**2. Glycerol: A byproduct of biodiesel production**

Glycerin is the principal byproduct of biodiesel production, with waste streams containing at least 95% glycerol; however its purity can vary depending on the efficiency of the production process [8].

Glycerol is generated by the process for obtaining biodiesel from vegetable oils or animal fats. This process often uses the addition of catalysts, such as sodium hydroxide, and alcohols, such as methanol or ethanol, with reactors maintained under heat and agitation (Figure 1).

However, during the transesterification process a high volume of glycerol is produced as a byproduct: for 90 m3 of produced biodiesel, approximately 10 m3 of glycerin is generated [8]. Pure glycerol can be used in many different applications, mainly in textile, chemical, pharma‐ ceutical and food industries. However, to use it in these applications is necessary a degree of purity higher than 95% [9]. To achieve this, the crude glycerin must be submitted to a purifi‐ cation process, often resulting in high financial costs.

**Figure 1.** Representation of Biodiesel production by Transesterification Process (adapted from [10]).

Due to the significant amount of unwanted glycerol generated in the biodiesel manufacturing process [11], traditional markets have not found the capacity to absorb it, as described above. Depending on the purity, the main uses for glycerol are direct burning for energy production, inputs for various industrial segments and as raw material in animal feed [8].
