Environmental Engineering Reference
In-Depth Information
• On some sites, it may be advantageous to pump the flow to the reac-
tor. This can ensure a more uniform flow rate but it has to be demon-
strated that there will be effective plume capture under all seasonal
groundwater conditions.
The concept of an in-ground reactor adds flexibility to the design of reac-
tive treatment zones, allowing more precise control of the reaction envi-
ronment and easy chemical recharging or recovery and replacement of the
active material should this be required. Also, several reactors may be linked
in series to treat mixed contaminants.
The use of in-ground reactors allow the full armory of chemical engi-
neering reactor technology to be applied to what is often regarded as a
civil engineering/environmental science problem. This will bring many
new ideas.
Significant remaining problems are: the design of in-ground reactors for
high flow situations and the monitoring of PRB performance. If costs are to
be kept to the minimum, monitoring intervals must be as long as possible—
this requires confidence in PRB performance and the proactive design for
long monitoring intervals. In steady state, PRB performance can be mod-
eled and sampling within a reactor system may allow confidence that per-
formance will remain satisfactory for months/years to come. However, there
can be complicating factors such as desorption of contaminants as a result
of competitive sorption between contaminant species leading to the release
of sorbed contaminants as rather short spikes at concentrations higher than
their original input concentrations. Procedures need to be developed to iden-
tify impending changes (Jefferis, 2005; Birke et al., 2007).
13.8 Conclusion
13.8.1 Long-Term Performance and Longevity of PRBs
Long-term performance studies and lessons learned from established sys-
tems over two decades (e.g., ITRC, 2005, 2011; Carey et  al., 2002; Birke and
Parbs, 2006; Birke et al., 2007) show that 80%-90% of all ZVI PRBs work suc-
cessfully and many PRBs are performing well after more than a decade of
operation. The life of a PRB is expected to range from 10 to 30 years. A PRB
“failure” is usually attributed to poor site characterization and/or hydraulic
design. The long-term performance data show that conventional F&G sys-
tems are more prone to performance limitations, caused by preferential flow
paths, clogging of pore space due to mineral precipitation and/or gas accu-
mulations (gas plugging by hydrogen, methane, etc.), and bypassing.
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