Peracetic acid (PAA)-based pretreatment effectively improves medium-chain fatty acids (MCFAs) production from sewage sludge

Abstract

Peracetic acid (PAA), known for its environmentally friendly properties as a oxidant and bactericide, is gaining prominence in decontamination and disinfection applications. The primary product of PAA oxidation is acetate that can serve as an electron acceptor (EA) for the biosynthesis of medium-chain fatty acids (MCFAs) via chain elongation (CE) reactions. Hence, PAA-based pretreatment is supposed to be beneficial for MCFAs production from anaerobic sludge fermentation, as it could enhance organic matter availability, suppress competing microorganisms and furnish EA by providing acetate. However, such a hypothesis has rarely been proved. Here we reveal that PAA-based pretreatment leads to significant exfoliation of extracellular polymeric substances (EPS) from sludge flocs and disruption of proteinic secondary structures, through inducing highly active free radicals and singlet oxygen. The production of MCFAs increases substantially to 11,265.6 mg COD L^-1, while the undesired byproducts, specifically long-chain alcohols (LCAs), decrease to 723.5 mg COD L^-1. Microbial activity tests further demonstrate that PAA pretreatment stimulates the CE process, attributed to the up-regulation of functional genes involved in fatty acid biosynthesis pathway. These comprehensive findings provide insights into the effectiveness and mechanisms behind enhanced MCFAs production through PAA-based technology, advancing our understanding of sustainable resource recovery from sewage sludge.

Keywords: Extracellular polymeric substances (EPS); Medium-chain fatty acids (MCFAs); Metabolic activity; Peracetic acid (PAA); Sewage sludge.

Graphical abstract

Fig. 1

a–b, Changes of soluble proteins (a) and carbohydrates (b) within 24 h of PAA oxidation. c, Soluble proteins and carbohydrates contents after pretreatment with significance test results. d–h, EEM spectra after pretreatment: Control (d), 10 (e), 15 (f), 20 (g), and 25 mg PAA per g TSS (h). i, Peak fluorescence intensity of EEM spectra. “∗” represents p < 0.05, “∗∗” represents p < 0.01, “∗∗∗” represents p < 0.001. Error bars represent standard deviations of triplicate experiments.

Fig. 2

a–b, Contents of proteins (a) and polysaccharides (b). c, Peak fluorescence intensity of extracted EPS. d, FTIR spectra of soluble and bound EPS fractions. e–f, changes of sludge structure reflected by SEM, i.e., the control (e) and PAA group (f).

Fig. 3

a, The second-derivative fitting curves of FTIR spectra from 1700 to 1600 cm⁻¹, with samples being the soluble, LB, and TB fractions of control and PAA-pretreated WAS. b, XPS C 1s, N 1s, and S 2p analysis of aqueous phase obtained from the control and PAA-pretreated WAS. c, The area (%) of secondary structures determined by second-derivative fitting.

Fig. 4

Concentrations evolution of ethanol (a), MCFAs (b), LCAs (c), and product distribution (d) in the control and PAA-pretreated reactors during anaerobic sewage sludge fermentation with ethanol as ED. “∗” represents p < 0.05, “∗∗” represents p < 0.01, “∗∗∗” represents p < 0.001. Error bars represent standard deviations of triplicate experiments.

Fig. 5

Microbial community evolutions of the experimental (i.e., 15, 20, and 25 mg PAA per g TSS) and control reactors. a, Microbial abundance analysis at a phylum level. b, Heatmap of functional microorganisms and typical non-functional bacteria based on a genus level.

Fig. 6

a–c, ESR spectra detected from the PAA group (i.e., 25 mg per g TSS), including DMPO-OH (a), DMPO-O₂⁻ (b), and TEMP-¹O₂ (c). Note: ∗ and • stand for DMPOX and DMPO-OH adducts. d, ORP variations for the control and PAA groups during pretreatment, with the inner bar chart representing Fe element content detected by EDS.

Fig. 7

a, Degradation of BSA during a four-day fermentation. b, Accumulative SCFAs production during a three-day fermentation. c–d, MCFAs production with ethanol and acetate as substrates during an eight-day fermentation for the control (c) and experimental (d) groups. e, Predictions of microbial functions in the control and PAA systems. Error bars represent standard deviations of triplicate experiments.

Fig. 8

a, Proposed metabolic pathways for WAS degradation and CE with ethanol as ED, with functional enzyme numbers listed and upgrading functional enzymes marked in red. b, Predicted functional enzyme abundances of CE microorganisms from the control, 15, 20, and 25 mg PAA per g TSS groups.