Evaluation of Slow Release Substrates for Anaerobic Bioremediation

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Title: Evaluation of Slow Release Substrates for Anaerobic Bioremediation
Author: Rodriguez, Ximena
Advisors: Morton Barlaz, Committee Co-Chair
Francis de los Reyes, Committee Co-Chair
Robert C. Borden, Committee Chair
Abstract: A variety of anaerobic bioremediation processes are being developed for the in-situ treatment of groundwater contaminants. The most common approach for enhancing in-situ anaerobic biotransformation processes is to flush a readily biodegradable, soluble substrate through the treatment zone. The substrate serves as a carbon source for cell growth and as an electron donor for energy generation. While this approach can be very effective, operating costs can be high because of problems associated with clogging of pumps, piping and mixers, as well as the labor for extensive monitoring and process control. An alternative approach for stimulating anaerobic biotransformation processes is to generate a biological reducing zone (BRZ) by injecting one or more low solubility, slowly biodegrading substrates throughout the proposed treatment interval. We evaluated a variety of common substrates to assess their potential use in a BRZ. Initial screening studies were conducted by incubating 50 mg of the target substrate with anaerobic digester sludge and monitoring gas production over time. All of the vegetable oils generated significant gas production while the petroleum based materials did not result in significant gas production in any incubation. The organic acids and carbohydrates resulted in rapid gas production with no initial inhibition of methanogenesis. The surfactants resulted in the most rapid and complete gas production. Sucrose esters of fatty acids (SEFAs) are readily biodegradable at low concentrations but can be inhibitory to microbiological processes at higher concentrations. Follow-up studies examined the suitability of soybean oil and a sucrose fatty acid ester (SEFA) for stimulating reductive dechlorination in flow through columns. The influent and effluent from each column were periodically monitored for chlorinated solvents (PCE, TCE, DCE isomers, VC), electron acceptors and donors (oxygen, nitrate, phosphate, sulfate, methane, ethene and ethane), pH, and dissolved organic carbon. Some columns received 200 mg/L Na2SO4 in the influent to evaluate the effect of high background sulfate levels on substrate longevity and reductive dechlorination, with pH ~ 6. Three columns received dilute HCl (0.01 N) in the influent to inhibit biological activity, having a pH of ~2. After 15 months of operation, the carbon content of the soybean oil amended columns was significantly higher than the SEFA (S-270) amended columns or unamended control columns indicating a one time addition of a low solubility organic substrate can support anaerobic bioremediation processes for an extended time period. PCE removal was low in all of the columns because of the very short hydraulic retention time and high PCE loading. cis-DCE production was significant in all soybean oil treated columns. Fewer degradation products and less methane were produced for the columns treated with sulfate, which is in agreement with thermodynamics and several studies regarding the inhibition of dechlorinating activity in the presence of excess sulfate. A similar study was conducted for perchlorate and TCA reduction, using liquid and hydrogenated soybean oil in intermittent flow columns. Initial microcosm and columns results were favorable, showing perchlorate reduction in added carbon treatments. In general, TCA was not significantly depleted, presumably because of the short contact time of the fluid inside the column. Extensive perchlorate and nitrate reduction were observed in columns treated with liquid and hydrogenated soybean oil but not in the untreated control column. This work successfully demonstrated the potential application of emulsified soybean oil into groundwater systems contaminated with chlorinated compounds. By incorporating liquid soybean oil and solid soybean wax emulsions, the longevity and reducing capabilities of the treatment area can be controlled. The overall benefits include the in-situ approach, low environmental impact, and effective removal of chlorinated compounds.
Date: 2004-05-19
Degree: MS
Discipline: Civil Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/541


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