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 FeCl3, Poly-FeCl3, Fe3O4, FeOOH, and Fe5HO8·4H2O on AD. The results showed that FeCl3 and Poly-FeCl3 decreased methane yield by 9.90% and 11.92%, respectively, whereas Fe3O4, FeOOH, and Fe5HO8·4H2O increased it by 18.54%, 15.23%, and 15.09%. The analysis suggested that flocculating salts FeCl3 and Poly-FeCl3 groups increased sludge particle size, decreased SCOD concentrations by 10.21% and 12.41%, as well as F420 by 16.88% and 28.63%, respectively, thereby inhibited the methanogenesis process. In contrast, Fe3O4, FeOOH, and Fe5HO8·4H2O 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 Fe3O4 likely promoted methanogenesis through a stable Fe(III)/Fe(II) cycle and electron transfer. Although FeOOH and Fe5HO8·4H2O also underwent Fe(III)/Fe(II) conversion, their promoting effects were weaker than that of Fe3O4, 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.
Bai et al. (Mon,) studied this question.