Effects of Fe/S ratio on the kinetics and microbial ecology of an Fe(III)-dosed anaerobic wastewater treatment system

Highlights: • Fe/S ratio was critical for organics oxidation kinetics, microbial ecology and sludge production. • Higher COD oxidation rates were achieved with increased Fe/S ratios. • SRB and IRB both contributed to organics oxidation. • Relative abundance of IRB had a positive correlation with COD oxidation rate. • Biogenic ferrous and sulfide mostly precipitated as FeS and FeS₂. -- Abstract: Effects of Fe(III)/sulfate (Fe/S) ratio on organic carbon oxidation kinetics and microbial ecology of a novel Fe(III)-dosed anaerobic wastewater treatment system were investigated in this study. Fixed-film batch bioreactors under three Fe/S molar ratios (1, 2, and 3) yielded COD oxidation rates that increased with the Fe/S ratio, and estimated Michaelis-Menten model parameters V_max ranging in 0.47–1.09 mg/L⋅min and K_m in 2503-3267 mg/L. Both iron and sulfate reducing bacteria contributed to the organics oxidation, and the produced sludge materials contained both biomass (32–45 wt.%) and inorganic precipitates from biogenic ferrous iron and sulfide (68-55 wt.%). Spectroscopic and chemical elemental analyses indicated that the inorganic fraction of the sludge materials contained both FeS and FeS₂, and had Fe/S stoichiometric ratios close to 1. Microbiological analyses of the biofilm samples revealed that the major putative iron- and sulfate reducers were Geobacter sp. and Desulfovibrio sp. along with noticeable N-fixing and fermentative bacteria. The COD oxidation rate had a positive correlation with the relative abundance of iron reducers, and both increased with the Fe/S ratio. A conceptual framework was proposed to illustrate the effects of Fe/S ratio on organics oxidation rate, microbial ecology and their interplays.