Environmental Engineering Reference
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showed that the autochthonous microflora concentration increased by two
orders. The concentration of nutrients (N, P) in the bioreactor unit was suf-
ficient throughout the pilot testing. On the basis of the results of laboratory
work, it was decided to use autochthonous microflora for the groundwa-
ter decontamination process. The biofilter units of the pilot PRB were not
directly inoculated; autochthonous microflora present in groundwater was
enhanced by aeration and added nutrients.
13.7 Europe's Oldest PRB (Belfast, Monkstown, Northern
Ireland)
In 1994, Europe's first full-scale, first ZVI PRB and first PRB to use an in-
ground reaction chamber (in-ground reactor) was designed and set up in the
United Kingdom. Although the design has been widely adopted and devel-
oped, it should be recognized that the initial concept was designed to meet
specific constraints of the original site. This was an operational industrial
site in Belfast used for the manufacture of electronic components. Historic
spillages of chlorinated solvents had led to an intense-though-localized con-
taminant source. Details of the site setting and initial performance of the
reactor are given elsewhere (Jefferis, 2002, 2005; Parbs and Birke, 2005; Birke
et al., 2007). The principal contaminant at the site was TCE and the high-
est identified concentration was 390 mg/L. Other chlorinated solvents were
present but at much lower concentrations.
In Belfast, the site geology and location placed a number of restraints on
the reactor design (Jefferis, 2005):
• The contaminant source extended to within a few meters of the site
boundary. Beyond the boundary, there was a public road, and it was
not practicable to extend the reactive zone into the road. The reactive
treatment zone therefore needed to be very compact.
• The solvent source was underlain by a thin layer of clay that pre-
vented its migration to a greater depth. If this layer was penetrated
by a reactive gate, the free solvents would sink and pollute a lower
aquifer stratum. In such an event, however, they would be ultimately
retained by a thick clay layer at about 10 m depth that underlies the
site and dips toward the proposed funnel.
• The groundwater perched on the thin clay layer was shallow and
showed seasonal variations in depth. It would be difficult to achieve
any significant depth of horizontal flow in a reactive treatment zone
without deepening the gate and thus penetrating the underlying
thin clay layer.
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