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Residual glycerol from biodiesel manufacturing, waste or potential source of bioenergy: a review

This review provides a summary of the research conducted on the use of crude glycerol, the major byproduct of the
biodiesel industry, as substrate for anaerobic co-digestion and production of biogas. In general, for every 100 kg biodiesel
produced, approximately 10 kg crude glycerol is generated. Because this glycerol is expensive to purify for use in food,
pharmaceutical, or cosmetic industries, biodiesel producers must seek alternative methods for its disposal. Several studies
have demonstrated that the use of crude glycerol as a C source for fermentation and biogas generation is a promising
alternative use for this waste material. The high C content of glycerol increases the C:N ratio in the mixture, avoiding
the inhibition of the process by the excess of N increasing methane production of digesters by 50 to 200%. Anaerobic codigestion
of glycerol and a variety of residual biomasses may be a good integrated solution for managing these wastes and
simultaneously producing a source of bioenergy in an environmentally friendly way. On the other hand, after anaerobic
treatment of glycerol, an organic matter-rich solid waste is generated (digestate). The incorporation of digestates from
glycerol co-digestion to soils constitutes an important source of organic matter and nutrients for plants. However, the
potential of digestates as an organic soil amendment has not been sufficiently studied. The utilization of glycerol as a
potential source of energy, rather than as a waste, seems to be a convenient way of lowering the costs of biodiesel production
and making this emerging industry more competitive.