3.2. Current techniques available in converting WEO into biodiesel
Despite the changes in WEO chemical and physical properties as compared to fresh edible oil, both of the oil feedstocks can still be converted to biodiesel using similar method; that is via catalytic reaction using alkali catalyst, acid catalyst and enzyme or via non-catalytic reaction in supercritical transesterification reaction. Nevertheless, the high content of FFAs and water in WEO has caused alkaline-catalyzed transesterification reaction less efficient. A more suitable method would be via non-catalytic supercritical reaction or the two-step transesterification method that consists of acid-catalyzed reaction followed by alkali-catalyzed reaction. Table 6 summarizes the recently introduced transesterification techniques to convert WEO into biodiesel as reported in the literature.