Continuously feeding fenton sludge into anaerobic digesters: Iron species change and operating stability

•Fenton sludge as an Fe(III) source of dissimilatory iron reduction improved anaerobic treatment.•Fenton sludge could be consumed during anaerobic digestion to a certain extent.•Continuously feeding Fenton sludge did not deteriorate anaerobic treatment.•Iron content in the digester decreased when stopping feeding Fenton sludge.•Feeding Fenton sludge increased the electron transfer capacity of microorganisms.