Fe Salts Hinder and Fe Oxides Help: Divergent Mechanisms in Sewage Sludge Anaerobic Digestion

Anaerobic digestion (AD) is an important method for sewage sludge (SS) stabilization and methane recovery. Fe compounds are widely present in SS because they are commonly used for phosphorus removal and organic matter (OM) capture in wastewater treatment plants. Endogenous Fe occurs in different forms, but the roles of these forms in SS AD remain unclear. This study systematically compared the effects of FeCl 3 , Poly-FeCl 3 , Fe 3 O 4 , FeOOH, and Fe 5 HO 8 ·4H 2 O on AD. The results showed that FeCl 3 and Poly-FeCl 3 decreased methane yield by 9.90% and 11.92%, respectively, whereas Fe 3 O 4 , FeOOH, and Fe 5 HO 8 ·4H 2 O increased it by 18.54%, 15.23%, and 15.09%. The analysis suggested that flocculating salts FeCl 3 and Poly-FeCl 3 groups increased sludge particle size, decreased SCOD concentrations by 10.21% and 12.41%, as well as F 420 by 16.88% and 28.63%, respectively, thereby inhibited the methanogenesis process. In contrast, Fe 3 O 4 , FeOOH, and Fe 5 HO 8 ·4H 2 O enhanced methane production by promoting OM hydrolysis, with SCOD concentrations increased by 12.71%, 8.99%, and 7.47%, respectively. XRD, CV, and EIS results showed that Fe 3 O 4 likely promoted methanogenesis through a stable Fe(III)/Fe(II) cycle and electron transfer. Although FeOOH and Fe 5 HO 8 ·4H 2 O also underwent Fe(III)/Fe(II) conversion, their promoting effects were weaker than that of Fe 3 O 4 , possibly because the lack of a bulk mixed-valence structure reduced the efficiency of continuous electron transfer. This study highlights that the chemical form of Fe in SS fundamentally determines its effects on AD performance.