Zero valent iron simultaneously enhances methane production and sulfate reduction in anaerobic granular sludge reactors

Zero valent iron (ZVI) packed anaerobic granular sludge reactors have been developed for improved anaerobic wastewater treatment. In this work, a mathematical model is developed to describe the enhanced methane production and sulfate reduction in anaerobic granular sludge reactors with the addition of ZVI. The model is successfully calibrated and validated using long-term experimental data sets from two independent ZVI-enhanced anaerobic granular sludge reactors with different operational conditions. The model satisfactorily describes the chemical oxygen demand (COD) removal, sulfate reduction and methane production data from both systems. Results show ZVI directly promotes propionate degradation and methanogenesis to enhance methane production. Simultaneously, ZVI alleviates the inhibition of un-dissociated H2S on acetogens, methanogens and sulfate reducing bacteria (SRB) through buffering pH (Fe0 + 2H+ = Fe2+ + H2) and iron sulfide precipitation, which improve the sulfate reduction capacity, especially under deterioration conditions. In addition, the enhancement of ZVI on methane production and sulfate reduction occurs mainly at relatively low COD/SO42− ratio (e.g., 2–4.5) rather than high COD/SO42− ratio (e.g., 16.7) compared to the reactor without ZVI addition. The model proposed in this work is expected to provide support for further development of a more efficient ZVI-based anaerobic granular system. •A model is developed for ZVI-anaerobic granular sludge treating sulfate wastewater.•The model is successfully calibrated and validated using three independent data sets.•Model results confirm that ZVI promotes propionate degradation and methanogenesis.•ZVI also alleviates the inhibition of un-dissociated H2S on acetogens, MA and SRB.