Permeable Reactive Barriers in Europe - Permeable Reactive Barrier: Sustainable Groundwater Remediation - page 301

of direction from horizontal to vertical flow tended to homogenize the flow
both in terms of concentration and across the cross-sectional area of the reac-
tor. Flow heterogeneity across a reactor can seriously comprise its perfor-
mance (Jefferis, 2002, 2005).
Because of the relatively low permeability and heterogeneity of the adja-
cent soil, it was decided that the flow to the reactor should be collected
via an upstream, high-permeability, collector and that downstream of the
reactor, there should be a similar distributor. The collector and distributor
were formed from gravel-filled piles taken down to the top surface of the
thin clay layer and capped with clay to prevent surface water ingress. A
polymer-supported, gravel-backfilled, slurry trench was the preferred col-
lector and distributor. In 1994, there were still concerns about the effect of
a polymer remaining on the iron filings and as there was insufficient time
to carry out the necessary research, augered piles were used instead of
the slurry wall. The reactor was fitted with sampling points at 1.5 m inter-
vals throughout the iron filings bed depth so that its performance could
be monitored. Monitoring points were also installed in the collector and
distributor piles.
Iron filings in contact with water in an oxygen-free environment produce
hydrogen. This hydrogen was vented from the reactor via a vent tube fitted
with a spark arrester and mounted in a tall lighting standard. Finally, the
internal geometry of the reactor was arranged so that the pipework connec-
tions to the gravel-filled collector and distributor piles could be made from
within the clean environment of the reactor shell without the need for any
hand excavation or for anyone to enter the excavations.
The reactor has performed as designed and there has been substantial
reduction in the source and in the downstream plume. A major uncertainty
at the design stage was the flow through the reactor and field measurement
proved difficult. Tests were undertaken with several tracer materials. These
showed that there was spare flow capacity in the reactor because the first
in-ground reactor had been designed with a reasonable margin of safety.
However, this spare capacity was not wasted; rather, it was exploited to treat
water pumped from the plume downstream of the reactor—a plume of con-
taminants that had developed prior to installation of the PRB. This proved
very effective and significantly reduced the extent of the plume.
Those working on potential future PRB sites need to carefully consider
the constraints of their sites. PRBs are not a “one design fits all technology.”
However, adoption of the in-ground reactor concept brings many engineer-
ing benefits and the design has since been widely used elsewhere. Although
in-ground reactors have been considered since 1994, the benefits and prob-
lems of the ground reactor do not appear to have been fully recognized.
These advantages include
• Providing a controlled reactor zone—a basic tenet of chemical
engineering.