Continuous ferric iron-dosed anaerobic wastewater treatment: Treatment performance, sludge characteristics, and microbial composition

Abstract

This study examined the treatment performance of an innovative Fe(III)-dosed anaerobic bioreactor treating synthetic wastewater under varied Fe(III)/sulfate (Fe/S) ratios. The treatment system was operated continuously for 500 days with a constant hydraulic retention time. COD removal efficiency was attributed to organics oxidation coupled to both iron and sulfate reduction, and observed as 84 ± 4 %, 86 ± 4 % and 89 ± 2 % under Fe/S molar ratios of 0.5, 1 and 2 respectively. The increasing COD removal efficiency was corroborated by a similar increasing trend of COD removal rate that ranged from 468 ± 25 mg/L/d to 497 ± 12 mg/L/d. Effluent quality was also regulated by the Fe/S ratio with sulfide levels decreasing from 0.42 ± 0.3 mg/L under Fe/S ratio 0.5 to 0.03 ± 0.02 mg/L under ratio 2 due to increasing quantity of ferrous iron for forming ferrous sulfide precipitates. The sludge samples contained organics (biomass) and inorganic precipitates (FeS and FeS2) with the inorganic fraction increasing from 60 % to 80 % as the Fe/S ratio increased from 0.5 to 2. Microbiological analyses of the sludge samples showed Geobacter sp., Geothrix sp. and Ignavibacteria sp. as putative iron reducers, and Desulfovibrio sp., Desulfobulbus sp., Desulfatirhabdium sp., Desulforhabdus sp. and Desulfomonile sp. as putative sulfate reducers. Relative abundances of the iron reducers to the sulfate reducers increased with Fe/S ratio. This suggests faster carbon oxidation kinetics by the iron reducers than the sulfate reducers given the overall higher organics oxidation kinetics under the higher Fe/S ratios.