Electricity production and the analysis of the anode microbial community in a constructed wetland-microbial fuel cell

Abstract

The objective of this study is to assess bioelectricity generation, pollutant removal (COD, ammonium, nitrate) and the bacterial communities on anodes in constructed wetlands coupled with microbial fuel cells (CW-MFCs), through feeding the systems with three different types of synthetic wastewater (system 1: normal wastewater; system 2: ammonium-free wastewater; system 3: nitrate-free wastewater). Three CW-MFCs were operated with different wastewater concentrations and hydraulic retention times (HRTs) over a long time period (6 months). The results indicate that the maximum open circuit voltage (775.63 mV) and maximum power density (0.628 W m^-3) were observed in system 3 (period 3), and that bioenergy production was inhibited in system 2, when feeding with ammonium-free wastewater continuously. COD removal rates in the three systems were similar during each period and ranged from 82.2 ± 6.8% to 98.3 ± 2.2%. Ammonium removal occurred at the air cathode of the CW-MFCs through nitrification, and a higher level of ammonium removal was found in system 1 (period 3) compared with the others. Meanwhile, denitrification occurred at the anaerobic anode of the CW-MFCs, and a large amount of nitrate was removed effectively. The highest nitrate removal rate was 98.8 ± 0.5% in system 2 (period 3). Additionally, four genera related to electricity generation were detected at the anode: Geothrix; Desulfovibrio; Desulfobulbus; and Geobacter. The relative abundances of Desulfovibrio, Desulfobulbus and Geothrix gradually increased during the three periods in system 3, which might be beneficial for bioelectricity generation. Further investigations are needed to optimize the CW-MFC performance and explain the mechanism behind the pollutant degradation and electron motion in the CW-MFCs.

Fig. 1. The cell voltage performances of the three systems. (A) Cell voltage variations of the three systems during periods 1, 2 and 3. (B), (C) and (D) Comparisons of the average voltages between the three systems in periods 1, 2 and 3. The error bars indicate standard deviation. An asterisk (*) denotes significant difference (p < 0.05) between system 1 and another system.

Fig. 2. The cell polarization curves and power densities of the three systems. (A), (B) and (C) Polarization curves from periods 1, 2, and 3, respectively. (D) A comparison of the maximum current density and maximum power density values between the three systems during periods 1, 2 and 3.

Fig. 3. Differences in microbial community structure distributions on the surfaces of anode electrode samples between the three systems during all periods, as indicated by weighted fast UniFrac PCoA based on phylogenetic lineages.

Fig. 4. The relative abundances of 16S rDNA sequences in anode biofilm samples from the CW-MFCs supplemented with different types of synthetic wastewater during three periods at the phylum level.

Fig. 5. A heat map graph of the hierarchy clusters for the top 100 genera. The color intensity in each panel respects the similarity characteristic between the four samples. Red and blue colors mean the good or poor enrichment, respectively, of a genus.