Comparison of different valent iron on anaerobic sludge digestion: Focusing on oxidation reduction potential,dissolved organic nitrogen and microbial community

? ORP value from ?278.71 to ?379.80 mV showed indiscernible effects on methane yield. ? Fe(II) and Fe(III) promoted more degradation of proteins and amino acids than Fe⁰. ? The highest enrichment of Geobacter was noted in samples added with Fe⁰. ? Cysteine was accumulated during iron enhanced anaerobic sludge digestion. ? Both iron content and valence were important for methane production. This study compared effects of three different valent iron (Fe⁰, Fe(II) and Fe(III)) on enhanced anaerobic sludge digestion, focusing on the changes of oxidation reduction potential (ORP), dissolved organic nitrogen (DON), and microbial community. Under the same iron dose in range of 0?160 mg/L after an incubation period of 30 days (d), the maximum methane production rate of sludge samples dosed with respective Fe⁰, Fe(II) and Fe(III) at the same concentration showed indiscernible differences at each iron dose, regardless of the different iron valence. Moreover, their behavior in changes of ORP, DON and microbial community was different: (1) the addition of Fe⁰ made the ORP of sludge more negative, and the addition of Fe(II) and Fe(III) made the ORP of sludge less negative. However, whether being more or less negative, the changes of ORP may show unobservable effects on methane yield when it ranged from ?278.71 to ?379.80 mV; (2) the degradation of dissolved organic nitrogen, particularly proteins, was less efficient in sludge samples dosed with Fe⁰ compared with those dosed with Fe(II) and Fe(III) after an incubation period of 30 d. At the same dose of 160 mg/L iron, more cysteine was noted in sludge samples dosed with Fe(II) (30.74 mg/L) and Fe(III) (27.92 mg/L) compared with that dosed with Fe⁰ (21.75 mg/L); (3) Fe⁰ particularly promoted the enrichment of Geobacter, and it was 6 times higher than those in sludge samples dosed with Fe(II) and Fe(III) at the same dose of 160 mg/L iron.