Performances and mechanisms of MFC treating organic wastewater at various COD: sulfate ratio

Abstract

In this study, three identical two compartment single chamber air-breathing microbial fuel cells (MFC) were used to treat sulfate-rich wastewater simultaneously with electricity generation at the COD:SO42- ratio of 1, 3, and 6 in MFC1, MFC3, and MFC6, respectively. COD, sulfate, and sulfide removal, electricity generation, and mechanisms in MFCs were investigated. The MFCs were continuously operated at a hydraulic retention time of 24 hr in the first compartment. Glucose equivalent to 3,000 mgCOD/L and sulfate concentrations of 3,000, 1,000, and 500 mgSO42-/L were fed into MFC1, MFC3, and MFC6, corresponding to the COD:SO42- ratio of 1, 3, and 6, respectively. For the first compartments, COD removal efficiencies were 56.06 ± 10.67, 62.49 ± 11.21, and 63.22 ± 11.57% in MFC1, MFC3, and MFC6, respectively. Sulfate removal was 1,209 ± 455, 964 ± 93, and 492 ± 44 mgSO42-/L in MFC1, MFC3, and MFC6, respectively, whereas dissolved sulfide concentrations of 400 ± 69, 265 ± 59, and 119 ± 32 mgS2-/L were observed in MFC1, MFC3, and MFC6, respectively. From the microbial community analysis with 16S rRNA gene amplicon sequencing (MiSeq, Illumina), Tolumunas spp. were predominant species in all of the MFCs. These microorganisms were the fermenters that can ferment glucose into VFAs and acetate, which can be further consumed by sulfate-reducing bacteria (SRB, Desulfovibrio spp.) and methanogens (Methanoregulaceae and Methanosaetaceae). SRB (17.32% of total sequences) were the most abundant in MFC1 whereas methanogens (4.13% of total sequences) were the highest in MFC6. For the second compartment of MFCs, the COD removal efficiencies were 0.15 ± 9.83, 7.98 ± 10.23, and 9.98 ± 16.50% in MFC1, MFC3, and MFC6, respectively, whereas sulfate removal was negligible. Sulfide removal was 49.51 ± 57.74, 24.08 ± 13.74, and 15.69 ± 21.30 mgS2-/L in MFC1, MFC3, and MFC6, respectively. The maximum power generation of 9.33, 1.79, and 1.41 mW/m2 were achieved on the first day of operation. Then, both OCV and voltage across the electrodes decreased over time, resulting from sulfur accumulation on the anode electrodes as suggested by the results from scanning electron microscopy equipped with energy-dispersive X-ray (SEM/EDX). In addition, nonexo-electrogenic microorganisms on the anode electrodes could also increase the voltage losses in the systems. The main mechanism of electrical generation in all MFCs was abiotic sulfide oxidation. Klebsiella, Tolumonas, and Methanoseata were the predominant microorganisms on the anode electrodes, which were not exo-electrogenic microorganisms.