Magnetite-mediating direct interspecies electron transfer is not pivotal role in directing electrons toward methane production from sulfate containing wastewater

Sulfate reduction can compete for electrons with methane production, thereby reducing methane production. Several factors, such as magnetite addition, carbon sources, and the chemical oxygen demand (COD)/SO₄²⁻ ratio, can influence the direction of electron flow toward methanogenesis. However, their contributions to methane production remain unclear. In this study, the addition of magnetite significantly enhanced the maximum methane production rate (R_max). The enrichment of Methanothrix and Methanobacterium in the conductive material groups indicated the establishment of a methanogenesis process through direct interspecies electron transfer (DIET). Although the DIET pathway enhanced R_max in the conductive material groups, this increase did not translate into a higher methane yield in this study. The contribution of DIET pathway to methane yield was negligible compared with the effect of COD/SO₄²⁻ ratio and carbon sources. The relative contribution of COD/SO₄²⁻ ratio exceeded 40 % in directing electron flow.