Chenjie Wu

Date of Graduation

2014

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Civil and Environmental Engineering

Committee Chair

Lian-Shin Lin

Committee Member

Harry Finklea

Committee Member

Hoil Park

Committee Member

Jennifer Weidhaas

Committee Member

John Zondlo

Abstract

Recent developments of bioelectrochemical systems (BESs) have shown promising advancements in applying these innovative technologies for municipal wastewater treatment. These systems promise several distinct energy and environmental benefits over the existing activated sludge processes including electricity conservation and production, less biological sludge production, significant reduction in greenhouse gas emission, and potential useful chemical production from wastewater treatment. The goal of this research is to evaluate the feasibility of using a two-chambered BES to generate an environmental friendly oxidizer, hydrogen peroxide, from wastewater treatment. Five research objectives were proposed to achieve the research goal by filling several identified knowledge gaps: 1) determine optimal conditions for H2O2 production with the graphite felt electrode material using electrolysis tests; 2) analyze anode biofilm to evaluate its electrochemical properties; 3) quantify H2O2 production and organics removal from wastewater treatment using a two-chambered BES and to identify rate limiting factors; 4) characterize microbial ecology of the anode biofilm and its relationships with current and H2O2 production; and 5) optimize the BES performance through potential control. Our research demonstrated that electroconductive biofilm was successfully developed using wastewater and acid mine drainage as the inoculation source. Such biomass was successfully used as biocatalysts to achieve the treatment purpose of COD removal and H2O2 production under operating conditions investigated. The optimal potential for hydrogen peroxide production on the graphite electrode was found at -0.5 V (vs Ag/AgCl) using electrolysis tests. Hydrogen peroxide concentration increased fairly linearly with time when pH was stable around 7. The concentration of H2O2 decreased when pH started to increase. The results suggested that H2O2 decomposition rate exceeds its production rate as the pH reaches 12 given the BES settings in this study. The biofilms were gradually established with time. The qPCR analysis showed that sulfate reducing bacteria constituted approximately 40% of the total microbial population after 30 days of enrichment. A decrease in charge transfer resistance with time indicates that the establishment of the conductive biofilm could contribute to the improvement of the kinetics of electrochemical reactions. Different redox potentials were found through CV scans on anode with biofilm formation which may indicate temporal evolution of biofilm reactions at the anode and shift in microbial metabolic functions over time.

The highest production of hydrogen peroxide in the system was 70 mg/L during a 12-hour period. The removal rate of COD and sulfate could reach above 90% during a 5-day recirculation operation. Many obstacles will need to be overcome for using a BES to produce H2O2 for industrial uses. The main problem was the low production yield. High internal resistance was found in our BESs with ohmic resistance 0.77 Ω/cm2, charge transfer resistance 2.77 Ω/cm2 and large diffusion resistances. The presence of sulfate in wastewater will compete with the anode for electrons, which would sacrifice current production. Further investigation will be needed to test our hypothesis that the rate-limiting step is the biochemical oxidation of the organics by the microorganisms in the anode chamber. Several species of sulfate reducing bacteria was found in the biofilm community. The presence of sulfate reducing bacteria indicates a success development of biofilm with electron transfer ability. The controlled potentials had significant effect on the sulfate reducing bacteria population. When BES was operated under no potential control condition at the beginning of the experiments, the percentage of sulfate reducing bacteria was around 38%. When poised with a control potential at anode, the percentage of sulfate reducing bacteria increased significantly. Potential control of the anode could improve removal efficiency of COD and sulfate. However, hydrogen peroxide production did not increase during control potential experiments. The best performance of BESs was observed at potential control of -0.1 V (highest current density, high COD and sulfate removal rate, detection of hydrogen peroxide production), indicating this anode potential favor the organics oxidation in the anode and electron flow from the anode to cathode for H2O2 production.

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