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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Civil Engineering

Year Degree Awarded

2017

Month Degree Awarded

September

First Advisor

Caitlyn S. Butler

Second Advisor

John E. Tobiason

Third Advisor

Klaus Nüsslein

Subject Categories

Environmental Engineering | Environmental Microbiology and Microbial Ecology

Abstract

Bioelectrochemical systems(BESs)/ microbial fuel fells (MFCs) are a well-studied potential technology for bioremediation and decentralized wastewater treatment. However, progress has been somewhat stalled at the bench-scale. In well controlled experiments electron recovery is high. In natural environments, wastewaters are complex and anode-respiring bacteria can be outcompeted in the presence of competing microorganisms, leading to a loss in electron-recovery and power production. Furthermore, the cathode of the MFC plays a vital role in providing flexibility for treatment options but is an understudied part of MFCs.

Modelling Intracellular Competition in a Denitrifying Biocathode:

One potential MFC configuration uses an organic-oxidizing anode biofilm and a denitrifying cathode biofilm. However nitrite, a denitrification intermediate with environmental and public health impacts, has been reported to accumulate. In this study, before complete denitrification was achieved in a bench-scale, batch denitrifying cathode, nitrite concentrations reached 66.4 % ± 7.5 % of the initial nitrogen. Common environmental inhibitors such as insufficient electron donor, dissolved oxygen, insufficient carbon source, and pH, were considered as a cause of the accumulation. Improvement in these conditions did not mitigate nitrite accumulation. We present an activated sludge model with an integration of the Nernst-Monod model and indirect coupling of electrons (ASM-NICE) that effectively simulated the observed batch data, including nitrite-accumulation by coupling biocathodic electron transfer to intracellular electron mediators. The simulated half-saturation constants for mediated intracellular transfer of electrons during nitrate and nitrite reduction suggested a greater affinity for nitrate reduction when electrons are not limiting. The results imply that longer hydraulic retention times (HRTs) may be necessary for a denitrifying biocathode to ensure complete denitrification. These findings could play a role in designing full-scale MFC wastewater treatment systems to maximize total nitrogen removal.

Experimental Evaluation of Responses of Anode-Respiring Communities to Nitrate:

A poorly understood phenomenon with a potentially significant impact on electron recovery in MFCs is the role of competition between anode-respiring bacteria and microorganisms that use other electron acceptors. Nitrogen species are a major constituent of wastewater and nitrate can act as a competing electron acceptor in the anode. Studies investigating the impact of competition on population dynamics in mixed communities in the anode are lacking. Here, we investigated the impact of nitrate at different C/N ratios, 1.8, 3.7 and 7.4 mg-C/mg-N, on the electrochemical performance and the biofilm community in mixed-culture chemostat MFCs. The electrochemical performance of the MFC was not affected under electron donor non-limiting conditions, 7.4 mg-C/mg-N. At lower C/N ratios, electron donor limiting, electron recovery was significantly lower. The electrochemical performance recovered upon removal of nitrate at 3.7 mg-C/mg-N. Microbial community analysis showed a decrease of Deltaproteobacteria accompanied by an increase in Betaproteobacteria in response to nitrate at low C/N ratios, and no significant changes at 7.4 mg-C/mg-N. Transcriptional analysis showed increased transcription of nirK and nirS genes during nitrate flux suggesting that denitrification to N2 (and not facultative nitrate reduction by Geobacter spp.) might be the primary response to perturbation with nitrate.

Modelling Interspecies Competition in the Anode of a Microbial Fuel Cell:

MFCs offer great promise for simultaneous treatment of wastewater and energy recovery. Even though there have been extensive experimental studies of multi-species anode-respiring biofilms, models and process optimization studies have been scarce. The formulation and evaluation of models is a critical step in the application of MFCs to wastewater treatment and bioremediation. The purpose of this study was to formulate a model that could simulate the effect of influx of a competing electron acceptor such as nitrate on the anode biofilm community. A model was formulated considering two distinct communities of bacteria: an anode-respiring community (not capable of nitrate reduction) and a denitrifying community (not capable of anode-respiration). A competitive scenario involving the influx of acetate and nitrate at a C/N ratio of 1.8 mg-C/mg-N was used to calibrate the model using experimental data. Calibration results indicate that facultative reduction of nitrate by facultative anode-respiring bacteria could be an important factor playing a role in the robustness and resilience of the anode-biofilms to fluxes of nitrate. Sensitivity analyses revealed that the biofilm retention coefficient (biofilm detachment rate) and species-specific growth kinetic parameters could play a significant role in the robustness of anode communities to influx of nitrate. Further investigation of change in detachment rate in response to the presence of nitrate in bulk solution is required.

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