Method
This concept can be execuded by using the following techniques:
- extraction of air via vertical filters
- extraction of air via horizontal filters
- installation of a top-sealing
- discontinuous (steering all filters at the same time or individual per filter) injection via PLC steering
- purification of extracted air
- lowering ground water level to enhance the unsaturated zone (optional)
- purification of extracted ground water (optional)
- installation of a top-sealing (foil of hardened) (optional if underground is not hardened )
Scheme Soil air extraction
In the implementation of soil air extraction, volatile compounds are removed from the soil by sucking out the soil air. By lowering the concentrations in the soil air, a balance between the soil and the gas phase will be re-instated, which also leads to a lowering of concentrations in the soil. By continuously refreshing the soil air, the soil as well as the (upper) ground-water can be cleansed. The extracted air is cleansed above ground. The soil air is extracted via a drain or horizontal or vertical extraction filters which are placed in the unsaturated zone.
The technique is often implemented in combination with ground-water extraction and compressed-air injection in the saturated zone in order to increase the technique’s working area. If the technique is combined with compressed-air injection and/or ground-water extraction, then one or more injection lances or ground-water extraction wells must be installed. Besides for increasing the working area, ground-water extraction is also used to prevent the uncontrolled escape of pollutants via the ground-water.
The extracted soil air can be cleaned via an active carbon filter, a bio-filter or a catalyst. The choice of the cleaning technique that is to be implemented is determined by the type of pollutants and the pollution concentration in the gaseous effluent stream. For an oil-based pollutant with concentrations in excess of 5 g/m³ and a pollution load of more than 500 to 1000 kilogram, a catalytic oxidiser may be more suitable than an active carbon filter. In these circumstances, a regenerative active carbon filter is often used for halogenated hydrocarbons In other cases, an active carbon filter may be sufficient. The extracted ground-water is (after eventual remediation) discharged onto surface water - where diluted waste water is concerned - or into the sewer.
Implementation area and implementation conditions
Where the basic objective of soil air extraction is to create air flows in the soil, the permeability of the soil becomes one of the most important parameters. For this, the homogeneity and permeability of the soil, as well as the volatility of the pollution, will be critical for the attainability of soil air extraction. Experience in Flanders has shown that soil air extraction can be well implemented in permeability (hydraulic conductivity) greater than 10-3 m/s, but there are also a number of cases where it could be implemented at lower permeability (10-3 – 10-7 m/s).
In principle, soil air extraction can only be implemented for volatile compounds or biologically degradable compounds in the unsaturated zone. The saturated zone could possibly be increased by lowering the ground-water level. The level, up to which polluted substances can still be efficiently extracted via soil air, lies at a vapour tension of approximately 100N/m² (inc. mono-cyclical aromatics and chloroethanes)
A study by the OVAM shows that soil air extraction is, according to Flemish contractors of clean up operations, a technique that can be well implemented in sandy to loamy soils. The technique is not suitable for stronger loams to clay soils.
In addition to this, the addition of oxygen – as a result of refreshment – can lead to the break down of polluted substances (=bio-venting). One benefit of bio-venting for soil air extraction is that only that amount of air which is needed for break down needs to be extracted or refreshed. In general, this is considerably less than the volume required for soil air extraction.
Costs
The costs for soil air extraction are normally lower than those for compressed-air injection and are determined by the concentrations of the extracted air and the air purification. For the ‘active’ variant, extra costs for ventilation and energy have to be added.
For the placement of horizontal extraction drains, the same costs can be adopted as those for ground-water extraction.
Table: Costs for soil air extraction without air treatment, excluding piping (OVB, 2004)
|
Specification |
Costs |
|---|---|
|
Exploitation cost basic implementation: Max 500m³ per hour and 200 millibar subnormal pressure. |
€ 750 – 1500 per month |
|
Filters |
€ 50 – 100 per metre |
|
Drains |
€ 15 -200 per metre |
Environmental burden and measures to be implemented
Emissions can be more or less fully avoided by guiding the extracted air through a purification step. The following systems are available for cleaning extracted air:
- Active carbon filter (with or without regeneration)
- Bio-filter
- Catalytic oxidiser
The choice of the cleaning technique that is to be implemented is determined by the pollution level and the type of pollution in the air. A bio-filter is, for example, suitable for lower concentrations of pollutants like aromatics and benzene.
Emissions into the air during air purification are only present if gas purification has failed (e.g. filter malfunction). However, this can be simply avoided if there is efficient management.
On average, during clean up via soil air extraction, 50 to 250 m³/hour is discharged. Concentrations of polluted substances in the extracted air vary from 100 to 30.000 mg/m³.
In soil air extraction, polluted water may be released in the form of condensation water or extracted ground-water in order to lower the ground-water level. The released ground-water will need to be purified in accordance with the discharge norms. Low concentrations are attainable with the currently available purification techniques. After sealing, concentrations in the ground-water, namely in less homogeneous and fair to difficult to permeable soils, appear to rise once again (rebound). Thus, a few months after stopping clean up activities, the ground-water needs to be re-sampled before the remediation can be finally concluded.
If active carbon is used in the clean up of the gaseous effluent, this may be discharged as a waste stream. If there are high concentrations of polluted substances in then soil, the active carbon could be regenerated, which reduces the waste stream.
Noise will be a source of hindrance. In general, the extraction units are placed in a container, which can be made sound-proof – thus noise problems can be avoided.
In soil air extraction, the environmental burden is determined by energy use and the related air purification. This energy use depends on the volume of the extracted air. Further, the selected air clean up technique plays an important role in the energy used, the quantity of waste produced and the space that is taken.
In bio-venting, as for wet clean up, stimulation of biological processes is relatively favourable in terms of benefits to the environment. The potential energy use for the stimulation of biological processes does naturally need to be calculated.