Exploring Microbial Community Dynamics in Dual-Chamber Microbial Fuel Cells under Varied Organic Load Conditions Using Acidic Water as a Catholyte

Abstract

Microbial fuel cells (MFCs) are green technology capable of generating direct current while simultaneously treating wastewater. Although in most studies, catholytes used in MFC reactors are either phosphate buffer, direct oxygen or air cathodes. In this study, the effect of different organic loads in synthetic wastewater (equivalent to the organic load present in low- and medium-strength wastewater) as anolyte and acidic water as catholyte on cell voltage, current, power generation, and chemical oxygen demand (COD) removal efficiency with a mixed bacterial culture inoculum as biocatalyst have been accessed. Results showed a positive effect of using acidic water (pH 2) as catholyte with increasing organic load in each cycle on cell voltage, current density, and volumetric power density generation. Among all the cycles, Cycle 5 with 5.35 g/L COD load generated highest cell voltage of 205.55 mV, continuous current density, and volumetric power density of 60.45 mA/m2 and 84.50 mW/m3, respectively, with 93.85% COD removal efficiency and 77.38% Coulombic efficiency (CE). Although none of the cyclic voltammograms (CVs) showed any oxidation peak but reduction peaks were observed in Cycles 3, 4, and 5 with 1.60, 3.21, and 5.35 g/L organic load. Cycle 5 showed two sharp reduction peaks, which can be attributed to growth of mixed bacterial culture, and 16S microbiome profiling confirmed the dominance of two unclassified species, one of Sphingomonas and the other of Burkholderia, which may have contributed to the occurrence of the two sharp reduction peaks in Cycle 5. This study demonstrates that acidic water (pH 2) can be used as an effective catholyte along with species from Sphingomonas and Burkholderia genera in treatment of synthetic wastewater with COD equivalent to low- and medium-strength domestic wastewater and bioelectricity generation. Conventional wastewater treatment technologies are not sustainable to treat huge quantities of low- and medium-strength wastewater because they are cost-intensive and energy-driven processes that also release a large quantity of sludge, leading to sludge disposal problem. Moreover, they do not yield energy or any other valuable by-product. Results from our study shows that MFC can be effectively used to reduce 68.83% of total dissolved solids (TDS) and 93.85% of COD content of synthetic wastewater equivalent to low- and medium-strength wastewater, thereby efficiently treating the wastewater and also solving the problem of huge amounts of sludge generation and disposal. In the process of wastewater treatment, our study was also able to generate 84.50 mW/m3 of power density, making it an energy-yielding technology. Usage of an inexpensive mixed microbial culture inoculum as a biocatalyst (dominance of Sphingomonas and Burkholderia species) in this work showed that biocatalysts can be used as alternative to expensive metal catalysts in treatment of low- and medium-strength wastewater. During practical application of this study’s findings, synthetic wastewater can be replaced with any low- and medium-strength wastewater, be it of kitchen, domestic, or municipal origin, and acidic water as catholyte can be replaced with acidic wastewater effluents of any industry to simultaneously treat two type of wastewaters and generate bioelectricity.