A neglected contributor of thermal hydrolysis to sludge anaerobic digestion: Fulvic acids release and their influences

Carbon dots coupled bioelectrocatalysis for enhanced methane productivity in anaerobic co-digestion of sewage sludge and food waste: Focusing on enhancement mechanisms and microbial community succession

The limited electron transfer efficiency and poor stability of microorganisms are challenges in traditional bioelectrocatalytic food waste treatment. Herein, carbon dots (CDs) possess excellent biocompatibility and electrochemical properties. When combined with bioelectrocatalysis, CDs can regulate microbial community structure and enhance electron exchange capacity. The results demonstrated that compared with the control group (28.1 mL/g-VS/d), the CDs at a dosage of 0.50 g/g VS could increase CH4 production by a factor of 7.8. CDs not only increase CH4 production but also improve the digestate's stability, making it suitable for use as bio-fertilizer. Moreover, a significantly high Methanobacterium richness (11.6 %) signified an intensified utilization of hydrogen and formic acid pathways in CH4 production. Particularly, the biocompatible CDs could be absorbed by microorganisms, forming an environmental network that was more conducive to electron transfer with unabsorbed CDs and accelerating interspecies electron transfer. This work provides mechanistic insights into boosting CH4 production in AD.

Influences of released humic acids during thermal hydrolysis on sludge anaerobic digestion: New insights from the molecular weight of humic acids

Humic acids (HAs) would be excessively released during thermal hydrolysis pretreatment (THP) and deeply disturb anaerobic digestion (AD) of waste activated sludge (WAS). The molecular weights of HAs could affect HAs entering microbial cells, binding with digestive enzymes and participating in electron transfer, thereby determining its influences on sludge AD. Results in this study confirmed the different influences of HAs from diverse sources on sludge AD indeed had significant correlations with their molecular weights. The presence of commercial HAs (SAHA) inhibited methane production by 53.3% at 0.5 g/L while HAs extracted from raw sludge (WNHA) increased methane production by 20.5% at the same concentration, which attribute to the comprehensive impacts from their differences in functional group compositions and molecular weights. Moreover, comparing to WNHA, the HAs extracted from thermally hydrolyzed sludge (THHA) showed unchanged functional group compositions but reduced methane generation facilitation to 5.1%, which only be due to its decreased molecular weights. In-depth research indicated that HAs influences on enzymes were closely relative to its molecular weight. HAs with greater molecular weights presented more significant inhibition to extracellular enzymes while micromolecular HAs affected intracellular enzymes more. Furthermore, macromolecular HAs promoted sludge solubilization and acidification but hindered hydrolysis and methanogenesis, whereas micromolecular HAs promoted acidification but inhibited methanogenesis. This study underscored the importance of changes in molecular weight of HAs during sludge THP, offering insights into previous discrepancies in reports on HAs effects on sludge AD.

Response of fulvic acid linking to redox characteristics on methane and short-chain fatty acids in anaerobic digestion of chicken manure

Fulvic acids (FAs) is formed during the bioconversion of organic matter (OM) to biogas during anaerobic digestion (AD) and has a complex structure and redox function. However, the evolutionary mechanisms of FAs during AD and its interactions with acid and methane production have not been sufficiently investigated, especially at different stages of AD. Intermittent AD experiments by chicken manure and rice husk showed significant structural changes and reduced aromatization of FAs (e.g., O-H stretch6, 14.10-0%; SR, 0.22-0.60). The electron donating capacity (EDC) [9.76-45.39 μmole-/(g C)] and electron accepting capacity (EAC) [2.55-5.20 μmole-/(g C)] of FAs showed a tendency of decreasing and then increasing, and FAs had a stronger electron transfer capacity (ETC) in the methanogenic stage. Correlation analysis showed that the EDC of FAs was influenced by their own structure (C-O stretch2, C-H bend1, C-H bend4, and N-H bend) and also had an inhibitory effect on propionic production, which further inhibited acetic production. The EAC of FAs was affected by molecular weight and had a promoting effect on methane production. Structural equation modelling identified three possible pathways for AD. The C-O stretch2 structure of FAs alone inhibits the production of propionic. In addition, pH can directly affect the EDC of FAs. This study provides a theoretical basis for the structural and functional evolution of FAs in AD of chicken manure on the mechanism of methane production.