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.