S. Astals, V. Nolla and J. Mata-Alvarez. 2011. Anaerobic co-digestion between pig manure and crude glycerol at mesophilic and thermophilic conditions.

Abstract
The glycerol, the main by-product of the biodiesel production, is characterized by its low alkalinities and its high C/N ratios. These properties make the glycerol a perfect co-substrate to improve the efficiency of a manure-based anaerobic digester. The results obtained in this study showed that the co-digestion between pig manure and glycerol is feasible at mesophilic and thermophilic conditions, where the limiting concentration of glycerol in the food supply was 4% w/w and 3% w/w respectively. Comparing the biogas production obtained by co-digestion with the biogas production obtained in the reference digester a rise of a 380% at mesophilic conditions and of a 450% at thermophilic conditions was reached. The significant increase of the biogas production highlighted the high biodegradability of the glycerol and the synergy between both substrates in the co-digestion medium; where the dilution of the TAN in the food supply and the slight pH reduction decreased the free ammonia concentration of the digester. At the same time, an optimum C/N ratio balanced the microorganism’s nutrition. Finally, in order to study in depth the differences between the two processes the proteins, the lipids and the fibers were monitored.