104 Environmental Biotechnology
Figure 5.5 Bioventing
the polluting substances. The air flow through the soil is further driven by
vacuum extractors peripheral to the treatment zone, which increases the dissolved
oxygen levels of the soil water and thus facilitates uptake by the native micro-
organisms. Volatile compounds, which are either present as part of the original
contamination, or generated as byproducts of the biological treatment, are often
mobilised during processing and thus more easily extracted. However, in many
practical applications, the air extraction rate is adjusted to maximise decomposi-
tion underground, thus reducing a separate requirement for surface treatment of
volatile compounds.
As with the biosparger, control devices typically regulate the pressure, fil-
ters clean particles from the intake and the flow rate is monitored in operation,
with data loggers and telemetry systems again featuring in the more complex
applications.
Unsurprisingly, bioventing also requires extensive and comprehensive site
investigation before commencement, not least because the proper positioning
of the necessary system of pipework is essential to the proper functioning of
this technique.
Injection recovery
The injection and recovery method, for which a generalised diagram appears
in Figure 5.6, makes use of the movement of groundwater through the zone of
contamination to assist the remediation process. Although, as mentioned in the
introductory comments, this approach shares many functional similarities with
the preceding technologies, it is essentially more sophisticated and refined, with
the biological treatment being effectively divided into two complementary stages.
Thus, what may be considered a ‘virtual’ bioreactor is established within the soil
matrix, with the actual clean-up activity taking place both within the groundwater
and also externally to it.
The major characteristic of this technique is the two-well system sunk into the
ground, the ‘injection well’ and the ‘recovery well’, the former being located