Once separated from the glycerin, the biodiesel goes through a cleanup or pur­ification process to remove excess alcohol, residual catalyst, and soaps. This consists of one or more washings with clean water. It is then dried and sent to storage. Sometimes the biodiesel goes through an additional distillation step to produce a colorless, odorless, zero-sulfur biodiesel.

The glycerin byproduct contains unreacted catalyst and soaps that are neu­tralized with an acid. Water and alcohol are removed to produce 50-80% crude glycerin. The remaining contaminants include unreacted fats and oils. In large

biodiesel plants, the glycerin can be further purified, to 99% or higher purity, for sale to the pharmaceutical and cosmetic industries.

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