Environmental Engineering Reference
In-Depth Information
The generation of hydrogen and formation of a gas phase are significant
processes that may occur in ZVI PRBs. The molar amount of available hydro-
nium ions determines the extent of hydrogen generation. Since hydrogen car-
bonate and carbonic acid can be considered as the main source of hydronium
ions in natural waters, the extent of gas formation is strongly associated with
the dissolved inorganic carbon (DIC) concentration (Parbs et al., 2007; Ruhl
et al., 2012). A gradually increasing accumulation of precipitates in ZVI PRBs
together with the typical shift in pore size distribution toward smaller pores
renders the barrier more prone to gas plugging. Gas-consuming processes
play a major role in balancing gas production. Particularly, microorganisms
are able to reduce the accumulation of gases by hydrogenotrophic sulfate
reduction, denitrification, or acetogenesis. Thus, as no bioclogging events are
known until now (e.g., Henderson and Demond, 2007), microbial coloniza-
tion seems to favor the long-term performance of ZVI PRBs. Additionally,
dechlorinating strains may even bear contaminant degradation in the case
of cVOCs (Weber et al., 2013).
On this basis of past experience, CRBs are now the preferred solution in
North America. One of the main arguments for preferring the CRB design is
its lower sensitivity to design flaws. This means less risk when cleaning up
groundwater at complex sites with heterogeneous flow of pollutants and con-
taminant distribution (which in turn simplifies the characterization phase).
In Europe, however, modified F&G and EC-PRBs are the preferred solutions.
The criteria supporting this technological option are that the PRB can be con-
figured to suit site-specific features and that monitoring and maintenance
can be controlled more effectively. Some vendors propose a maintenance
strategy based on annual operations that can range from simple clearing of
clogged sections to replacement of the reactive medium (particularly recom-
mended for barriers based on the adsorption principle) or venting of gas
accumulations. This approach can only be considered if the design of the
barrier allows easy access to the treatment reactor. This optimized mainte-
nance strategy is sometimes backed up with guarantees on the performance
of the barrier, usually for periods of 10-30 years.
EC-PRBs, D&G PRBs, or modified (nonclassical) F&G PRBs have been
erected at former manufactured gas plant (MGP) and related sites to treat
PAHs and BTEX in more or less accessible reactive zones (in-ground/in situ
reactors, cartridges, etc.), and they have successfully been operated at differ-
ent sites across Europe (especially in the United Kingdom, France, Austria,
and Germany) between approx. 5 and 15 years in 2014. They are packed
with GAC and/or equipped with a biological treatment zone, where added
nutrients and/or microbes enhance microbiological degradation (Bio-PRBs).
Moreover, during the second working period of the German PRB R&D pro-
gram “RUBIN” from 2006 to 2012, it was verified that even emerging novel
contaminants, such as heterocyclic PAHs (NSO-PAHs), can effectively
be retained and/or destroyed in such PRBs as well. Approximately 10-20
PRBs have been set up in Europe at MGP sites so far; in contrast, there are
Search WWH ::
Custom Search