), has a heterogeneous composition or contains a high organic matter content ( 0.3 %), a high end content can be expected.
Degradable pollutants
In well permeable soils (> 3x10-5 m/s) with a low organic matter content (< 0.3 %), the background value is attainable for benzenes (BTEX compounds). For heavier oil pollutants (hydrocarbon chains > C15) the background value is not attainable, though all environmentally harmful risks can be eliminated. If the soil is less permeable (< 3x10-5 m/s), has a heterogeneous composition or contains a high organic matter content ( 0.3 %), a high end content can be expected.
Costs
The costs of in-situ clean up via soil air extraction are determined by the type of pollutants and the quantity of the pollutants. The exploitation costs for a basic implementation of max. 500 m³ per hour and 200 millibar sub-normal pressure, amounts to €750 – 1500 per month, without piping and air treatment.
For the placement of air extraction filters, one should consider € 50 to 100 per meter and € 15 to 200 per meter for a drain.
The following costs can be used for full-scale in-situ clean up via air extraction/injection (costs in euro/m³):
|
Total m³ |
< 1.000 m3 |
1.000 to 5.000 m3 |
< 5,000 m3 |
|
Benzenes (C6-C14) |
400 |
300 |
200 |
|
Heavier oil (C9-C30) |
300 |
200 |
150 |
|
Chlorine-based solvents |
500 |
350 |
300 |
Environmental burden and measures to be implemented
The environmental yield of in-situ clean up is generally high in comparison with clean up via excavation. If the pollutant is volatile or degradable, the clean up can be aimed at maximising biological break down. This increase in environmental yield brings little energy and clean up costs, but can lead to a longer clean up time.
The energy use of a soil air extraction unit for circa 200 m³/hour is around 2 kWh. No residue is produced with this technique.