Enhanced anaerobic digestion of waste activated sludge with periodate-based pretreatment

Abstract

The potential of periodate (PI) in sludge anaerobic digestion is not tapped, although it has recently attracted great research interest in organic contaminants removal and pathogens inactivation in wastewater treatment. This is the first work to demonstrate significant improvement in methane generation from waste activated sludge (WAS) with PI pretreatment and to provide underlying mechanisms. Biochemical methane potential tests indicated that methane yield enhanced from 100.2 to 146.3 L per kg VS (VS, volatile solids) with PI dosages from 0 to 100 mg per g TS (TS, total solids). Electron spin resonance showed PI could be activated without extra activator addition, which might be attributed to the native transition metals (e.g., Fe²⁺) in WAS, thereby generating hydroxyl radical (•OH), superoxide radicals (•O₂ ^-), and singlet oxygen (¹O₂). Further scavenging tests demonstrated all of them synergistically promoted WAS disintegration, and their contributions were in the order of •O₂ ^- > •OH > ¹O₂, leading to the release of substantial biodegradable substances (i.e., proteins and polysaccharides) into the liquid phase for subsequent biotransformation. Moreover, fluorescence and ultraviolet spectroscopy analyses indicated the recalcitrant organics (especially lignocellulose and humus) could be degraded by reducing their aromaticity under oxidative stress of PI, thus readily for methanogenesis. Microbial community analysis revealed some microorganisms participating in hydrolysis, acidogenesis, and acetoclastic methanogenesis were enriched after PI pretreatment. The improved key enzyme activities and up-regulated metabolic pathways further provided direct evidence for enhanced methane production. This research was expected to broaden the application scope of PI and provide more diverse pretreatment choices for energy recovery through anaerobic digestion.

Keywords: Anaerobic digestion; Energy recovery; Free radicals; Periodate; Recalcitrant organics.

Graphical abstract

Fig. 1

a, Cumulative methane from waste activated sludge (WAS) pretreated by varying periodate (PI) levels during anaerobic digestion. The dotted line represents the results of model simulated. b, The relationship between the maximum cumulative methane yield and PI levels. c, The confidence regions of 95% about the hydrolysis rate (k) and biochemical methane potential (B₀) based on the model simulation. Error bars represented standard deviation of triplicate tests.

Fig. 2

a, Soluble chemical oxygen demand (SCOD) release and the degree of sludge disintegration (%). b, Soluble proteins and polysaccharides release, short-chain fatty acids (SCFAs) generation. c, Fluorescence matrix profiles and fluorescence response (P_i,n) percentages of fermentation liquor at the end of 1-day pretreatment with different PI dosages. Asterisks (∗) indicate the significant differences with control (one-way ANOVA). ∗P < 0.05, significant; ∗∗P < 0.01, highly significant.

Fig. 3

a, Variations of oxidation-reduction potential (ORP) in the pretreatment systems with different periodate (PI) levels. b, ESR spectra of TEMP-¹O₂ and DMPO-•OH adducts in water, and DMPO-•O₂⁻ adduct in methanol. c, The schematic illustration of PI pretreatment. Asterisks (∗) indicate the significant differences with control (one-way ANOVA). ∗P < 0.05, significant; ∗∗P < 0.01, highly significant.

Fig. 4

a–b, Fluorescence excitation-emission matrix (EEM) spectra of mixed matrix of model lignocellulose and humus without PI (a) and with PI treatment (b). c, UV absorption spectra of mixed matrix of model lignocellulose and humus. d, Daily methane production from model lignocellulose and humus with and without PI treatment. Note: the dosage of PI was equal to the fermenter with 75 mg PI per g TS pretreatment. ∗P < 0.05, significant; ∗∗P < 0.01, highly significant.

Fig. 5

Microbial community evaluations of the control and experimental (with 75 mg PI per g TS pretreatment) digesters. a, Venn diagram based on OUTs. b, Alpha diversity comparisons. c, The circos plot of the phylum level. d, The microbial heatmap of the genus level.

Fig. 6

Schematic illustration of metabolic pathways for the expressions of key enzymes responsible for hydrolysis-acidogenesis, homoacetogenesis, and methanogenesis. Nodes represent intermediate compounds in the metabolism cycles, boxes represent the specific enzymes information, and blue and pink numbers in the parentheses represent the abundances of the encoding gene of functional enzymes in the control and experimental (with PI pretreatment) samples, while the variations of α-glucosidase, protease, acetate kinase, and F₄₂₀ coenzyme were directly indicated by their activities (Table S4).