Graduation Year

2020

Document Type

Thesis

Degree

M.S.E.V.

Degree Name

MS in Environmental Engr. (M.S.E.V.)

Degree Granting Department

Civil and Environmental Engineering

Major Professor

Daniel H. Yeh, Ph.D.

Committee Member

Christopher Alexander, Ph.D.

Committee Member

Albert Robert Rubin, Ph.D.

Keywords

Ammonium, Autotrophic, Decentralized, Nitrate, Nitrogen

Abstract

Denitrification experiments using biological cathodes (biocathodes) were conducted in three separate experiments. The first experiment was to observe whether the rate of denitrification in biocathodes could be increased with added ammonium at mass (mg/L) ratios of 0.100 (2.5/25), 0.05 (1.25/25), 0.0250 (0.625/25), and 0.0125 (0.3125/25) (parts ammonium to nitrate (NH4+-N/ NO3--N). The purpose of the ammonium present was to provide the microorganisms a starting nitrogen source for assimilation to biomass which, in theory, would encourage the removal of nitrate through dissimilatory denitrification. As mixed cultures were used, ratios tested were based on simplifying future operation of scaled-up biocathodes applied to decentralized treatment of nitrogen. If an ideal ratio could be estimated, future operators could add either ammonium or nitrate to increase the removal of nitrogen. Bench-scale testing was done using a standard H-type microbial fuel cell (MFC) design. The second and third experiments tested complex and simplified bio-electrochemical systems (BESs) scaled-up for on-site treatment of nitrate. The more complex system was a 2.9 L denitrifying biocathode MFC (DB-MFC) designed to remove both chemical oxygen demand (COD) and nitrate. The simplified system was a 2.9 L single-chamber denitrifying biocathode (DB-SC) electrochemically assisted using a direct current (DC) power supply. DB-SC was the largest reported BES of this type by volume and the only one to be operated in continuous flow was designed to solely remove nitrate.

The control reactor on the bench-scale performed the best for both total nitrogen (TN) and nitrate removal. When analyzing the average background ammonium in the control, a nitrate to ammonium mass ratio close to 0.00327 (0.0830/25) was measured. Results on the bench-scale indicated that this ratio was better suited for both nitrate and TN removal. None of the tested ratios performed better than the control reactor after the addition of ammonium. For the MFCs tested with higher concentrations of ammonium, correlating to mass ratios higher than 0.0103, no improvement to denitrification performance was observed when compared to the control and baseline tests for each reactor. Incidentally, ratios above 0.0103 exhibited slower removal of TN and almost no removal of nitrate. Further research is required to determine if a ratio below 0.0103 or near 0.00327 correlates to higher denitrification rates.

DB-MFC was able to remove up to 91% of the influent COD. However, due to low coulombic efficiency (CE), simultaneous treatment of COD and nitrate was not observed during operation. The highest average CE observed was 6.5 % ± 1.8 % when the anode was fed 500 mg/L of COD. The increase in size did not correlate to a higher overall cell potential which led to a lower CE than what was observed on the bench-scale. Furthermore, the square shape of the reactor led to poor mixing conditions in both the anode and cathode chambers. The main objective to remove the influent nitrate in the biocathode was not achieved. The protons and electrons provided by the additional COD oxidation were not enough for denitrification to be facilitated at the cathode. As nitrate removal was not observed, research moved on to the design of DB-SC for on-site treatment of nitrate.

DB-SC performed partial denitrification at an average rate of 6.2 mg N/(L*d). 25% of the influent nitrate, was partially denitrified to nitrite at a cathode potential of -1.8 V (vs. Standard Hydrogen Electrode (SHE)). The size and shape of the reactor did not affect the performance like what was observed in DB-MFC. Based on the results, DB-SC had more potential for applicability to on-site water treatment than DB-MFC.

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