Electrogenerated singlet oxygen and reactive chlorine species enhancing volatile fatty acids production from co-fermentation of waste activated sludge and food waste: The key role of metal oxide coated electrodes.

• Effects of metal oxide coated electrodes on EPT and WAS-FW Co-AF were investigated. • Electrogenerated RCS and 1O 2 enhanced WAS-FW disintegration and biodegradability. • RCS and 1O 2 both enriched acidogenic bacteria, while RCS inhibited methanogens. Electrochemical pretreatment (EPT) has shown to be superior in improving acidogenic co-fermentation (Co-AF) of waste activated sludge (WAS) and food waste (FW) for volatile fatty acids (VFAs). However, the influence of EPT electrode materials on the production of electrogenerated oxidants (such as singlet oxygen (1O 2) and reactive chlorine species (RCS)), as well as their effects on properties of electrodes, the microbial community structure and functional enzymes remain unclear. Therefore, this study investigated the effects of various metal oxide coated electrodes (i.e., Ti/PbO 2 , Ti/Ta 2 O 5 -IrO 2 , Ti/SnO 2 -RuO 2 , and Ti/IrO 2 -RuO 2) on EPT and subsequent Co-AF of WAS-FW. The results showed that EPT with Ti/PbO 2 , Ti/Ta 2 O 5 -IrO 2 , Ti/SnO 2 -RuO 2 and Ti/IrO 2 -RuO 2 electrodes generated 165.3–848.2 mg Cl 2 /L of RCS and 5.643 × 1011–3.311 × 1012 spins/mm3 of 1O 2 , which significantly enhanced the solubilization and biodegradability of WAS-FW by 106.4 %–233.6 % and 177.3 %–481.8 %, respectively. Especially with Ti/Ta 2 O 5 -IrO 2 as the electrode material, an appropriate residual RCS (2.0–10.4 mg Cl 2 /L) remained in Co-AF step, resulted in hydrolytic and acidogenic bacteria (e.g., Prevotella_7, accounting for 78.9 %) gradually become dominant rather than methanogens (e.g., Methanolinea and Methanothrix) due to their different tolerance to residual RCS. Meanwhile, the functional gene abundances of hydrolytic and acidogenic enzymes increased, while the methanogenic enzymes deceased. Consequently, this reactor produced the highest VFAs up to 545.5 ± 36.0 mg COD/g VS, which was 101.8 % higher than that of the Control (without EPT). Finally, the economic analysis and confirmatory experiments further proved the benefits of WAS-FW Co-AF with EPT. [Display omitted] [ABSTRACT FROM AUTHOR]