Trichloroethene Removal From Waste Gases in Anaerobic Biotrickling Filters Through Reductive Dechlorination
Trichloroethene (TCE) is a persistent soil and groundwater pollutant that is known to be toxic to humans. Ex situ treatment is currently considered the most viable option for remediation of TCE contaminated sites. Among ex situ techniques, soil vapor extraction (SVE) and dual-phase extraction (DPE) with air sparging are more attractive since they result in rapid remediation of sites. However, waste gas streams laden with TCE are thus generated, and these require treatment. While traditionally this has been done using incineration or adsorption onto activated carbon, in this dissertation it is proposed that treatment of waste gases containing TCE can be carried out in anaerobic biological reactors, after displacement of oxygen during SVE or DPE with nitrogen. These anaerobic bioreactors rely on the metabolism of novel microorganisms from the Dehalococcoides genus that reductively dechlorinate TCE all the way to non-toxic ethene via cis-1,2,-dichloroethene (cis-DCE) and vinyl chloride (VC), in combination with oxidation of hydrogen.
For providing the proof of concept of anaerobic TCE waste gas treatment, a lab-scale anaerobic biotrickling filter inoculated with a mixed culture containing Dehalococcoides spp. that harbor the TceA and VcrA reductive dehalogenases was setup and monitored for performance. A biotrickling filter configuration was chosen since it gives an easy means of providing lactate as an indirect source of hydrogen to Dehalococcoides spp. immobilized on the bed packing through its fermentation. TCE elimination capacities (EC) higher than those reported for aerobic gas-phase bioreactors treating TCE were observed. The distribution of the intermediates of TCE dechlorination was significantly affected by the recirculating liquid pH, with near neutral pHs resulting in the maximum conversion of removed TCE to ethene (>60% at loadings of 3 gTCE mbed-3 h-1).
Because complete conversion to ethene was not observed, a detailed modeling study followed focusing on the determination of biokinetic constants for the dechlorination of TCE, cis-DCE and VC as well as any inhibition that may exist between these compounds. Dechlorination data from experiments with differential reactor operation with single compounds only were fitted to the model to determine biokinetic constants. Experiments with multiple compounds were used to determine inhibition between various compounds. It was found that the Michaelis-Menten constants for all compounds were higher than when grown typically in suspended cultures (i.e. lower enzyme affinity). It was also observed that TCE competitively inhibits the dechlorination of cis-DCE, and also has an effect on the dechlorination of VC, suggesting that some modification to the process culture could alter the distribution of the intermediates.
To test the hypothesis that addition of a strain containing a different reductive dehalogenase (BvcA, responsible for dechlorination of cis-DCE and VC) could result in better conversion of TCE to ethene, a biotrickling filter inoculated with the original culture used and operating at high loads (8-9 gTCE mbed-3 h-1), was bioaugmented with Dehalococcoides sp. strain BAV1 that contains the BvcA enzyme. It was found that conversion of TCE to ethene was improved significantly (from <10% of removed TCE to 45%) after bioaugmentation. Quantitative polymerase chain reaction (qPCR) and reverse transcript qPCR analysis revealed that not only strain BAV1 was able to grow, but that the expression of bvcA genes was also very high.
Finally, the rate-limiting step after long-term operation of the biotrickling filters was evaluated after it was found that there was a rapid decrease in the reactor efficacy following excessive biomass growth. It was found that mass transfer limitations as a result of decrease in specific surface area of the packing from excessive biomass growth existed. Depending on the gas and the liquid flow rates, this limitation was either at the gas or the liquid films. Mass transfer limitations were confirmed by omission of the recirculating liquid, which resulted in higher ECs, suggesting that after significant biomass build-up, careful considered of possible mass transfer limitations is warranted. Overall, the present study provides the foundations for a possible new technique for remediation of TCE contaminated sites.