Yongtae Ahn

A Two-Stage Microbial Fuel Cell and Anaerobic Fluidized Bed Membrane Bioreactor (MFC-AFMBR) System for Effective Domestic Wastewater Treatment

Microbial fuel cells (MFCs) are a promising technology for energy-efficient domestic wastewater treatment, but the effluent quality has typically not been sufficient for discharge without further treatment. A two-stage laboratory-scale combined treatment process, consisting of microbial fuel cells and an anaerobic fluidized bed membrane bioreactor (MFC-AFMBR), was examined here to produce high quality effluent with minimal energy demands. The combined system was operated continuously for 50 days at room temperature (∼25 °C) with domestic wastewater having a total chemical oxygen demand (tCOD) of 210 ± 11 mg/L. At a combined hydraulic retention time (HRT) for both processes of 9 h, the effluent tCOD was reduced to 16 ± 3 mg/L (92.5% removal), and there was nearly complete removal of total suspended solids (TSS; from 45 ± 10 mg/L to <1 mg/L). The AFMBR was operated at a constant high permeate flux of 16 L/m2/h over 50 days, without the need or use of any membrane cleaning or backwashing. Total electrical energy required for the operation of the MFC-AFMBR system was 0.0186 kWh/m3, which was slightly less than the electrical energy produced by the MFCs (0.0197 kWh/m3). The energy in the methane produced in the AFMBR was comparatively negligible (0.005 kWh/m3). These results show that a combined MFC-AFMBR system could be used to effectively treat domestic primary effluent at ambient temperatures, producing high effluent quality with low energy requirements.

A multi-electrode continuous flow microbial fuel cell with separator electrode assembly design

Scaling up microbial fuel cells (MFCs) requires the development of compact reactors with multiple electrodes. A scalable single chamber MFC (130xa0mL), with multiple graphite fiber brush anodes and a single air-cathode cathode chamber (27xa0m2/m3), was designed with a separator electrode assembly (SEA) to minimize electrode spacing. The maximum voltage produced in fed-batch operation was 0.65xa0V (1,000xa0Ω) with a textile separator, compared to only 0.18xa0V with a glass fiber separator due to short-circuiting by anode bristles through this separator with the cathode. The maximum power density was 975xa0mW/m2, with an overall chemical oxygen demand (COD) removal of >90% and a maximum coulombic efficiency (CE) of 53% (50xa0Ω resistor). When the reactor was switched to continuous flow operation at a hydraulic retention time (HRT) of 8xa0h, the cell voltage was 0.21u2009±u20090.04xa0V, with a very high CEu2009=u200985%. Voltage was reduced to 0.13u2009±u20090.03xa0V at a longer HRTu2009=u200916xa0h due to a lower average COD concentration, and the CE (80%) decreased slightly with increased oxygen intrusion into the reactor per amount of COD removed. Total internal resistance was 33xa0Ω, with a solution resistance of 2xa0Ω. These results show that the SEA type MFC can produce stable power and a high CE, making it useful for future continuous flow treatment using actual wastewaters.

Domestic wastewater treatment using multi-electrode continuous flow MFCs with a separator electrode assembly design.

Treatment of domestic wastewater using microbial fuel cells (MFCs) will require reactors with multiple electrodes, but this presents unique challenges under continuous flow conditions due to large changes in the chemical oxygen demand (COD) concentration within the reactor. Domestic wastewater treatment was examined using a single-chamber MFC (130xa0mL) with multiple graphite fiber brush anodes wired together and a single air cathode (cathode specific area of 27xa0m2/m3). In fed-batch operation, where the COD concentration was spatially uniform in the reactor but changed over time, the maximum current density was 148u2009±u20098xa0mA/m2 (1,000xa0Ω), the maximum power density was 120xa0mW/m2, and the overall COD removal was >90xa0%. However, in continuous flow operation (8xa0h hydraulic retention time, HRT), there was a 57xa0% change in the COD concentration across the reactor (influent versus effluent) and the current density was only 20u2009±u200913xa0mA/m2. Two approaches were used to increase performance under continuous flow conditions. First, the anodes were separately wired to the cathode, which increased the current density to 55u2009±u200915xa0mA/m2. Second, two MFCs were hydraulically connected in series (each with half the original HRT) to avoid large changes in COD among the anodes in the same reactor. The second approach improved current density to 73u2009±u200913xa0mA/m2. These results show that current generation from wastewaters in MFCs with multiple anodes, under continuous flow conditions, can be improved using multiple reactors in series, as this minimizes changes in COD in each reactor.

Treating refinery wastewaters in microbial fuel cells using separator electrode assembly or spaced electrode configurations.

The effectiveness of refinery wastewater (RW) treatment using air-cathode, microbial fuel cells (MFCs) was examined relative to previous tests based on completely anaerobic microbial electrolysis cells (MECs). MFCs were configured with separator electrode assembly (SEA) or spaced electrode (SPA) configurations to measure power production and relative impacts of oxygen crossover on organics removal. The SEA configuration produced a higher maximum power density (280±6 mW/m(2); 16.3±0.4 W/m(3)) than the SPA arrangement (255±2 mW/m(2)) due to lower internal resistance. Power production in both configurations was lower than that obtained with the domestic wastewater (positive control) due to less favorable (more positive) anode potentials, indicating poorer biodegradability of the RW. MFCs with RW achieved up to 84% total COD removal, 73% soluble COD removal and 92% HBOD removal. These removals were higher than those previously obtained in mini-MEC tests, as oxygen crossover from the cathode enhanced degradation in MFCs compared to MECs.

Saline catholytes as alternatives to phosphate buffers in microbial fuel cells

Highly saline solutions were examined as alternatives to chemical buffers in microbial fuel cells (MFCs). The performance of two-chamber MFCs with different concentrations of saline solutions in the cathode chamber was compared to those with a buffered catholyte (50mM PBS). The use of a NaCl catholyte improved the CE to 43-60% (28% with no membrane) due to a reduction in oxygen transfer into the anolyte. The saline catholyte also reduced the membrane and solution resistance to 23Ω (41Ω without a membrane). The maximum power density of 491mW/m(2) (240mM NaCl) was only 17% less than the MFC with 50mM PBS. The decrease in power output with highest salinity was due to reduced proton transfer due to the ion exchange membrane, and pH changes in the two solutions. These results show that MFC performance can be improved by using a saline catholyte without pH control.

Reference and counter electrode positions affect electrochemical characterization of bioanodes in different bioelectrochemical systems.

The placement of the reference electrode (RE) in various bioelectrochemical systems is often varied to accommodate different reactor configurations. While the effect of the RE placement is well understood from a strictly electrochemistry perspective, there are impacts on exoelectrogenic biofilms in engineered systems that have not been adequately addressed. Varying distances between the working electrode (WE) and the RE, or the RE and the counter electrode (CE) in microbial fuel cells (MFCs) can alter bioanode characteristics. With well‐spaced anode and cathode distances in an MFC, increasing the distance between the RE and anode (WE) altered bioanode cyclic voltammograms (CVs) due to the uncompensated ohmic drop. Electrochemical impedance spectra (EIS) also changed with RE distances, resulting in a calculated increase in anode resistance that varied between 17 and 31u2009Ω (−0.2u2009V). While WE potentials could be corrected with ohmic drop compensation during the CV tests, they could not be automatically corrected by the potentiostat in the EIS tests. The electrochemical characteristics of bioanodes were altered by their acclimation to different anode potentials that resulted from varying the distance between the RE and the CE (cathode). These differences were true changes in biofilm characteristics because the CVs were electrochemically independent of conditions resulting from changing CE to RE distances. Placing the RE outside of the current path enabled accurate bioanode characterization using CVs and EIS due to negligible ohmic resistances (0.4u2009Ω). It is therefore concluded for bioelectrochemical systems that when possible, the RE should be placed outside the current path and near the WE, as this will result in more accurate representation of bioanode characteristics. Biotechnol. Bioeng. 2014;111: 1931–1939.

Influence of Electrode Spacing on Methane Production in Microbial Electrolysis Cell Fed with Sewage Sludge

Effect of electrode spacing on the performance of microbial electrolysis cells(MECs) for treating sewage sludge was investigated through lab scale experiment. The reactors were equipped with two pairs of electrodes that have a different electrode spacing (16, 32 mm). Shorter electrode distance improved the overall performance of MEC system. With the 16 mm of electrode distance, the current density was 3.04~3.74 A/m and methane production was 0.616~0.804 Nm/m, which were higher than those obtained with 32 mm of electrode spacing (1.50~1.82 A/m, 0.529~0.664 Nm/m). The COD removal was in the range of 34~40%, and the VSS reduction ranged 32~38%. As the current production increased, VSS reduction and methane production were increased possibly due to the improved bioelectrochemical performance of the system. Methane production was more affected by current density than VSS reduction. These results imply that the reducing the electrode spacing can enhance the methane production and recovery from sewage sludge with the decreased internal resistance, however, it was not able to improve VSS reduction of sewage sludge.

The Effect of Electrode Spacing and Size on the Performance of Soil Microbial Fuel Cells (SMFC)

Soil microbial fuel cells (SMFC) have gained a great attention as an eco-friendly technology that can simultaneously generate electricity and treat organic pollutants from the contaminated soil. We evaluated the effect of electrode spacing and size on the performance of SMFC treating soil contaminated with organic pollutants. Maximum power density decreased with increase in electrode distance or decrease in electrode size, likely due to higher internal resistance. The maximum voltage and power density decreased from 326 mV and 19.5 mW/m with 4 cm of electrode distance to 222 mV and 5.9 mW/m with 9 cm of electrode distance. In case of electrode size test, the maximum voltage and power density generated was 291 mV, 0.34 mW/m when both of anode and cathode area were 64 cm with 4 cm of electrode distance. The maximum voltage decreased by 19~29% when the anode area decreased to 16 cm while only 3~12% of voltage decreased with cathode area decrease. The maximum power density decreased by 49~68% with decreasing anode size, and by 29~47% with decreasing cathode size. These results showed that the anode area had more significant effects than the cathode area on the power generation of SMFC which has a high internal resistance due to a coexistence of soil and wastewater in the reactor.