Ferric iron amendment increases Fe(III)-reducing microbial diversity and carbon oxidation in on-site wastewater systems

► Onsite wastewater carbon mineralization was increased by ferric iron amendment. ► Methanogenesis was inhibited by ferric iron while mineralization increased by 100%. ► Increased mineralization was correlated with increasing Fe(III) reducing biomass. ► This strategy can be applied to existing onsite wastewater treatment systems.