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Bioaugmentation coupled with activated carbon for the treatment of petroleum hydrocarbons in groundwater systems
Abstract
The contamination of groundwater by petroleum hydrocarbons (PHCs) is a global concern with
negative human health and environmental impacts. The injection of an activated carbon (AC)
particulate amendment to create a permeable reactive barrier (PRB) to prevent additional
downgradient migration of a dissolved PHC plume from a source area has gained popularity. In
this remedial application the selected AC amendment is strategically injected across a dissolved
PHC plume and provides sufficient adsorption capacity to remove dissolved PHCs from the
groundwater that flows through the PRB. This concentrated mass of PHCs is considered to serve
as a haven for indigenous microorganisms to thrive and results in biodegradation of the PHCs and
regeneration of the adsorption capacity of the AC. Often anaerobic conditions develop within the
PRB which leads to depleted concentrations of electron acceptors. To overcome this limitation,
bacterial cultures are co-injected with the AC to increase the rate of biodegradation since some
indigenous microorganisms are perhaps not capable, in low abundance or not present to operate
effectivity under anaerobic conditions. The overarching objective of this research was to
investigate if an AC particulate amendment coupled with bioaugmentation (culture injection) can
synergistically enhance the biodegradation of PHCs. The hypothesis postulated was that the
combination of powdered AC (PAC) and enriched methanogenic cultures will enhance the
biodegradation of benzene, toluene and o-xylene (BTX) under anaerobic conditions.
To evaluate this hypothesis, data was collected from microcosm experiments representing static
conditions, and from continuous flow column experiments mimicking in situ conditions. Both the
microcosm and column experiments were conducted in anaerobic environments and included
controls and active treatment systems involving combinations of BTX, PAC, and/or
bioaugmentation (BA). The PAC utilized was a virgin coconut-based thermally activated product
with a mean particle size of 13.1 μm. The methanogenic cultures were enriched from nature and
have been shown to completely degrade benzene (DGG™-B), toluene (DGG™-T), and o-xylene
(DGG™-X). The aquifer material used in the experiments was collected from the University of
Waterloo Groundwater Research Facility at Canadian Forces Base (CFB) Borden.
Five different microcosm systems were constructed using artificial groundwater, aquifer material,
methanogenic cultures and PAC. A single-compound experiment used toluene only while a multicompound
experiment used BTX. The active systems included microcosms with and without BA
and with (A-PAC-BA and A-PAC) and without (A-BA and A) PAC. Control microcosm systems
included killed control (autoclaved with biocide), positive control (BA without aquifer material),
and starved control (no addition of toluene or BTX). A total of 300 microcosms bottles were
assembled and stored on their side in an anerobic glove chamber undistributed except during
sampling. Microcosms were sampled at selected timepoints over a period of nearly one year using
a repetitive and sacrificial strategy. Dissolved phase samples were used to determine pH, oxidative
reductive potential (ORP), dissolved oxygen (DO), BTX concentrations, dissolved inorganic
carbon (DIC) content, and sulfate and sulfide concentrations. Gas phase samples collected from
the microcosm headspace were analyzed for methane (CH4) and carbon dioxide (CO2). Solid phase
samples were also collected to determine bulk BTX concentrations, and deoxyribonucleic acid
(DNA) extractions were assayed by quantitative polymerase chain reaction (qPCR). Toluene or
BTX mass was replenished as needed in the active systems.
There was no evidence of biodegradation in either the killed or starved controls. Depletion of
toluene and o-xylene in conjunction with consumption of sulfate and production of methane and
carbon dioxide indicated biodegradation occurred in the positive control and all active
bioaugmented microcosm systems. In all microcosm systems the depletion of benzene was not
observed. As expected, the presence of PAC in the active microcosm systems considerably reduced
the aqueous concentrations of toluene or BTX. Bulk toluene or BTX concentration data provided
evidence for the regeneration of PAC sorption capacity because of biodegradation. qPCR results
support the depletion of toluene and o-xylene in the active bioaugmented systems with elevated
populations of key degraders (Desulfosporosinus (DSP) and Peptococcaceae (PEP)). During the
final ~30 days, a higher temporal resolution sampling strategy was implemented to collect data to
estimate compartmental mass distributions and system biodegradation rates. The estimated
biodegradation rate for A and A-PAC microcosms (no BA) were not statistically different at the
5% LOS, as well as the estimated biodegradation rate for A-BA and A-PAC-BA microcosms.
Based on the data set assembled from the microcosm experiments, there is no evidence that
supports the stated hypothesis. Specifically, the presence of PAC by itself or in combination with
BA did not increase the mass of toluene and o-xylene biodegraded or the rate of biodegradation.
Five different columns systems were used to examine BTX biodegradation under anaerobic
conditions. The active systems included columns with and without BA and with (A-PAC-BA and
A-PAC) and without (A-BA and A) PAC. A control column (killed control) containing PAC was
constructed from autoclaved materials. The columns were packed with aquifer material and a 6-
cm long PAC zone (0.5% by wt) was emplaced in the central part of the column to mimic a PRB
for some systems. An access port was used to inject cultures into the PAC zone. Anerobic artificial
groundwater augmented with BTX was used as the feed solution. Biocide was added to the feed
solution for the killed control column. The columns were run for a one-year acclimation period
followed by 9 months of a comprehensive sampling. Dissolved phase samples collected from the
influent and effluent were used to determine pH, ORP, DO, DIC content, and BTX, sulfate, CH4,
and CO2 concentrations. At the termination of the experiment, solid phase subsamples were
collected from each active column and used to determine bulk BTX concentrations, and DNA
extractions were assayed by qPCR.
There was no evidence of biodegradation in the killed control column. In the absence of BA (A vs
A-PAC columns), the PAC zone improved the biodegradation of toluene as supported by the
production of CH4 and CO2, and increased population of toluene degraders within the PAC zone.
While in both BA systems (A-BA and A-PAC-BA) toluene biodegradation was near complete
(>95% mass reduction), suggesting that the PAC zone did not enhance the biodegradation capacity
of toluene when combined with BA. Biodegradation of benzene (~25% mass reduction) occurred
in the A-PAC-BA system, despite a larger population of benzene degrading microbes in the A-BA
system. O-xylene biodegradation was the highest in the A-BA system (~90% mass reduction),
which was supported by the DNA results (>107 copies/g o-xylene degraders), along with
production of CH4 and CO2. Based on the data set assembled, there is evidence that supports the
stated hypothesis. Specifically, the presence of a PAC zone in the column by itself improved the
biodegradation of toluene (~95% versus ~70% mass reduction) and o-xylene (~25% versus <20 %
mass reduction) compared to the column with no PAC zone. The combination of PAC and BA
increased benzene biodegradation (~25% versus <20 % mass reduction) but decreased o-xylene
biodegradation (~90% versus ~80% mass reduction) compared to BA but without a PAC zone.
Taken together, experimental findings were not able to prove that PAC and BA work
synergistically to enhance the biodegradation of BTX. Nevertheless, PAC did not reduce the
biodegradation ability of the systems and can therefore still provide benefits when used for
groundwater remediation applications. Specifically, the use of PAC in combination with
bioaugmentation in field applications may provide benefits by containing the BTX mass in a more
spatially confined area for longer durations (> 20 years), which would provide more time for
biodegradation to occur.
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Cite this version of the work
Adam Schneider
(2024).
Bioaugmentation coupled with activated carbon for the treatment of petroleum hydrocarbons in groundwater systems. UWSpace.
http://hdl.handle.net/10012/20743
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