Comparison of anaerobic digesters performance treating palmitic, stearic and oleic acid: determination of the LCFA kinetic constants using ADM1.

In this work, the process performance of three parallel anaerobic digesters was evaluated while treating emulsified long-chain fatty acids (LCFA) (i.e., palmitic, stearic and oleic acid) at two different organic loading rates (OLR) (2 and 4 g L day). The digester treating palmitic acid achieved a biogas yield of 0.42 ± 0.05 L g COD with a digester effluent COD of 2.3 ± 0.4 g L and VFA concentrations below 250 mg L. Stearic acid showed lower biogas yield values (0.34 ± 0.05 L g COD) with low effluent COD (2.5 ± 0.4 g L) and VFA concentrations (< 350 mg L). On the contrary, anaerobic digestion of oleic acid was accompanied with increased effluent COD (4.5 ± 0.7 g L) and VFA concentrations (2.7 ± 1.5 g L). As such, COD removal efficiency remained at 89 ± 1% compared to 95 ± 1% for the digesters treating palmitic and stearic acid. During continuous digester operation, there was a decrease in VFA concentrations (around 1-2 g L), combined with increasing biogas yield values up to 0.51 ± 0.04 L g COD, possibly due to biomass adaptation to oleic acid. Digester foaming was recorded when the accumulated COD was higher than 200, 1200 and 500 mg COD g MLSS for the digester treating palmitic, stearic and oleic acid, respectively. ADM1 simulation allowed for the determination of the maximum specific LCFA consumption rate constant (km) which was equal to 4.8, 1.6 and 8.0 day for palmitic, stearic and oleic acid respectively.