title: Investigation of Bench-Scale Packed Bioreactor for Perchlorate Reduction in the Presence of Co-Contaminants and High Salinity
Creators: name: Ribeiro Rabello, Julia 
Publisher: name: University of Nevada, Las Vegas
Year: 2021

description: Perchlorate (ClO4-), commonly found as a salt, is naturally occurring in nature and is found in natural nitrate deposits (such as the Chilean nitrate). Perchlorate anion is considered an inhibitor of iodine by the thyroid gland, reducing the production of thyroid hormones. The latest action by the USEPA was to not determine a national drinking water standard for perchlorate. However, the states still have their own regulations on perchlorate concentration in drinking water. Contaminated sites with high concentrations of perchlorate (ppm levels) are associated with historic perchlorate manufacture and use, while those with low concentrations (ppb levels) are found in areas where perchlorate was used to manufacture rockets and ammunition. Many technologies are capable of treating perchlorate in contaminated waters, however, biological reduction is the most cost-effective. This thesis focus on the determination of the potential use of in-situ bioremediation for remediation of groundwater contaminated with competing electron-accepting anionic compounds. The specific objectives of this thesis were to evaluate the effectiveness of the use of molasses as electron donor/carbon source for the removal of contaminants of concern (COCs), determine the removal efficiency of COCs under different groundwater dilutions, examine the impact of contact time in the contaminants’ degradation, evaluate if effluent chloride concentrations reflect the amount of chlorate and perchlorate degraded within the columns and investigate the microbial ecology developed within the packed-bed reactor. Four packed-bed reactors, four-foot-tall and two-inches diameter each, were built using transparent Plexiglas. A mixture of 60% sand and 40% soil was used as a media for the bioreactors. Two bioreactors used soil from a deep horizon (95’-105’), while the other two bioreactors used soil from an intermediate horizon (75’-85’). Initially, the bioreactors were fed with diluted groundwater (1:4) and further on undiluted contaminated groundwater was applied. In addition, the solution was amended with molasses as the carbon source, Urea/diammonium phosphate, sodium bicarbonate, and vitamin B-12. Effluent samples were collected three times a week and analyzed for the contaminants of concern. The results of groundwater and soil analyses revealed that the contaminated site has very high concentrations of Cr(VI), nitrate, chlorate, and perchlorate; the highest concentrations reported to date for a perchlorate contaminated site. The groundwater and soil analyses showed that the highest contamination was from chlorate, followed by perchlorate, nitrate, and Cr(VI). It was found that TOC concentrations are directly proportional to the amount of molasses added, due to the higher consumption by the bacteria to degrade the contaminants and the higher abiotic reduction of chromium. Therefore, the bacteria present are able to use molasses as a carbon and energy source. Excellent degradation of Cr(VI) was observed during the column testing. Even in the presence of high total dissolved solids (TDS), Cr(VI) was fully reduced in the effluent. This achievement is highly due to the combination of abiotic and biotic reduction. Also, the degradation of Cr(V) was found to be very feasible with the use of molasses. Denitrification was found to be a carbon limited process and be negatively impacted by the presence of high TDS. The results also showed that effluent chlorate concentration in the columns is a function of the initial influent concentration, the retention time within the column, the presence of sufficient electron donor/carbon source, plus the competition of nitrate and chromium which degraded first. Perchlorate degradation in the bioreactor studies occurred simultaneously with Cr(VI), nitrate, and chlorate degradation. Overall, the main factors that impacted perchlorate reduction was TDS concentrations, EBCT, carbon availability, and potentially high residual nutrient ammonia.