The only biotechnological in situ technology available for treating unsaturated soil is bioventing. The process scheme is shown in Figure 12.5  and is based on vacuum-enhanced soil vapor extraction. The pressure difference in the subsurface causes an inflow of atmospheric air and therefore an oxygen supply, as needed for aerobic contaminant degradation. Depending on site conditions, nutrients may need to be added, e.g., by sprinkling nutrient solutions on top of the soil or by installing horizontal infiltration drainage above the contaminant soil zone.
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One of the most important tasks of process design is to ensure a sufficient air flow regime within the soil. In particular, the geometry of the exfiltration wells, the necessity of active or passive air injection wells, and the need for ground sealing need to be considered. High contaminant concentrations may clog the soil pores, leading to reduced efficiency of oxygen supply. If this occurs, a pulsed soil vapor extraction method may be of advantage.
If the contaminants to be treated are volatile, the extracted soil vapor has to be treated, e.g., by sorption of the contaminants on activated carbon or by biodegradation within a biofilter. Bioventing is applicable to treating petroleum hydrocarbons, aromatic hydrocarbons, and other comparable contaminants. Because soil vapor extraction dries the soil, the water budget of the soil can be adjusted with the nutrient addition. Optimum biodegradation requires a water content of 40%-60% of the maximum water holding capacity. A lower water content reduces the biodegradation rates, a higher water content leads to water-saturated zones in which air flow is not possible and aerobic biodegradation is prevented.
Bioventing is easy to monitor (Section 12.4). Under optimal conditions degradation rates of about 0.2-20 mg kg-1 d-1 for petroleum products in soils of medium permeability can be achieved. The degradation rates in the processes for the treatment of the saturated soil are influenced by many more parameters; hence; these rates diverge to a much higher extent.
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