After the transesterification reaction, the post-processing steps needed to purify biodiesel according to the existing regulations and norms are the same as those involved in the biodiesel production from currently edible vegetable oils. Although the final product composition in terms of esters may be different depending on the feedstocks used, their physical properties are similar and no significant differences are expected between the two variants.

After the reaction is finished, the mixture is allowed to separate into an upper layer of methyl esters and a lower layer of glycerol diluted with methanol. Glycerol is removed by allowing the two phases to form and settle. Then, any unreacted alco­hol is air-stripped or vacuum-distilled away from the esters phase and recycled back to the reactor.

Depending on the process, water can be used to wash catalyst residues and sodium soaps from the methyl esters. Moreover, small amounts of concentrated phosphoric acid (H3PO4) can be added to the raw methyl esters to break down cata­lyst residues and sodium soaps. Predojevic [78] studied different puri fi cation steps of biodiesel obtained from waste frying oils, by a two-step alkali-catalyzed transesterification reaction, concluding that the best results are obtained when using silica gel and phosphoric acid treatments (with a yield of 92%) and the lowest yields (89%) are obtained using hot water. Also, Sabudak and Yildiz [83] applied three different purification methods to biodiesel produced from waste frying oils (water washing with distilled water, dry wash with addition of magnesol, and an ion — exchange resin) concluding that the most effective one is the ion-exchange resin.

The same situation occurs for the storage of biodiesel, where potential problems of decomposition may occur. For example, Lin and Lee [62] studied the oxidative stability of marine fish-oil biodiesel showing that the addition of antioxidant significantly retards the fuel deterioration over time, although it increases the kine­matic viscosity and carbon residue at the beginning of the storage period. These authors also concluded that the operating temperature is a dominant factor in the deterioration of the fuel characteristics.