A novel concept of dosing iron ions using Fe3O4 engineered nanoparticles is used to improve biogas production in anaerobic digestion processes. Since small nanoparticles are unstable, they can be designed to provide ions in a controlled manner, and the highest ever reported improvement of biogas production is obtained. The nanoparticles evolution during operation is followed by an array of spectroscopic techniques. ; Financial support was provided by the Spanish Ministerio de Medio Ambiente y Medio Rural y Marino (Project Exp. 007/RN08/03.1). The continuation of this research is being supported by the Bill and Melinda Gates Foundation (Grand Challenges Exploration, Proposal OPP1044410). ; Peer Reviewed