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