Principle
The following techniques can be used:
Figure: Bio-sparging diagram
Bio-sparging is a technique in which air is injected beneath the ground-water table. By way of this injection, the water is enriched with oxygen and the biological break down is stimulated. The volatilisation of pollutants also takes place. It is typical for the injected air quantities to be lower than those in sparging due to the volatilisation of pollutants.
Possible volatilised compounds can also be broken down in the unsaturated zone. If volatile degradable pollutants are present and there are objects that could possibly be endangered in the surroundings (houses), a soil air extraction system must be installed in order to combat dispersion of volatilised compounds. The extracted air must be purified.
Air is injected by using compressors with pressure valves (up to 10 bar) or blowers (up to 0.8 to 1 bar), whereby the injection pressure (mostly varied between 0.5 and 2 bar) is determined by the depth beneath the ground-water table and the permeability of the soil.
Pulsating injection of air provides better results (greater radius of influence) than continuous injection. In general, vertical air injection filters are used. Horizontal air injection filters, in long filter lengths (> 2 metres), lead to unequal dispersion of the inserted air.
The radius of influence is determined by the soil construction (stratification), but can be varied by the depth of the filter in relation to the ground-water table and via the injection pressure and the quantity of air that is injected. In general, as a rule of thumb, it is accepted that the radius of influence of a compressed-air filter is directly proportionate to the depth of the filter under the ground-water table (up to depths of circa 10 m beneath ground-water). If greater certainty is require with regards to the radius of influence, this can be done via a short-term field experiment.
The volume of air to be injected is determined by the oxygen demand from the soil volume that is influenced by air injection and by the total load of pollutants present. The oxygen demand can be determined via oxygen measurements in the ground-water (respiration). In general, injection volumes of 5 to 20 m³/hour per filter are implemented during air injection.
One benefit of this technique is that water does not have to be extracted. However, there is still very little known about the extent to which air injection causes a flow of ground-water. To date, investigations have shown that only a limited flow of ground-water is caused by sparging.
Implementation area and implementation conditions
Pollutant type
This technique can be used for pollutants that are degradable in aerobic conditions (such as oil pollutants with a chain length up to C30 and aromatics).
In addition to this, volatile compounds (oil pollutants smaller than C12 and chloroethanes) with a vapour tension of at least 100 N/m³ or a Henri Coefficient of 0.01 (dimensionless) can also be removed with this technique as a result of evaporation. Non-volatile floating layers must be removed prior to clean up.
Soil type
Soil must have a minimum permeability of 0.1 m/day. For permeability in excess of 1 m/day, compressed-air injection can be well implemented. However, a heterogeneous soil construction can have a negative impact on the clean up result because less permeable layers are more difficult for air to flow through and because air run off via these layers leads to an uncontrolled situation.
Thus in heterogeneous conditions it could be more beneficial to begin with a pilot-plant, so that uncertainties can be further investigated.
As a result of air being injected, iron present in the soil will oxidise (from Fe(II) to Fe(III)). Practical data does not a show negative impact of high iron contents in the ground-water on the effectiveness of compressed-air injection.
With the help of compressed air injection, concentrations in the soil and the ground-water can be reduced to 15% of the start concentration within two to four years. After sealing, the sparging, concentrations in the ground-water, namely in less homogeneous and fair to difficult-to-permeate 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.
The contents in the soil can be less efficiently reduced than in bio-venting/soil air extraction because the scale of aeration and the density of air channels is smaller.
Costs
Bio-sparging is a relatively cheap technique for adding oxygen to ground-water, for the stimulation of aerobic pollutant break down. This explains why the basic technique is successful and why so much research has been dedicated to it.
The costs of implementing compressed-air injection amount to an average of 12.5 to 25 euro per m³, excluding air extraction and purification.
The placement of air injection filters costs circa 250 to 500 euro. The placement of an air injection installation costs circa 5000 to 10,000 euro.
The cost of a possible attainability test for (bio) sparging amounts to circa 12.500 to 20.000 euro.
Environmental burden and measures to be implemented
For the injection of air into soil under high pressure, a relatively large quantity of energy is required; 10 to 50 kWh per m³ of soil. For the insertion of air at lower pressures via blowers, the energy use is considerably less (5-20 kWh per m³).
The energy consumption of soil air extraction for about 200 m³/hour is about 2 kWh.
No residue is produced during this technique.
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.