Both biosparging and bioslurping are not restricted to treatment of the saturated zone, but are also used for treatment of the unsaturated zone. Biosparging is the injection of atmospheric air into the aquifer (Fig. 12.6) [4, 5], which results in the formation of small branched channels through which air moves to the unsaturated zone. Outside these channels all processes are limited by diffusion. Therefore, highly branched channels are desired, which can be achieved by pulsing the air injection. Biosparging enhances the in situ stripping of volatile contaminants, desorption of contaminants, and their degradation by enriching the groundwater with oxygen. Because the contaminants are transported to the unsaturated zone, biosparging is usually combined with soil vapor extraction. Biosparging is applicable if the sparging point can be installed below the zone of contamination, because air flows upward, forming a cone. The angle of the cone and the degree of branching of the channels in a given soil depend mainly on the injection pressure, which should be only a little higher than the pressure of the water column. Usually the radius of influence of a biosparging well is determined by a pilot test at the site. At various distances from the sparging point, additional wells are installed to monitor groundwater level and
oxygen saturation. An increase of the groundwater table is observed only in the beginning of the treatment, and after the air channels are formed the groundwater returns to its original level. Biosparging is very sensitive to soil inhomogeneities. Zones of lower permeability may deflect the air channels. Zones of high permeability may act like open pipes channeling the air flow. In both cases the soil above these zones remains untreated. Additional nutrients to enhance microbiological degradation may be infiltrated with the same biosparging wells or with separate infiltration wells.
Bioslurping (also known as vacuum-enhanced recovery) is the only technology that also treats free product phases floating on top of the groundwater [3, 6]. Bioslurping wells (gas-tight at the well head) are mainly screened within the groundwater fluctuation zone. A suction pipe is directed within the free phase. By applying a vacuum, a mixture of free product, soil vapor, and groundwater is extracted. Above ground all three media (free product, waste air, and water) are separated. Free product is collected, and water and waste air are cleaned separately. The treatment plant can be small and run at comparatively low cost because only a small amount of groundwater and soil air is extracted. The main advantage of bioslurping is the horizontal flow of the free product. Compared to conventional recovery systems, e.g., by hydraulic gradients toward a pumping well, bioslurping does not enhance the smearing of the free phase to greater depths of the aquifer. The accompanying extraction of soil vapor leads to enrichment of the soil with oxygen, comparable to bio-venting. Hence biodegradation in this zone is enhanced. Bioslurping can be com bined with infiltration of nutrient salts into the vadose zone or into the saturated zone.
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