BACKGROUND During anaerobic digestion of wastes, volatile fatty acid (VFA) production has been studied, but little attention has been paid to alcohols and lactic acid. Thermodynamically, lactic acid and alcohols are better substrates to produce methane than other VFA. This research identified the metabolites produced during the fermentation of the organic fraction of municipal solid waste and studied their methanization in a second stage. Analysis of the methane production curves is provided and explained according to Gibbʼs free energy of acetogenesis and methanogenesis. Based on the Michaelis–Menten model, a kinetic analysis is presented. RESULTS The specific methane production of the identified acids and alcohols are acetic acid, 343 NL/kgCOD; ethanol, 296; methanol, 181; butyric acid, 108; lactic acid, 64. Lactic and butyric acids present a long adaptation phase followed by a fast and short methane production. Acetic acid and ethanol finished the reaction in less than 16 h. The highest methane production rates (Vmax) were for butyric acid, methanol, and ethanol with 2136, 1934, and 1928 NmL L–1, respectively. The best affinity (lowest Km) values were for acetic and lactic acids and ethanol with 1.2, 1.9, and 2.0 gCOD L–1. CONCLUSION Together, acetic acid and ethanol represent 92% of all fermentation products. No endogenous methane production was observed during the methanization from acetic acid and ethanol. The highest specific methane production belongs to acetic acid (98% of the theoretical) and the lowest corresponds to lactic acid. Except for propionic acid, the methanization of the selected substrates follows the Michaelis and Menten model. © 2021 Society of Chemical Industry (SCI).

Thermodynamically, lactic acid and ethanol allow the best energy recovery among all common metabolites for methane production. The main objective of this work was to favor the fermentation of OFMSW for better lactic acid and ethanol production. First, OFMSW fermented with three different inocula: Naturally occurring microorganisms (NOM), yeast (Saccharomyces cerevisiae), and UASB sludge. Second, methanization of digestates from the fermentations. OFMSW fermentation with NOM and UASB sludge + NOM mainly produces lactic acid; yeast fermentation mainly produces ethanol, lactic acid, and volatile fatty acids. Methane production increased with decreasing substrate concentration, and higher methane production was obtained with the digestate from NOM fermentation. Every one of the fermented digestates produced more methane than unfermented OFMSW. Higher concentrations of propionic and butyric acids were observed for higher digestate concentrations from yeast and NOM after 30-day methanization. Methane produced from unfermented OFMSW was not inhibited as propionic, and butyric acids concentrations were the lowest of all four cases. Statement Of NoveltyThis research proposes a simple fermentation method to produce the best eligible substances for further methane production. According to Jojoa-Unigarro and González-Martínez ( 2021), ethanol and acetic acid are the preferred substrates for methanization because of the potential inhibition caused by hydrogen partial pressure and acetogenesis of volatile fatty acids can be avoided. Although lactic acid needs to undergo acetogenesis before methanization, the hydrogen partial pressure does not affect this step. The novelty of this research lies in the use of UASB sludge and commercial yeast as inoculum to produce mainly ethanol and lactic acid through fermentation. This proposal also considers that undesired metabolites such as propionic and butyric acids are produced in lower concentrations when using these inocula for fermentation before methanization. The inevitable side production of acetic acid is welcome.

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Thermodynamically, lactic acid and ethanol allow the best energy recovery among all common metabolites for methane production. The main objective of this work was to favor the fermentation of OFMSW for better lactic acid and ethanol production. First, OFMSW fermented with three different inocula: Naturally occurring microorganisms (NOM), yeast (Saccharomyces cerevisiae), and UASB sludge. Second, methanization of digestates from the fermentations. OFMSW fermentation with NOM and UASB sludge + NOM mainly produces lactic acid; yeast fermentation mainly produces ethanol, lactic acid, and volatile fatty acids. Methane production increased with decreasing substrate concentration, and higher methane production was obtained with the digestate from NOM fermentation. Every one of the fermented digestates produced more methane than unfermented OFMSW. Higher concentrations of propionic and butyric acids were observed for higher digestate concentrations from yeast and NOM after 30-day methanization. Methane produced from unfermented OFMSW was not inhibited as propionic, and butyric acids concentrations were the lowest of all four cases.

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