Effectiveness of electromagnetic in situ magnetite capture in anaerobic sequencing batch treatment of dairy effluent under electro-syntrophic conditions

Abstract

Adding conductive material has proven to be an effective way to promote direct interspecies electron transfer (DIET) and accelerate methanogenesis in anaerobic digestion processes. This study developed an automated sequencing batch reactor with a bottom-installed electromagnet and evaluated its effectiveness in enhancing the retention of magnetite (20 mM Fe) and biomass and maintaining enhanced methanogenesis. The reactor with electromagnetic in situ magnetite capture (RE) maintained better methanogenic performance and sludge settling than the reactor without it (RC) throughout the 19-month experiment at increasing organic loading rates (OLRs) (0.25–2.0 g chemical oxygen demand/L·d), particularly at higher OLRs. The washout loss of magnetite was not completely prevented in RE, although it was delayed compared with RC, suggesting the need for further research to minimize the loss of magnetite (and biomass aggregated therewith) for practical applications. DIET-based electro-syntrophic associations via magnetite-mediated electrical connections between cells appear to have developed in both reactors. Several (putative) electro-syntrophic microbes, for example, Methanothrix, Geobacter, Mesotoga, Smithella, Thermovirga, and Coriobacteriaceae species, occurred in both reactors; however, their relative abundance and distribution and the microbial community structure were significantly different between RC and RE, reflecting the effect of in situ magnetite capture on the development of methanogenic microbial community.