Bioaugmentation coupled with activated carbon for the treatment of petroleum hydrocarbons in groundwater systems

The contamination of groundwater by petroleum hydrocarbons (PHCs) is a global concern with \nnegative human health and environmental impacts. The injection of an activated carbon (AC) \nparticulate amendment to create a permeable reactive barrier (PRB) to prevent additional \ndowngradient migration of a dissolved PHC plume from a source area has gained popularity. In \nthis remedial application the selected AC amendment is strategically injected across a dissolved \nPHC plume and provides sufficient adsorption capacity to remove dissolved PHCs from the \ngroundwater that flows through the PRB. This concentrated mass of PHCs is considered to serve \nas a haven for indigenous microorganisms to thrive and results in biodegradation of the PHCs and \nregeneration of the adsorption capacity of the AC. Often anaerobic conditions develop within the \nPRB which leads to depleted concentrations of electron acceptors. To overcome this limitation, \nbacterial cultures are co-injected with the AC to increase the rate of biodegradation since some \nindigenous microorganisms are perhaps not capable, in low abundance or not present to operate \neffectivity under anaerobic conditions. The overarching objective of this research was to \ninvestigate if an AC particulate amendment coupled with bioaugmentation (culture injection) can \nsynergistically enhance the biodegradation of PHCs. The hypothesis postulated was that the \ncombination of powdered AC (PAC) and enriched methanogenic cultures will enhance the \nbiodegradation of benzene, toluene and o-xylene (BTX) under anaerobic conditions. \nTo evaluate this hypothesis, data was collected from microcosm experiments representing static \nconditions, and from continuous flow column experiments mimicking in situ conditions. Both the \nmicrocosm and column experiments were conducted in anaerobic environments and included \ncontrols and active treatment systems involving combinations of BTX, PAC, and/or \nbioaugmentation (BA). The PAC utilized was a virgin coconut-based thermally activated product \nwith a mean particle size of 13.1 μm. The methanogenic cultures were enriched from nature and \nhave been shown to completely degrade benzene (DGG™-B), toluene (DGG™-T), and o-xylene \n(DGG™-X). The aquifer material used in the experiments was collected from the University of \nWaterloo Groundwater Research Facility at Canadian Forces Base (CFB) Borden. \nFive different microcosm systems were constructed using artificial groundwater, aquifer material, \nmethanogenic cultures and PAC. A single-compound experiment used toluene only while a multicompound \nexperiment used BTX. The active systems included microcosms with and without BA \nand with (A-PAC-BA and A-PAC) and without (A-BA and A) PAC. Control microcosm systems \nincluded killed control (autoclaved with biocide), positive control (BA without aquifer material), \nand starved control (no addition of toluene or BTX). A total of 300 microcosms bottles were \nassembled and stored on their side in an anerobic glove chamber undistributed except during \nsampling. Microcosms were sampled at selected timepoints over a period of nearly one year using \na repetitive and sacrificial strategy. Dissolved phase samples were used to determine pH, oxidative \nreductive potential (ORP), dissolved oxygen (DO), BTX concentrations, dissolved inorganic \ncarbon (DIC) content, and sulfate and sulfide concentrations. Gas phase samples collected from \nthe microcosm headspace were analyzed for methane (CH4) and carbon dioxide (CO2). Solid phase \nsamples were also collected to determine bulk BTX concentrations, and deoxyribonucleic acid \n(DNA) extractions were assayed by quantitative polymerase chain reaction (qPCR). Toluene or \nBTX mass was replenished as needed in the active systems. \nThere was no evidence of biodegradation in either the killed or starved controls. Depletion of \ntoluene and o-xylene in conjunction with consumption of sulfate and production of methane and \ncarbon dioxide indicated biodegradation occurred in the positive control and all active \nbioaugmented microcosm systems. In all microcosm systems the depletion of benzene was not \nobserved. As expected, the presence of PAC in the active microcosm systems considerably reduced \nthe aqueous concentrations of toluene or BTX. Bulk toluene or BTX concentration data provided \nevidence for the regeneration of PAC sorption capacity because of biodegradation. qPCR results \nsupport the depletion of toluene and o-xylene in the active bioaugmented systems with elevated \npopulations of key degraders (Desulfosporosinus (DSP) and Peptococcaceae (PEP)). During the \nfinal ~30 days, a higher temporal resolution sampling strategy was implemented to collect data to \nestimate compartmental mass distributions and system biodegradation rates. The estimated \nbiodegradation rate for A and A-PAC microcosms (no BA) were not statistically different at the \n5% LOS, as well as the estimated biodegradation rate for A-BA and A-PAC-BA microcosms. \nBased on the data set assembled from the microcosm experiments, there is no evidence that \nsupports the stated hypothesis. Specifically, the presence of PAC by itself or in combination with \nBA did not increase the mass of toluene and o-xylene biodegraded or the rate of biodegradation. \nFive different columns systems were used to examine BTX biodegradation under anaerobic \nconditions. The active systems included columns with and without BA and with (A-PAC-BA and \nA-PAC) and without (A-BA and A) PAC. A control column (killed control) containing PAC was \nconstructed from autoclaved materials. The columns were packed with aquifer material and a 6- \ncm long PAC zone (0.5% by wt) was emplaced in the central part of the column to mimic a PRB \nfor some systems. An access port was used to inject cultures into the PAC zone. Anerobic artificial \ngroundwater augmented with BTX was used as the feed solution. Biocide was added to the feed \nsolution for the killed control column. The columns were run for a one-year acclimation period \nfollowed by 9 months of a comprehensive sampling. Dissolved phase samples collected from the \ninfluent and effluent were used to determine pH, ORP, DO, DIC content, and BTX, sulfate, CH4, \nand CO2 concentrations. At the termination of the experiment, solid phase subsamples were \ncollected from each active column and used to determine bulk BTX concentrations, and DNA \nextractions were assayed by qPCR. \nThere was no evidence of biodegradation in the killed control column. In the absence of BA (A vs \nA-PAC columns), the PAC zone improved the biodegradation of toluene as supported by the \nproduction of CH4 and CO2, and increased population of toluene degraders within the PAC zone. \nWhile in both BA systems (A-BA and A-PAC-BA) toluene biodegradation was near complete \n(>95% mass reduction), suggesting that the PAC zone did not enhance the biodegradation capacity \nof toluene when combined with BA. Biodegradation of benzene (~25% mass reduction) occurred \nin the A-PAC-BA system, despite a larger population of benzene degrading microbes in the A-BA \nsystem. O-xylene biodegradation was the highest in the A-BA system (~90% mass reduction), \nwhich was supported by the DNA results (>107 copies/g o-xylene degraders), along with \nproduction of CH4 and CO2. Based on the data set assembled, there is evidence that supports the \nstated hypothesis. Specifically, the presence of a PAC zone in the column by itself improved the \nbiodegradation of toluene (~95% versus ~70% mass reduction) and o-xylene (~25% versus <20 % \nmass reduction) compared to the column with no PAC zone. The combination of PAC and BA \nincreased benzene biodegradation (~25% versus <20 % mass reduction) but decreased o-xylene \nbiodegradation (~90% versus ~80% mass reduction) compared to BA but without a PAC zone. \nTaken together, experimental findings were not able to prove that PAC and BA work \nsynergistically to enhance the biodegradation of BTX. Nevertheless, PAC did not reduce the \nbiodegradation ability of the systems and can therefore still provide benefits when used for \ngroundwater remediation applications. Specifically, the use of PAC in combination with \nbioaugmentation in field applications may provide benefits by containing the BTX mass in a more \nspatially confined area for longer durations (> 20 years), which would provide more time for \nbiodegradation to occur.

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