Ultra-high sulfate loads and iron precision dosing for hydrogen sulfide management: A mass balance assessment and microbial structure implications in anaerobic bioreactors

Abstract

This article presents an original and detailed study of semi continuous lab-scale anaerobic bioreactors subjected to Sulfate Loads Rates (SLRs) >130 mgSO42-/day.Lreactor. At these sulfate inputs, hydrogen sulfide yield by bacterial conversion can reach concentrations above 400 mg/L in the liquid phase and more than 25,000 ppm in gas phase, respectively. Both values comprise significant technical challenges in biogas plants as well as health and environmental concerns. In order to control sulfide in liquid phase, two bioreactors were treated with opposite ferric dosing strategies: one underwent five consecutive cycles of shock precipitation of hydrogen sulfide, reducing peak levels from ~400 mg/L to <40 mg/L; the other involved the application of a more continuous and fine-tuned dosing strategy to manage sulfide concentrations across different stepwise levels: 400-300, 300-200, 200-100, and 100 to <10 mg/L. A third bioreactor, without iron addition, stabilized at ~250 mg/L H2S and 10,000 ppm in biogas over 140 days. A detailed sulfur mass balance enabled analysis of sulfate-to-sulfide conversion rates, Henry’s constants between liquid and gas phases, and Fe3+/S2- molar ratios for both strategies. Values of 0.99 and 0.65 were obtained, matching with the stoichiometry of FeS during shock precipitation and FeS+S0 and/or Fe2S3 in stepwise precipitation, respectively. High-throughput sequencing of the 16S rRNA gene revealed subtle yet significant changes in the microbial community structures within bioreactors subjected to high sulfate loads and iron addition. Furthermore, the authors characterized the late-stage microbial response following methanogenic process inhibition and the cessation of biogas production.