Hydrogen production in single-chamber microbial electrolysis cell under high applied voltages

High-performance nanofiltration concentrate treatment by a five-chamber bioelectrochemical system

A combination of bioelectrochemical systems and electrodialysis has been considered an effective strategy for removing salts from the nanofiltration (NF) concentrate of electroplating wastewater; however, the recovery efficiency of multivalent metals is low. Herein, a new process based on microbial electrolysis desalination and chemical-production cell with five chambers (MEDCC-FC) has been proposed for the simultaneous desalination and recovery of the multivalent metals from NF concentrate. The MEDCC-FC was found to be significantly superior to the MEDCC with the monovalent selective cation exchange membrane (MEDCC-MSCEM) and MEDCC with the cation exchange membrane (MEDCC-CEM), in terms of the elevated desalination efficiency, multivalent metal recovery efficiency, current density, and coulombic efficiency, and decreased energy consumption and membrane fouling. Within 12 h, the MEDCC-FC provided the desirable outcome, indicated by a maximum current density of 6.88 ± 0.06 A/m2, desalination efficiency of 88 ± 10%, metals recovery efficiency of >58%, and total energy consumption of 1.17 ± 0.11 kWh for the per kg total dissolved solids removal. Mechanistic studies revealed that the integration of CEM and MSCEM in the MEDCC-FC promoted the separation and recovery of multivalent metal. These findings revealed that the proposed MEDCC-FC was promising in treating NF concentrate of electroplating wastewater towards advantages of effectiveness, economic viability, and flexibility.

Efficient hydrogen production in single-chamber microbial electrolysis cell with a fermentable substrate under hyperalkaline conditions

Hydrogen production from food waste is of great significance for energy conversion and pollution control. The aim of this study was to investigate the glucose fermentation from food waste and hydrogen (H2) production in the single-chamber microbial electrolysis cell (MEC) under hyperalkaline conditions. Single-chamber MECs were tested with 1 g/L glucose as substrate under different pH values (i.e., 7.0, 9.5, and 11.2) and applied voltages (i.e., 0.8, 1.2, and 1.6 V). With pH increase from 7.0 to 11.2, H2 production with methanogenesis inhibition was significantly improved in the MEC. At pH of 11.2, the maximum current density reached 180 ± 9 A/m3 with the H2 purity of 93.3 ± 1.2% and average H2 yield of 7.72 ± 0.23 mol H2/ mol glucose under 1.6 V. Acetate from glucose fermentation was the largest electron sink within 12 h. Methanobacterium alcaliphilum dominated the archaeal communities with the relative abundance of > 99.0% in the cathodic biofilms. The microbial communities and mcr A gene copy numbers analyses showed that high pH enhanced the acetate production from glucose fermentation, inhibited syntrophic acetate-oxidizing with hydrogenotrophic methanogenesis in the anodic biofilms, and inhibited hydrogenotrophic methanogenesis in the cathodic biofilms. Our results of hyperalkaline conditions provide a feasible way to harvest H2 efficiently from fermentable substrates in the single-chamber MEC.

High current density with spatial distribution of Geobacter in anodic biofilm of the microbial electrolysis desalination and chemical-production cell with enlarged volumetric anode

The aim of this study was to establish the relationship between spatial distribution of Geobacter and electric intensity in the microbial electrolysis desalination and chemical-production cell (MEDCC) and to investigate the effect of enlarged volumetric anode on the performance of MEDCC. The MEDCC was constructed with nine carbon brush anodes (length × diameter = 11 cm × 3 cm) as enlarged volumetric anode, and operated by feeding with 1 g/L acetate as substrate and 35 g/L NaCl as artificial seawater under the applied voltages of 1.2-4.5 V. Spatial distribution of Geobacter in the anodic biofilm was determined according to the bacterial community analysis on 27 biofilm samples from the top, middle and bottom layers of anodes (i.e., with distance of 4.5, 10, and 15.5 cm to the cathode, respectively). Results showed that the enlarged volumetric anode significantly improved the performance of MEDCC. The maximum desalination rate and current density reached 338.5 ± 21.8 mg/L∙h and 55.7 ± 3.7 A/m² in the MEDCC, respectively. The electric intensity values decreased with the distance from the anode to the cathode and formed an uneven distribution in the anode chamber. The samples in the top layer of anodes had the highest average 16S rRNA gene copy number of Geobacter of 1.55 × 10⁷ copies/μL, which was 18 times higher than that in the bottom layer of anodes. A linear relation was established between the spatial distribution of Geobacter and electric intensity (R² = 0.994-0.999). The electric intensity gradient created the uneven spatial distribution of Geobacter in the biofilms of volumetric anode. Results from this study could be useful to enrich Geobacter in the anodic biofilm thus to improve the performance of MEDCC.

Enhanced sulfur recovery and sulfate reduction using single-chamber bioelectrochemical system

The aim of this study was to investigate the feasibility of sulfate removal and elemental sulfur (S⁰) recovery in the single-chamber bioelectrochemical system (S-BES). The performance of S-BES was compared with that of dual-chamber bioelectrochemical system (D-BES). The S-BES was constructed with graphite felt as the anode and graphite brush as the cathode. The D-BES was constructed with proton exchange membrane as the separator between anode and cathode chambers. With an applied voltage of 1.0 V and 1 g/L acetate as the substrate, the S-BES and D-BES were tested by feeding with 480 mg/L SO₄^2- in the phosphate buffer. Results showed that the maximum current density of 37.6 ± 4.5 mA/m³ was reached in the S-BES, which was higher than that in the D-BES (i.e., 22.2 ± 2.6 mA/m³). The SO₄^2- removal was much higher in the S-BES than in the D-BES (99.5% vs. 57.2%). In the effluent and the electrodes of S-BES, S⁰ was identified with Raman and X- Ray diffraction analyses. The S⁰ recovery on the anode was 13.7 times of that on the cathode of S-BES, indicating that S⁰ was mainly produced on the anode. The measured total S⁰ recovery reached 67.5% in the S-BES. High relative abundance of Desulfurella (47.1%) and Geobacter (26.1%) dominated the community in the anode biofilm of S-BES. The excellent performance of S-BES may be attributed to the neutral pH in the solution and the synergistic reaction between the anode and cathode. Results from this study should be useful to enhance the S-BES applications in treating wastewater containing sulfate.

Enhanced anaerobic digestion of swine manure via a coupled microbial electrolysis cell

Microbial electrolysis cell coupled anaerobic digestion (MEC-AD) is a new technology in energy recovery and waste treatment, which could be used to recycle swine manure. Here, different applied voltage effects were studied using MEC-AD with swine manure as a substrate. The maximum cumulative biogas and methane yields, both occurring with 0.9 V, were 547.3 mL/g total solid (TS) and 347.7 mL/g TS, respectively. The increased energy can counterbalance the electrical input. First order, logistic, gompertz, and back-propagation artificial neural network (BP-ANN) models were used to study cumulative biogas and methane yields. The BP-ANN model was superior to the other three models. The maximum degradation rate of hemicellulose, cellulose, and lignin was 60.97%, 48.59%, and 31.59% at 0.9 V, respectively. The BP-ANN model establishes a model for cumulative biogas and methane yields using MEC-AD. Thus, MEC-AD enhanced biogas and methane production and accelerated substrate degradation at a suitable voltage.