Integrating microbial electrochemical technologies with anaerobic digestion to accelerate propionate degradation

•The use of a pre-enriched inoculum promoted a shorter lag time for this AD-MET system.•Reactors inoculated with anaerobic sludge showed a more robust behaviour.•Geobacter is suggested to play a key in these propionate-degrading reactors.•Hydrogenotrophic pathways are the major contributor to methane production.•MET can be used to tackle excessive volatile fatty acid (VFA) accumulations in AD.•The direct energy improvement of this hybrid system is not very noticeable.