Deciphering the effect of propionic acid on anaerobic digestion of sludge: gas production and microbial community structure
Résumé
Anaerobic digestion (AD) is a common way to valorize sewage sludge by producing biogas (mainly CO2 and CH4), a renewable energy source. This bioconversion process is catalyzed by microbial communities, which are prone to suboptimal functioning by various abiotic factors such as the intermediate compounds like volatile fatty acids.
In this study, we focused on the effects of propionic acid on both the gas production and the microbial community structure, in order to identify potential microbial biomarkers of inhibition.
Series of batch AD microcosms were conducted in mesophilic conditions. In addition to the feeding (mixed sewage sludge) and the inoculum (mesophilic digested sewage sludge), either propionic acid, sodium propionate, sodium chloride or hydrochloric acid was added at the beginning of the incubation, to be able to discriminate between the effect of the propionate, of the pH decrease, and of the counterion. The AD performance was monitored by measuring the methane production over the digestion time. The microbial community composition was determined by 16S rDNA metabarcoding.
A significant decrease in the methane production rate was observed from 1 g/L propionic acid amendment. At an added concentration of 1.49 g/L, the microbial community composition was significantly modified, in particular, that of the Obligate Hydrogen Producing Acetogens, who degrade propionate into acetate, hydrogen and inorganic carbon (CO2). In addition, the proportion of archaea strongly decreased with initial concentrations of propionic acid. The observed inhibition was largely due to the pH decrease caused by the dissociation of the propionic acid.
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