Weihua He

Simultaneous water desalination and electricity generation in a microbial desalination cell with electrolyte recirculation for pH control

A recirculation microbial desalination cell (rMDC) was designed and operated to allow recirculation of solutions between the anode and cathode chambers. This recirculation avoided pH imbalances that could inhibit bacterial metabolism. The maximum power density was 931±29mW/m(2) with a 50mM phosphate buffer solution (PBS) and 776±30mW/m(2) with 25mM PBS. These power densities were higher than those obtained without recirculation of 698±10mW/m(2) (50mM PBS) and 508±11mW/m(2) (25mM PBS). The salt solution (20g/L NaCl) was reduced in salinity by 34±1% (50mM) and 37±2% (25mM) with recirculation (rMDC), and by 39±1% (50mM) and 25±3% (25mM) without recirculation (MDC). These results show that electrolyte recirculation using an rMDC is an effective method to increase power and achieve efficient desalination by eliminating pH imbalances.

COD removal characteristics in air-cathode microbial fuel cells

Exoelectrogenic microorganisms in microbial fuel cells (MFCs) compete with other microorganisms for substrate. In order to understand how this affects removal rates, current generation, and coulombic efficiencies (CEs), substrate removal rates were compared in MFCs fed a single, readily biodegradable compound (acetate) or domestic wastewater (WW). Removal rates based on initial test conditions fit first-order kinetics, but rate constants varied with circuit resistance. With filtered WW (100Ω), the rate constant was 0.18h(-)(1), which was higher than acetate or filtered WW with an open circuit (0.10h(-)(1)), but CEs were much lower (15-24%) than acetate. With raw WW (100Ω), COD removal proceeded in two stages: a fast removal stage with high current production, followed by a slower removal with little current. While using MFCs increased COD removal rate due to current generation, secondary processes will be needed to reduce COD to levels suitable for discharge.

Continuous electricity generation by a graphite granule baffled air–cathode microbial fuel cell

A baffled air-cathode microbial fuel cell (BAFMFC) was designed and operated under continuous flow. With glucose fed as substrate, an average voltage of 652 mV was obtained under the external resistance of 1000 Omega (30 degrees C). The maximum power density was 15.2 W/m(3) with the chemical oxygen demand (COD) removal rate of 88.0%. The overall resistance was 13.7 Omega while ohmic internal resistance was 10.8 Omega. Average COD removal rate was 69.7-88.0%, when COD loading varied from 4.11 kg COD/(m(3)NACd) to 16.0 kg COD/(m(3)NACd). The liquid from corn stover steam explosion process (COD=7160+/-50mg/L) was treated by BAFMFC, and the maximum power density was 10.7 W/m(3) with the average COD removal rate was 89.1%. The present study indicated BAFMFC can be comparable to the traditional anaerobic baffled reactor in COD removal rate for high-concentration wastewater and have an advantage in energy harvest from wastewater.

A horizontal plug flow and stackable pilot microbial fuel cell for municipal wastewater treatment

An application-oriented stackable horizontal MFC (SHMFC) was designed and proved to be capable for sewage treatment and simultaneously energy recovery. The system consisted of multiple stackable 250L modules, which is the largest single MFC module by far. Domestic wastewater was fed into SHMFC in horizontal advection. During the stable operation period, a maximum current 0.435±0.010A in each module was observed under the external resistance of 1Ω and the maximum power density was 116mW. The effluent COD was 70±17mgL(-1) with a removal rate of 79±7% and the effluent TN was 13±3mgL(-1) with a removal rate of 71±8%. From the comparison between SHMFC module (250L) and 4-cm cubic MFC (28mL), the internal resistance distribution changes and the contact resistance becomes assignable and even limiting factor in the enlargement.

A 90-liter stackable baffled microbial fuel cell for brewery wastewater treatment based on energy self-sufficient mode

A 90-liter stackable pilot microbial fuel cell was designed and proved to be capable for brewery wastewater treatment and simultaneous electricity harvested. The system was stacked by 5 easily-stackable modules, and operated in an energy self-sufficient manner for more than 6 months. Tests were conducted under two different influent strengths (diluted wastewater, stage 1; raw wastewater, stage 2). The COD, SS removal efficiencies were 84.7% and 81.7% at stage 1, 87.6% and 86.3% at stage 2. The system produced enough energy (0.056 kWh/m(3) at stage 1, 0.097 kWh/m(3) at stage 2) to power the pumping system (0.027 kWh/m(3) at both stages), net electrical energy of 0.021 kWh/m(3) and 0.034 kWh/m(3) were harvested. These results show that this pilot-scale system could be used to effectively treat real wastewater with zero energy input.

Application of nitrogen-doped carbon powders as low-cost and durable cathodic catalyst to air-cathode microbial fuel cells

Given few in-depth studies available on the application of nitrogen-doped carbon powders (NDCP) to air-cathode microbial fuel cells (ACMFCs), a low-cost and durable catalyst of NDCP was prepared and used as cathodic catalyst of ACMFCs. Compared to the untreated carbon powders, the N-doped treatment significantly increased the maximum power density (MPD) of ACMFC. A two-step pretreatment of heat treatment and hydrochloric acid immersion can further obviously increase the MPD. With a reasonably large loading of catalyst, the MPD of NDCP based ACMFC was comparable to that of carbon-supported platinum (Pt/C) based ACMFC, while the cost was dramatically reduced. The pretreatment increased the key nitrogen functional groups, pyridinic-like and pyrrolic-like nitrogen. A third new key nitrogen functional group, nitrogen oxide, was discovered and the mechanism of its contribution was explained. Compared to the inherent deterioration problem of Pt/C, NDCP exhibited high stability and was superior for long-term operation of ACMFCs.

Degradation of raw corn stover powder (RCSP) by an enriched microbial consortium and its community structure.

A microbial consortium with a high cellulolytic activity was enriched to degrade raw corn stover powder (RCSP). This consortium degraded more than 51% of non-sterilized RCSP or 81% of non-sterilized filter paper within 8 days at 40°C under facultative anoxic conditions. Cellulosome-like structures were observed in scanning electron micrographs (SEM) of RCSP degradation residue. The high cellulolytic activity was maintained during 40 subcultures in a medium containing cellulosic substrate. Small ribosomal gene sequence analyses showed the consortium contains uncultured and cultured bacteria with or without cellulolytic activities. Among these bacteria, some are anaerobic others aerobic. Analyses of the culture filtrate showed a typical anoxic polysaccharide fermentation during the culturing process. Reducing sugar concentration increased at early stage followed by various fermentation products that were consumed at the late stage.

Effects of sulfide on microbial fuel cells with platinum and nitrogen-doped carbon powder cathodes

Because of the advantages of low cost, good electrical conductivity and high oxidation resistance, nitrogen-doped carbon (NDC) materials have a potential to replace noble metals in microbial fuel cells (MFCs) for wastewater treatment. In spite of a large volume of studies on NDC materials as catalysts for oxygen reduction reaction, the influence of sulfide on NDC materials has not yet been explicitly reported so far. In this communication, nitrogen-doped carbon powders (NDCP) were prepared by treating carbon powders in nitric acid under reflux condition. Sodium sulfide (Na(2)S) was added to the cathodic electrolyte to compare its effects on platinum (Pt) and NDCP cathodes. Cell voltages, power density and cathodic potentials were monitored without and with Na(2)S and after Na(2)S was removed. The maximum cell voltage of the MFCs with Pt cathode decreased by 10% in the presence of Na(2)S that did not change the performance of the MFC with NDCP cathode, and the maximum power density of the MFC with NDCP cathode was even 11.3% higher than that with Pt cathode (222.5 ± 8 mW m(-2) vs. 199.7 ± 4 mW m(-2)).

High current densities enable exoelectrogens to outcompete aerobic heterotrophs for substrate

In mixed‐culture microbial fuel cells (MFCs), exoelectrogens and other microorganisms compete for substrate. It has previously been assumed that substrate losses to other terminal electron acceptors over a fed‐batch cycle, such as dissolved oxygen, are constant. However, a constant rate of substrate loss would only explain small increases in coulombic efficiencies (CEs, the fraction of substrate recovered as electrical current) with shorter cycle times, but not the large increases in CE that are usually observed with higher current densities and reduced cycle times. To better understand changes in CEs, COD concentrations were measured over time in fed‐batch, single‐chamber, air‐cathode MFCs at different current densities (external resistances). COD degradation rates were all found to be first‐order with respect to COD concentration, even under open circuit conditions with no current generation (first‐order rate constant of 0.14 ± 0.01 h−1). The rate of COD removal increased when there was current generation, with the highest rate constant (0.33 ± 0.02 h−1) obtained at the lowest external resistance (100 Ω). Therefore, as the substrate concentration was reduced more quickly due to current generation, the rate of loss of substrate to non‐exoelectrogens decreased due to this first‐order substrate‐concentration dependence. As a result, coulombic efficiencies rapidly increased due to decreased, and not constant, removal rates of substrate by non‐exoelectrogens. These results show that higher current densities (lower resistances) redirect a greater percentage of substrate into current generation, enabling large increase in CEs with increased current densities. Biotechnol. Bioeng. 2014;111: 2163–2169.

Microbial fuel cells with an integrated spacer and separate anode and cathode modules

A new type of scalable MFC was developed based on using alternating graphite fiber brush array anode modules and dual cathode modules in order to simplify construction, operation, and maintenance of the electrodes. The modular MFC design was tested with a single (two-sided) cathode module with a specific surface area of 29 m2 m−3 based on a total liquid volume (1.4 L; 20 m2 m−3 using the total reactor volume of 2 L), and two brush anode modules. Three different types of spacers were used in the cathode module to provide structural stability, and enhance air flow relative to previous cassette (combined anode–cathode) designs: a low-profile wire spacer; a rigid polycarbonate column spacer; and a flexible plastic mesh spacer. The best performance was obtained using the wire spacer that produced a maximum power density of 1100 ± 10 mW m−2 of cathode (32 ± 0.3 W m−3 based on liquid volume) with an acetate-amended wastewater (COD = 1010 ± 30 mg L−1), compared to 1010 ± 10 mW m−2 for the column and 650 ± 20 mW m−2 for the mesh spacers. Anode potentials were unaffected by the different types of spacers. Raw domestic wastewater produced a maximum of 400 ± 8 mW m−2 under fed batch conditions (wire-spacers), which is one of the highest power densities for this fuel. Over time the maximum power was reduced to 300 ± 10 mW m−2 and 275 ± 7 mW m−2 for the two anode compartments, with only slightly less power of 250 ± 20 mW m−2 obtained under continuous flow conditions. In fixed-resistance tests, the average COD removal was 57 ± 5% at a hydraulic retention time of 8 h. These results show that this modular MFC design can both simplify reactor construction and enable relatively high power generation from even relatively dilute wastewater.