Anaerobic co-digestion of pig manure and crude glycerol at mesophilic conditions: Biogas and digestate

► Satisfactory anaerobic co-digestion between pig manure and glycerol during 200days. ► Evaluation of the organic compounds removal (proteins, lipids, carbohydrates and fibers) and the biogas flow rates. ► Analysis of the stability of the digestates through chemical and respirometrics methods. Crude glycerol derived from biodiesel production is characterized by its high concentration of organic carbon and its solubility in water; properties that make it a suitable co-substrate to improve the efficiency of a manure digester. An increase of about 400% in biogas production was obtained under mesophilic conditions when pig manure was co-digested with 4% of glycerol, on a wet-basis, compared to mono-digestion. The increase in biogas production was mainly a consequence of the increase in organic loading rate. However, the differences could also be related to the synergy between both substrates and the carbon-to-nitrogen ratio. Moreover, the analysis of the macro-compounds, protein, lipids, carbohydrates and fibers, showed lower removal efficiencies in the co-digester as the microorganisms obtained nutrients from the soluble carbohydrates provided by the glycerol. The digestate stability, evaluated through a respirometric assay, showed that co-substrate addition does not exert a negative impact on digestate quality.