Application of zero valence iron for anaerobic digestion of skim latex wastewater: advantages and challenges

Aqueous sulfide, a product of the sulfate-reducing process in undissociated form, is a potent inhibitor of methanogens. Furthermore, the biogas generated from the anaerobic digestion of skim latex wastewater (SLW) typically contains > 10,000 ppm H₂S. In this study, we investigated the role of zero-valent iron (ZVI) as a sink for both sulfides generated from the reduction of sulfate in SLW and CO₂ from acetoclastic methanogenesis. The ZVI-based anaerobic digestion was performed in a fed-batch reactor fed with SLW for 30 days at an organic loading rate (OLR) of 2.34 g COD/L d and a COD/SO₄²⁻ ratio of around 3. The concentrations of undissociated sulfide in the reactor added with ZVI were 9.1 ± 3.3 mg/L S²⁻. In contrast, the concentrations of undissociated sulfide in a control reactor were 157.2 ± 44.4 mg/L S²⁻, which were closer to the levels that can be toxic to methanogens. The ZVI addition promoted the precipitation of iron sulfide and carbonate at a medium pH of 7.9. The biogas generated from the ZVI-based reactor had 94–96% CH₄, 4–6% CO₂, and undetectable amounts of H₂S. However, the methane production in the ZVI-based anaerobic digester decreased by 7.9%, possibly due to the buildup of partial pressure of H₂ and subsequent propionate accumulation.