Study on the electricity generation performance of microbial fuel cells in wastewater treatment

A classic two-chamber microbial fuel cell was designed, and its power generation performance for glucose simulated wastewater under the condition of inoculation with anaerobic sludge was investigated. The experiment mainly investigated the power generation performance and wastewater treatment efficiency of the battery system under different electrode materials and different COD concentrations. The results showed that the battery had the best power generation performance under the operating conditions of initial COD of 1000mg/L and graphite as electrode, with a maximum current density of 4.4mA/m2. Under different COD concentrations, the removal rate of COD in wastewater by the system was stable at 70%. In addition, the experiment also investigated the power generation performance and wastewater treatment efficiency of the battery system after aerobic sludge replaced air as the electron acceptor. Under this condition, the power generation performance of the microbial fuel cell was significantly improved, with an output current density of about 17.3mA/m2, and its COD removal rate in wastewater reached 82%.

Microbial fuel cells (MFCs) are devices that use microorganisms as catalysts to oxidize organic and inorganic substances and generate electricity [1]. As environmental problems become increasingly severe and the demand for energy increases with rapid economic development, microbial fuel cells are gaining more and more attention. Logan et al. [2] used urban domestic sewage as a nutrient to construct a new type of microbial fuel cell that can recover electricity while treating sewage, thereby reducing the cost of sewage treatment to a certain extent.

However, most of the current research on microbial fuel cells at home and abroad focuses on single-container microbial fuel cells, with the focus on reducing the internal resistance of microbial fuel cells, thereby improving the power generation performance of microbial fuel cells [3]. In traditional wastewater treatment processes, the biological treatment part is mainly composed of aerobic biological treatment and anaerobic biological treatment. This matches the structure of traditional dual-chamber microbial fuel cells. Therefore, dual-chamber microbial fuel cells are an ideal model for applying to actual wastewater treatment processes to achieve wastewater treatment and energy recovery.

Based on the above viewpoints, this study designed a classic mediator-free dual-chamber microbial fuel cell. The battery system was used to treat simulated domestic sewage and the power generation performance of the system in the process was investigated, providing a scientific basis for the application of microbial fuel cells in actual wastewater treatment processes.

1 Experimental setup and methods

1.1 Construction of a mediator-free dual-chamber microbial fuel cell system

The microbial fuel cell system is shown in Figure 1. The cell is made of plexiglass and mainly consists of two parts: the cathode chamber and the anode chamber. The mixed liquid in the anode chamber is continuously stirred by a constant temperature magnetic heating stirrer to ensure that the nutrients and microorganisms are fully mixed. The cathode chamber is aerated and oxygenated by a small air pump. The single chamber is cylindrical with an effective volume of 2009mL (Φ80mm×400mm). Both electrodes are made of graphite with an effective area of ​​350cm2. The positive and negative chambers are connected by a proton exchange membrane (Nafion 117, DuPont), and the effective area of ​​the connection is about 13cm2. The external circuit load is an adjustable resistance box (ZX97E) (1-1000000Ω). The voltage signal generated by the fuel cell is automatically collected by an external data acquisition system (personal Daq/56).

1.2 Experimental conditions

The inoculated sludge in the anode chamber was digested sludge from the sludge digestion tank of the Gaobeidian Wastewater Treatment Plant in Beijing. The sludge was cultured at room temperature for 7 days using glucose simulated wastewater with a COD of about 200 mg/L to restore the activity of the sludge and enrich the bacteria. The substrate was a nutrient reserve solution prepared with glucose, with a pH value of about 7[4-5] and a COD of about 1000 mg/L. Before entering the reactor, the anaerobic sludge and substrate needed to be passed through nitrogen for a certain period of time to remove the dissolved oxygen. The anaerobic state in the anode chamber was maintained throughout the experiment. After a power generation cycle was completed, the stirring was stopped. After the sludge in the mixed liquid was completely settled, the supernatant was discarded and new nutrients were added. The operating temperature of the entire microbial fuel cell was basically maintained at about 35°C[6-7]. During the entire experiment, the external circuit resistance was kept constant at about 100Ω.

1.3 Analysis items and methods

Determination of chemical oxygen demand (COD): Determined in accordance with the provisions of GB/T 11914-89.

Determination of electrochemical properties: The output voltage U of the battery is automatically recorded by the data acquisition system. The current in the circuit is I=U/Rw, where Rw is the external resistance. Current density ρ(I)=I/A, where A is the effective area of ​​the electrode. Power density ρ(P)=ρ(I)×U.

2 Test results

2.1 Effects of different electrode materials on the power generation performance of microbial fuel cells

For microbial fuel cells, the electrode material is directly related to the electron transfer rate and internal resistance of the battery, and has a significant impact on its power generation performance. This experiment mainly investigated two electrode materials. One is graphite with low cost and good mechanical strength. Since the reaction surface of the graphite electrode is flat, carbon fiber paper (abbreviated as carbon paper) (GEFC-GDL3, Beijing Jinneng) is selected as the comparative electrode material. Compared with traditional graphite electrodes, carbon paper has the advantages of small volume, light weight and high porosity. The comparative experiments of the two electrode materials were carried out under the conditions of COD of 1000mg/L and external resistance of 100Ω. When the COD concentration in the substrate is 1000mg/L, the comparison of the power generation performance of the two is shown in Figure 2.

As shown in Figure 2, the stability of electricity generation by graphite electrodes is better than that of carbon paper electrodes. This trend becomes more obvious in the later stage. The average current density of the external circuit of graphite electrodes is 30% higher than that of carbon paper electrodes, reaching 4.4 mA/m2. In addition, under the conditions of using carbon paper and graphite as electrodes, the removal rate of COD in simulated wastewater by the battery system is maintained at more than 70%, and the effluent COD is maintained at 300 mg/L. Therefore, for graphite and carbon paper, no matter what electrode material is used, it has no significant effect on the wastewater treatment effect of microbial fuel cells.

2.2 Power generation performance of microbial fuel cells under different COD conditions

This experiment mainly investigated the power generation performance of microbial fuel cells in the process of treating urban sewage with different COD concentrations. Considering that the COD concentration in urban sewage is generally not high, this experiment mainly considered the power generation performance of the anode (anaerobic end) of the fuel cell system when the COD was 200, 400, 600, 800, 1000, and 1500 mg/L. The output current density of the microbial fuel cell at different COD concentrations is shown in Figure 3. As can be seen from Figure 3, at different COD concentrations, the battery system has a certain current output. With the increase of COD concentration, the output current density of the microbial fuel cell system continues to increase, especially when the substrate concentration increases from 400g/L to 1000mg/L, the output current density of the system has a sharp increase, and the current density has increased from 1.6mA/m2 to 4.5mA/m2. However, as the COD concentration continues to increase, when the substrate concentration increases from 1000mg/L to 1500mg/L, the growth trend becomes gentle, and the output current does not increase significantly. The fuel cell system has a good removal effect on COD in simulated wastewater at different COD concentrations. During the entire experiment, the COD removal rate of the battery system was stable at about 70%. From the experimental results, it can be seen that the battery system has a good treatment effect on urban sewage of different concentrations, especially for low-concentration urban sewage (200~400mg/L). The COD concentration of the effluent can reach below 100mg/L after being treated by the microbial fuel cell system. Therefore, the use of microbial fuel cells can not only treat wastewater to meet environmental protection requirements but also recover part of the electrical energy.

2.3.1 Power generation rules of dual-chamber microbial fuel cells

In order to study the power generation law of the dual-chamber microbial fuel cell, the cumulative power generation of the battery under the above different COD conditions was analyzed, and the experimental results are shown in Figure 4. As shown in Figure 4, when the initial COD concentration is 1000 mg/L, the cumulative power generation of the microbial fuel cell is close to the maximum value, about 26C. When the initial COD is lower than this value, the cumulative power generation increases with the increase of the substrate concentration, and when the concentration exceeds this value, the cumulative power generation no longer increases significantly with the increase of the substrate concentration. This change law just conforms to the Monod equation of the enzymatic reaction, and this equation is used to describe the relationship between the microbial fuel cell and the initial COD:

From the nonlinear regression analysis in Figure 4, the half-saturation constant KS = 1211 mg/L can be obtained, and the maximum power generation of the microbial fuel cell Qmax = 27.52C.

2.3.2 COD degradation law of dual-chamber microbial fuel cell

In order to study the feasibility of using microbial fuel cells in actual wastewater treatment, it is necessary not only to examine the power generation performance, but also the treatment effect of the system on pollutants in wastewater. In order to analyze and evaluate the wastewater treatment capacity of the battery system, this experiment examined the wastewater treatment capacity of the battery system for different COD concentrations. The experimental results are shown in Figure 5.

As shown in Figure 5, the degradation law of COD in wastewater by microbial fuel cells conforms to the first-order kinetic equation of enzymatic reaction with respect to substrate concentration, and the regression rate of the fitting curve is high. For the entire experimental process, the operation time of the entire battery system remains unchanged at about 24h. Therefore, it can be concluded from the regression equation that the degradation rate constant of COD in wastewater by the microbial fuel cell system is k=0.215h-1.

2.4 Application of aerobic biological treatment in microbial fuel cell systems

In order to organically combine the microbial fuel cell with the actual wastewater treatment process, the feasibility of applying the system to the actual wastewater treatment process was further investigated. In this experiment, 1000mL of aerobic sludge was inoculated at the cathode (aerobic end) of the battery system, and aerobic sludge and its metabolites were used as electron acceptors instead of the usual air cathode. The aerobic sludge was taken from the sludge return tank of the Beixiaohe Wastewater Treatment Plant in Beijing. The sludge had good sedimentation properties, rich biological phase, and MLSS of 5.87g/L. After 24h of aeration to restore activity, 1000mL of glucose simulated wastewater was added, and the COD was about 500mg/L. Intermittent reaction was adopted in the cathode, and the hydraulic retention time was controlled at about 12h.

The power generation performance of the microbial fuel cell after aerobic sludge is used as the cathode is shown in Figure 6. After aerobic sludge is used as the cathode, the output current density of the microbial fuel cell is about 17.3mA/m2, which is about 4 times the current density when air is used as the cathode alone. After adding aerobic sludge to the cathode (aerobic end) of the battery, the effluent COD concentration is about 60mg/L, and the COD removal rate of the aerobic end in the wastewater reaches 82%. From the experimental results, it can be seen that using aerobic sludge instead of air as an electron acceptor can greatly improve the power generation performance of the microbial fuel cell. At the same time, the anaerobic end (anode) and aerobic end (cathode) of the microbial fuel cell are organically combined with aerobic and anaerobic biological treatment in traditional wastewater treatment.

While achieving a very ideal wastewater treatment effect, it also recovers a certain amount of electricity, meeting the requirements of wastewater treatment resource utilization.

3 Conclusion

(1) The microbial fuel cell has a stable current output when the COD concentration is 200~1500mg/L. The optimal COD concentration is 1000mg/L, at which the system output current density is 4.4mA/m2. The COD removal rate of the battery system in wastewater is stable at around 70% under different COD concentrations, achieving a relatively ideal treatment effect.

(2) Compared with graphite and carbon paper, graphite electrode has better power generation performance and mechanical strength, and is more suitable for practical engineering applications.

(3) After aerobic sludge was used as the electron acceptor instead of air at the cathode of the battery system, the power generation performance of the battery was greatly improved, with an output current density of about 17.3 mA/m2. At the same time, the COD removal rate in the wastewater reached 82%, achieving the simultaneous recovery of electrical energy while treating wastewater.

(4) The electrodes investigated in this experiment are mainly low-cost and mechanically strong graphite electrodes, and considering the cost of wastewater treatment, no modification is made to the graphite electrodes. If the graphite electrodes are modified, the current output density of the battery system can reach tens to hundreds of mA/m2.