The role of emulsification as pre-treatment on the anaerobic digestion of oleic acid: process performance, modeling, and sludge metabolic properties

Oleic acid is the most abundant long-chain fatty acid (LCFA) in nature and the main product of lipid hydrolysis. In this study, the effect of emulsification pre-treatment on the anaerobic digestion of oleic acid was studied using fed-batch mesophilic anaerobic digesters. A high-shear mixer was used to disintegrate oleic acid with whey protein as the emulsifier. The anaerobic digesters were operated with gradually increasing the organic loading rate (OLR) (from 1 to 8 g L⁻¹ d⁻¹), while the influent oleic acid concentration remained at 5 g L⁻¹. COD removal was between 80 and 90% for both digesters; however, the methane yield was significantly higher for the digester treating emulsified oleic acid (0.37 NL g⁻¹COD removed), compared to 0.27 NL g⁻¹COD for the control digester without emulsification pre-treatment. Non-emulsified oleic acid coalesced inside the digester and formed floating particles with high lipid-content (63% w/w TS). The anaerobic sludge exposed to emulsified oleic acid increased its methanogenic activity. The anaerobic digestion model (ADM1) predicted well the biogas production rate, the concentrations of volatile fatty acids (VFAs), the digesters pH, and the biogas methane content. The maximum specific consumption rate constant (km) for oleic acid was 4 times higher (i.e., from 2 to 8 d⁻¹) with the applied emulsification pre-treatment. The application of the ADM1 to the experimental results contributed into elucidating the kinetics of oleic acid biodegradation and quantifying the effect of emulsification pre-treatment.