Co-Digestion of Swine Manure and Crude Glycerine: Increasing Glycerine Ratio Results in Preferential Degradation of Labile Compounds

Co-digestion of swine manure and crude glycerine from biodiesel production has been successfully attempted by many authors reporting substantial increments in biogas production. However, the effectiveness of this approach has been questioned recently. The addition of glycerol may cause an improvement in biogas production but at the expense of disturbing the degradation of manure. In the present paper, the organic transformations undergone in the anaerobic digestion of pig manure at increasing amounts of glycerine (2-8%(v/v)) were analysed using spectroscopy techniques (Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (H-1 NMR)). An increase in biogas production was observed with the addition of glycerine up to 8 %, resulting in a volumetric production of methane per litre of reactor (Lr) of 1.4 L CH4/Lr d. However, the subsequent failure of the system was observed at this glycerine content due to the inhibitory effect caused by high H2S concentration and foam formation. FTIR and H-1 NMR analysis performed on digestate samples showed that the addition of the co-substrate also caused the preferential degradation of glycerine and accumulation of proteins and aliphatic compounds. A post-stabilisation stage was necessary to complete the degradation process. Modifications in organic matter continued under this last stage although in the previous digestion period, a competition for substrate between sulphate reducing bacteria and methanogens was observed.