Abstract
Microbial electrolysis cells (MECs) are widely considered as a next generation wastewater treatment system. However, fundamental insight on the temporal dynamics of microbial communities associated with MEC performance under different organic types with varied loading concentrations is still unknown, nevertheless this knowledge is essential for optimizing this technology for real-scale applications. Here, the temporal dynamics of anodic microbial communities associated with MEC performance was examined at low (0.5 g COD/L) and high (4 g COD/L) concentrations of acetate or propionate, which are important intermediates of fermentation of municipal wastewaters and sludge. The results showed that acetate-fed reactors exhibited higher performance in terms of maximum current density (I: 4.25 ± 0.23 A/m2), coulombic efficiency (CE: 95 ± 8%), and substrate degradation rate (98.8 ± 1.2%) than propionate-fed reactors (I: 2.7 ± 0.28 A/m2; CE: 68 ± 9.5%; substrate degradation rate: 84 ± 13%) irrespective of the concentrations tested. Despite of the repeated sampling of the anodic biofilm over time, the high-concentration reactors demonstrated lower and stable performance in terms of current density (I: 1.1 ± 0.14 to 4.2 ± 0.21 A/m2), coulombic efficiency (CE: 44 ± 4.1 to 103 ± 7.2%) and substrate degradation rate (64.9 ± 6.3 to 99.7 ± 0.5%), while the low-concentration reactors produced higher and dynamic performance (I: 1.1 ± 0.12 to 4.6 ± 0.1 A/m2; CE: 52 ± 2.5 to 105 ± 2.7%; substrate degradation rate: 87.2 ± 0.2 to 99.9 ± 0.06%) with the different substrates tested. Correlating reactor’s performance with temporal dynamics of microbial communities showed that relatively similar anodic microbial community composition but with varying relative abundances was observed in all the reactors despite differences in the substrate and concentrations tested. Particularly, Geobacter was the predominant bacteria on the anode biofilm of all MECs over time suggesting its possible role in maintaining functional stability of MECs fed with low and high concentrations of acetate and propionate. Taken together, these results provide new insights on the microbial community dynamics and its correlation to performance in MECs fed with different concentrations of acetate and propionate, which are important volatile fatty acids in wastewater.
Highlights
Microbial electrolysis cells (MECs) offer an alternative approach to effectively treat various organic waste streams with recovery of the inherent energy as hydrogen
High concentration reactors showed relatively stable current density irrespective of the substrate tested (Figure 1B), while it was dynamic in the low concentration reactors (Figure 1A)
That at the end of each batch cycle the anolyte solution was emptied and replaced with fresh autoclaved solution, and current generation was immediately observed for both acetate- and propionatefed MECs. These results suggest that the main microbial functions were taking place at the anode and the slight turbidity observed in the anolyte solution of the acetate- and propionate-fed reactors was possibly due to biofilm detachment
Summary
Microbial electrolysis cells (MECs) offer an alternative approach to effectively treat various organic waste streams with recovery of the inherent energy as hydrogen. In MECs, certain microorganisms known as exoelectrogens transport the electrons generated during the oxidation of organics in wastewater to the anode. Accumulation of volatile fatty acids (VFAs) such as acetate and propionate while treating high strength wastewater, is an important concern that leads to loss in methane production and process failure of methanogenic systems such as AD (Fernandez et al, 2000; Hashsham et al, 2000; Goux et al, 2015). Acetate and propionate represent the main VFAs, and their concentrations fluctuate resulting in a diverse and temporally fluctuating microbial communities. For MECs to become a viable anaerobic technology, it should adequately treat different concentrations (i.e., low and high) of acetate and propionate generated from various waste streams having different organic strength
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