Examination of Bioelectrochemical Systems with Different Configurations for Wastewater Treatment

Open Access
Author:
Ren, Lijiao
Graduate Program:
Environmental Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 05, 2014
Committee Members:
  • Bruce Ernest Logan, Dissertation Advisor
  • Bruce Ernest Logan, Committee Chair
  • John Michael Regan, Committee Member
  • William D Burgos, Committee Member
  • Li Li, Committee Member
Keywords:
  • microbial fuel cells (MFCs)
  • microbial electrolysis cells (MECs)
  • wastewater treatment
  • energy recovery
Abstract:
Bioelectrochemical systems (BESs) are emerging technologies that use microorganisms to convert the chemical energy stored in biodegradable substrates, such as wastewater, to direct electrical current or energy storage chemicals. BESs have undergone tremendous development resulting in substantial advances in performance during the past decade, in terms of increases in energy output, feasibility of scaling-up the reactors, and reduction in system costs. However, optimization of system operation and the development of evaluation methods are needed to facilitate commercialization of BESs for wastewater treatment. The research findings reported here addressed several different aspects of BES operation and performance for wastewater treatment: examination of what factors affected power production of multi-electrode microbial fuel cells (MFCs) with hydraulic and electrical connections; investigation of how substrate competition between exoelectrogens and other microorganisms affected coulombic efficiencies (CEs) and chemical oxygen demand (COD) removals in MFCs; development of an energy-efficient, high-quality wastewater treatment system that combined MFCs with a secondary treatment system; and the use of a high-throughput screening method to evaluate chemical effects and wastewater treatability. The power production of four hydraulically connected MFCs was compared with the reactors wired using individual electrical circuits to that obtained when the anodes and the cathodes were wired together. Based on the polarization tests, the same power was produced by the combined and individual circuit MFCs when operated in fed-batch mode. In continuous flow mode, where the acetate concentration significantly affected MFC performance, slightly lower power was produced by the MFCs containing the electrodes wired together compared to those with individual circuits. Parasitic current flow between adjacent MFCs showed no appreciable impact on reactor performance. These results demonstrated that the acetate concentration differences had more of an effect on the performance of multi-electrode systems than parasitic current, and thus that there was no need for electrolyte isolation between adjacent reactors. The competition for acetate through current generation by exoelectrogens and aerobic microbial respiration was examined in single-chamber air-cathode MFCs by measuring COD removals at different current densities. COD removal was always first-order with respect to COD concentration, even under open circuit conditions. As a result, the COD lost to non-exoelectrogenic processes was not constant over time, but instead decreased with COD concentration. With current generation, substrate was consumed at a faster rate, reducing the bulk COD concentration faster and therefore reducing the rate that COD was lost to aerobic respiration. Thus, the COD degradation rates and CEs improved with increases in current densities due to the greater substrate uptake by exoelectrogens. This demonstrated that exoelectrogens could outcompete aerobic heterotrophs for acetate at high current densities, resulting in increased COD removal rates and CEs that were achieved by operating the MFCs at lower resistances. A two-stage laboratory-scale treatment process, consisting of multiple MFCs followed by an anaerobic fluidized bed membrane bioreactor (AFMBR), was examined for the treatment of domestic wastewater at ambient temperatures. Four multi-electrode MFCs were separated into two flow lines, with two MFCs connected hydraulically in series, avoiding large COD changes in each reactor that has been shown to adversely affect power generation. The MFC-AFMBR produced a high-quality effluent (COD, 16 mg/L; TSS, < 1mg/L), with no need for membrane cleaning even after 50 d of operation, at a high flux of 16 L/m2/h. The energy produced by the MFCs (0.0197 kWh/m3) was theoretically sufficient to meet the energy demands (0.0186 kWh/m3) for the system operation. These results showed that a two-stage MFC-AFMBR system could be used to effectively treat domestic wastewater at ambient temperatures, with a low energy requirement. Mini-microbial electrolysis cells (mini-MECs) were tested as a high-throughput method to examine the effects of different chemicals on current production and the treatability of different wastewaters. Amorphous ferric hydroxide [Fe(OH)3] addition increased current densities for both mixed cultures and pure culture of Geobacter sulfurreducens in a bicarbonate buffer. Scanning electron microscopic images and electrochemical tests showed that this improved performance resulted from formation of a highly porous structure on the cathode surface, which reduced the cathode overpotential and its diffusion resistance. Tween 80 did not increase the current in MECs, although it has previously been shown to improve power production in MFCs. The addition of DNA also did not improve MEC performance. Various refinery wastewater samples, with appreciably different solution characteristics, resulted in large differences in current production and treatability in mini-MECs. All de-oiled wastewater samples showed good performance, with one sample producing results comparable to those obtained using domestic wastewater. The other refinery wastewater samples produced less current or even failed to generate any current, due to low biodegradability or high initial pH. The most successful approach for starting up MECs was pre-acclimation with domestic wastewater, as this improved electricity production, treatability, and reduced start-up time. These results showed the feasibility of using MECs as a treatment or pre-treatment method for certain types of refinery wastewaters.