Anaerobic digestion of chicken manure: Sequences of chemical structures in dissolved organic matter and its effect on acetic acid production

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The use of chicken manure (CM) leads to serious environmental pollution due to the existence of bacteria and insect pests. Anaerobic digestion (AD) is one of the important technologies of CM treatment. However, methane production is limited by the accumulation of short-chain fatty acids (SCFAs) from AD. Therefore, the study explored the possible formation mechanism of acetic acid by understanding the effect of sequences of chemical structure variation in DOM on acetic acid production. The chemical structures of DOM were observed. The tyrosine-like substances (C1, 53.53–29.99%) and humic-like substances (C3, 18.38–5.96%) showed a tendency to decrease. Tryptophan-like substances (C2, 28.09–64.04%) showed the increasing trend. The results indicated that C2 was unwilling to biodegrade. In DOM, the order of biodegradability was C2< C1< C3. AD resulted in the enrichment of N–H in-plane (0–22.75%) and COO- stretch (7.53–18.57%) and the loss of O–H stretch (19.39–13.72%), C–H stretch (4.56%-0), CC stretch (12.04–9.61%) and C–O stretch (10.02–5.03%). Two-dimensional correlation spectroscopy is applied to investigate the sequences of chemical structures in DOM, the order is as follows: CC stretch > COO- stretch > N–H in-plane > C–O stretch. The result confirmed that protein was rapidly decomposed and utilized, which would result in the increase of microorganism metabolism and hydrolysis rate, polysaccharide was hydrolyzed to form phenol and carboxylic acid. Four possible pathways were identified in AD by the structural equation model. C1and hydroxyl can promote propionic and butyric acid formation by the pathway of valeric or iso-butyric acid production and further effected acetic acid production. This study proposed the possible formative mechanisms of acetic acid according to sequences of chemical structures variation in DOM during AD, which can provide the theoretical basis for directional regulating the conversion of different chemical structures of DOM into acetic acid in AD.