In-sewer iron dosing enhances bioenergy recovery in downstream sewage sludge anaerobic digestion: The impact of iron salt types and thermal hydrolysis pretreatment

Dosing iron salts is a widely adopted strategy for sewer odor and corrosion management, and it can affect bioenergy recovery during anaerobic digestion (AD) of sludge in downstream wastewater treatment plants. However, the different impacts of in-sewer iron salt dosing on AD, depending on the types of iron and digestion conditions, remain unclear. Therefore, this study investigated the impact of in-sewer ferrous (Fe(II)) and ferrate (Fe(VI)) dosing on bioenergy recovery in both conventional AD and AD with thermal hydrolysis pretreatment (THP). The results showed that in-sewer Fe(VI) dosing notably enhanced methane production in AD more than in-sewer Fe(II) dosing, with cumulative methane yields of 197.1±1.9 mLCH₄∙gVSadded⁻¹ for Fe(VI) and 186.5±10.4 mLCH₄∙gVSadded⁻¹ for Fe(II), respectively. Microbial analyses and iron particle characterizations suggested that the superior promotion with Fe(VI) dosing may be attributed to the smaller particle sizes and higher iron oxide content of Fe(VI) resultant products. This led to a greater enhancement in direct interspecies electron transfer (DIET) between syntrophic bacteria and methanogens, as indicated by the upregulation of Methanosaeta and key functional genes involved in CO₂-utilizing methanogenesis. Additionally, in THP-AD, the methane production enhancement caused by in-sewer iron dosing (35.5 mLCH₄∙gVSadded⁻¹) exceeded that in conventional AD (26.9 mLCH₄∙gVSadded⁻¹), although organic degradation during THP was unaffected. As THP-AD gains popularity for improved bioenergy recovery from sludge, our findings suggest that in-sewer iron dosing supports this advancement. Furthermore, in-sewer Fe(VI) dosing appears more promising within integrated wastewater management strategies, facilitating energy- and carbon-neutralization of urban water systems.