•MFC and flocculation process were combined to treat the mixture of raw swine wastewater and its denitrification effluent.•When the mixed ratio was 40:60, MFCs removed 99.1 ± 0.1% of ammonia and produced a maximum power density of 37.5 W m−3.•MFC effluent was further treated by flocculation process with an overall COD removal efficiency of 96.6 ± 0.2%.•The economic benefit created by MFCs was sufficient to offset the cost of flocculation process.Microbial fuel cells (MFCs) provide a cost-effective method for treating swine wastewater treatment and simultaneously producing electricity, yet they need to be combined with other wastewater treatment processes to improve the effluent water quality. In this paper, we constructed single-chamber air-cathode MFCs with a compact configuration for nitrogen and COD removal and high electricity production and combined them with a low-cost flocculation process to discharge higher quality wastewater. We show that MFCs could remove ammonia at a rate of 269.2 ± 0.5 g m−3 d−1 (99.1± 0.1% ammonia removal efficiency) with a maximum power density of 37.5 W m−3 and 21.6% of coulombic efficiency at a 40:60 ratio of raw swine wastewater to denitrification effluent of swine wastewater. Up to 82.5 ± 0.5% COD could be removed with MFCs, from 2735 ± 15 mg L−1 to 480 ± 15 mg L−1, and flocculation further reduced levels to 90 ± 1 mg L−1 for a 96.6 ± 0.2% overall COD removal efficiency of the combination technology. Cost analysis of the combined MFC and flocculation process showed a net economic benefit of $ 0.026 m-3. In summary, this novel combination wastewater treatment method provides an effective way to treat swine wastewater to low pollutant levels in the effluent at low cost (a net gain).Download high-res image (140KB)Download full-size image

•A biofilm with a live outer-layer and dead inner-core was observed under low current.•A single and completely viable biofilm was observed under high current.•Switching biofilms between currents produced an outcome associated with the new one.Understanding how current densities affect electrogenic biofilm activity is important for wastewater treatment as current densities can substantially decrease at COD concentrations greater than those suitable for discharge to the environment. We examined the biofilm’s response, in terms of viability and enzymatic activity, to different current densities using microbial electrolysis cells with a lower (0.7 V) or higher (0.9 V) added voltage to alter current production. Viability was assessed using florescent dyes, with dead cells identified on the basis of dye penetration due to a compromised cell outer-membrane (red), and live cells (intact membrane) fluorescing green. Biofilms operated with 0.7 V produced 2.4 ± 0.2 A m−2, and had an inactive layer near the electrode and a viable layer at the biofilm-solution interface. The lack of cell activity near the electrode surface was confirmed by using an additional dye that fluoresces only with enzymatic activity. Adding 0.9 V increased the current by 61%, and resulted in a single, more homogeneous and active biofilm layer. Switching biofilms between these two voltages produced outcomes associated with the new current rather than the previous biofilm conditions. These findings suggest that maintaining higher current densities will be needed to ensure long-term viability electrogenic biofilms.Download high-res image (413KB)Download full-size image

To increase the power generation of microbial fuel cells (MFCs), anode modification with carbon materials (activated carbon, carbon nanotubes, and carbon nanohorns) was investigated.Maximum power densities of a stainless-steel anode MFC with a non-modified electrode (SS-MFC), an activated carbon-modified electrode (AC-MFC), a carbon nanotube-modified electrode (CNT-MFC) and a carbon nanohorn-modified electrode (CNH-MFC) were 72, 244, 261 and 327 mW m−2, respectively. The total polarization resistance measured by electrochemical impedance spectroscopy were 3610 Ω for SS-MFC, 283 Ω for AC-MFC, 231 Ω for CNTs-MFC, and 136 Ω for CNHs-MFC, consistent with the anode resistances obtained by fitting the anode polarization curves.Single-wall carbon nanohorns are better than activated carbon and carbon nanotubes as a new anode modification material for improving anode performance.

•The G. sulfurreducens biofilm at the highest electrochemical activity was much thinner than the final biofilm.•Fast accumulation of inactive cells caused the lower current generation in the thick biofilm.•Highly effective biofilm required for high absolute-mass and relative-ratio of live cells rather than thick thickness.It is widely believed that the high electrochemical performance of Geobacter sulfurreducens relies on its thick and conductive biofilms in bioelectrochemical systems. However, in this study, the G. sulfurreducens biofilm reached the highest electrochemical activity with a biofilm thickness of ∼20 μm, and then the electrochemical activity decreased with increasing thickness until the biofilm growth ceased at a thickness of ∼45 μm. The electrochemical analysis and the metabolic spatial variability showed that in the first 5 cycles the live cells grew fast, which led to a rapid drop of charge transfer resistance and further contributed to high current generation, however, from cycle 5 to 12, a great many inactive cells accumulated in the inner layer of biofilm, which resulted in high diffusion resistance. Thus, although the G. sulfurreducens can always form thick biofilms, its highest electrochemical activity reached at a much thinner thickness, suggesting that the live-cell mass rather than the biofilm thickness is responsible for the high current generation.

Sustained current generation by anodic biofilms is a key element for the longevity and success of bioelectrochemical systems. Over time, however, inactive or dead cells can accumulate within the anode biofilm, which can be particularly detrimental to current generation. Mixed and pure culture (Geobacter anodireducens) biofilms were examined here relative to changes in electrochemical properties over time. An analysis of the three-dimensional metabolic structure of the biofilms over time showed that both types of biofilms developed a live outer-layer that covered a dead inner-core. This two-layer structure appeared to be mostly a result of relatively low anodic current densities compared to other studies. During biofilm development, the live layer reached a constant thickness, whereas dead cells continued to accumulate near the electrode surface. This result indicated that only the live outer-layer of biofilm was responsible for current generation and suggested that the dead inner-layer continued to function as an electrically conductive matrix. Analysis of the electrochemical properties and biofilm thickness revealed that the diffusion resistance measured using electrochemical impedance spectroscopy might not be due to acetate or proton diffusion limitations to the live layer, but rather electron-mediator diffusion.

•A simple model to simulate the power loss of scale up microbial fuel cell.•Leading-out terminal could result in more than 47.1% of power loss.•Leading-out terminal of anode is one of the key factors for scaling up MFC.Low power output and high cost are two major challenges for scaling up microbial fuel cell (MFC). The ohmic resistance of anode increasing as MFCs scale up can be one of main reasons for power density decrease. We present a simple model to simulate power loss and potential drop distribution caused by ohmic resistance of carbon mesh anodes with different dimensions and various leading-out terminals. We also conduct experiments to confirm the simulation work and the large impact of anode ohmic resistance on large-scale MFCs by varying leading-out configurations. The simulation results show that the power loss with an anode size of 1 m2 can be as high as 4.19 W at current density of 3 A m−2, and the power loss can be decreased to 0.04 W with optimized configuration of leading-out terminals and to 0.01 W by utilizing brass mesh as anode material. The experiments well confirm the simulation results with the deviations less than 11.0%. Furthermore, the experiment results also show that more than 47.1% of the power loss from small-scale to large-scale MFC comes from bad-leading-out terminal. These results demonstrate that leading-out terminal of anode is one of the key factors for scaling up MFC.

•Increasing hydraulic pressure reduces the power generation of MFCs.•Power density decrease is due to the performance decline in both cathode and anode.•High hydraulic pressure temporarily restrains the metabolism of anodic bacteria.•High hydraulic pressure raises cathodic charge transfer and diffusion resistances.Scaling up of microbial fuel cells (MFCs) without losing power density requires a thorough understanding of the effect of hydraulic pressure on MFC performance. In this work, the performance of an activated carbon air-cathode MFC was evaluated under different hydraulic pressures. The MFC under 100 mmH2O hydraulic pressure produced a maximum power density of 1260±24 mW m–2, while the power density decreased by 24.4% and 44.7% as the hydraulic pressure increased to 500 mmH2O and 2000 mmH2O, respectively. Notably, the performance of both the anode and the cathode had decreased under high hydraulic pressures. Electrochemical impedance spectroscopy tests of the cathode indicated that both charge transfer resistance and diffusion transfer resistance increased with the increase in hydraulic pressure. Denaturing gradient gel electrophoresis of PCR-amplified partial 16S rRNA genes demonstrated that the similarity among anodic biofilm communities under different hydraulic pressures was ≥90%, and the communities of all MFCs were dominated by Geobacter sp. These results suggested that the reduction in power output of the single chamber air-cathode MFC under high hydraulic pressures can be attributed to water flooding of the cathode and suppression the metabolism of anodic exoelectrogenic bacteria.

Much energy is stored in wastewaters. How to efficiently capture this energy is of great significance for meeting the world’s energy needs, reducing wastewater handling costs and increasing the sustainability of wastewater treatment. The microbial fuel cell (MFC) is a recently developed biotechnology for electrical energy recovery from the organic pollutants in wastewaters. MFCs hold great promise for sustainable wastewater treatment. However, at present there is still much research needed before the MFC technique can be practically applied in the real world. In this review, we analyze the opportunities and key challenges for MFCs to achieve sustainability in wastewater treatment. We especially discuss the problems and challenges for scaling up the MFC systems; this is the most critical issue for realizing the practical implementation of this technique. In order to achieve sustainability, MFCs may also be combined with other techniques to yield high effluent quality or to recover more commercial value (i.e., by producing energy-rich or high value chemicals) from wastewaters. However, research in this area is still on-going and many problems need to be settled before real-world application. Advances are required in respect of efficiency, economic feasibility, system stability, and reliability.废水中蕴含着大量能量, 如何高效地回收利用这些能量对于满足世界能源需求, 降低废水处理成本, 提高污水处理的可持续性具有重要意义。 微生物燃料电池(MFC)是近年来发展起来的一种从废水的有机污染物中提取能量的新型生物技术, 有望实现废水处理的可持续性发展。 然而, 目前MFC 技术离实际应用还有很长的距离。 MFC 系统的扩大化问题是阻碍该技术实际应用的关键。 本文详细讨论了MFC 扩大化过程中的主要问题和挑战, 并提出了未来的发展方向。 MFC 与其他技术结合可以实现较高的出水水质或获得高商业价值的化学品, 然而该方面的研究才刚刚起步, 要实现其实际应用还需要解决许多问题, 包括如何提高生产效率, 提高经济可行性, 提升系统的定性和可靠性等。

•Ni–P coated Ni foam is prepared for catalyzing hydrogen production in MECs.•7.9 at.% P in the Ni–P coating exhibits the smallest hydrogen evolution overpotential.•Ni–P cathode obtains super hydrogen production rate and low methane generation in MECs.This study employed nickel foam (Ni foam), Ni–P coated nickel foam (Ni–P) and stainless steel (SS) as cathode materials for hydrogen production in single chamber microbial electrolysis cells (MECs), to replace noble metal catalysts such as Pt. The hydrogen evolution performance of the Ni–P cathode was significantly superior to Ni foam and SS. LSV and EDS tests showed that about 8 at.% of P in the Ni–P coating exhibited the smallest hydrogen evolution overpotential. The current density and hydrogen production rate using different cathode materials in lab-scale MECs showed similar trend as the electrochemical performance of cathodes. The maximum improvement of hydrogen production rate using the Ni–P cathode was 7.5% higher than Ni foam, 110% higher than SS, while hydrogen production rate of Ni–P reached 2.29 ± 0.11 L H2/L/d at an applied voltage of 0.9 V. Owing to the improved hydrogen recovery rate, the methane generation, another challenge for the practicability of MECs, was effectively inhibited on the Ni–P cathode. This study provides useful information for the preparation of high-efficient and low-cost cathode materials in MECs.