Anaerobic digestion of long-chain fatty acids (oleic, palmitic, stearic) with whey protein as the emulsifier

Long chain fatty acids are suitable for biogas production, however, they are often responsible for process inhibition, due to their high toxicity even at low concentrations. In this study, anaerobic co-digestion of whey protein emulsified with oleic, palmitic or stearic acid were examined in continuous (batch-fed) stirred tank reactors. The mixture consisted of 5 g/L LCFA and 20 g/L whey protein. Emulsification was performed with a high shear emulsifier while the mixture was pre-heated at 75 oC to optimize the procedure. The anaerobic reactors had a working volume of 2 L and were operated at 39 oC. Anaerobic co-digestion of oleic acid displayed high biogas yield (0.47 L/gCOD), compared to palmitic (0.42 L/gCOD) and stearic acid (0.30 L/gCOD). Biogas production from palmitic acid was stable, with a biogas yield close (90%) to the theoretically expected values. This was not the case however for stearic which displayed negligible biodegradability. Oleic acid, on the contrary, was highly degradable (100%), although resulted inhibition of the acetoclastic methanogens. Based on the results of this study it is can be concluded that stearic acid degradation is the rate-limiting step of the anaerobic digestion process, and this attributed to its low solubility and thus bioavailability.