Currently, world oil crop production is about 139,000,000 ton [141]. In particular, developing countries (97,370,185 ton) and developed coun­tries (41,193,308 ton) are the largest producers, while least developed countries contribute 4,141,535 ton. Most of this oil is used for deep — frying processes, after which it becomes a disposal problem. Disposal methods often contaminate environmental water and contribute to world pollution. Due to high oxidative thermal stress, such waste frying fats should not be used for human food [142]. Also, since 2002, the EU has enforced a ban on feeding these mixtures to animals, because during frying many harmful compounds are formed, which could result in the return of harmful compounds back into the food chain through the animal meat [143].

Used oils can be recycled through conversion into soap by saponifi­cation and reused as lubricating oil or hydraulic fluid. Nevertheless, bio­fuel production seems to be the most attractive alternative for waste oil treatment. Certainly, it will not solve the energy problem, because only a small percentage of diesel demand can be supplied by this source [20], but it will decrease the dependence on fossil oil while reducing an envi­ronmental problem.

For economic reasons, used frying oil is an interesting feedstock for biodiesel production. In this sense, Nye et al. were the first to describe the transesterification of used frying oil using excess of alcohol under both acidic and basic conditions. The best result was obtained using methanol with catalysis by KOH [144]. The tests were carried out using frying margarine and partially hydrogenated soybean oil. The reaction was carried out at 50°C for 24 h, using methanol in a methanol-triglyceride molar ratio of 3.6:1 and 0.4% KOH. At the same time, Mittelbach et al. investigated the use of waste oils to produce biodiesel and found that the increase in the amount of polymers during heating of the oil is a good indicator for the suitability for biodiesel production [42]. They proposed a low-temperature process (40°C) under alkaline catalysis and excess of methanol [145]. Considering used olive oil, better results were also obtained using KOH and methanol instead of NaOH and ethanol, which decreases transesterification rates. The reaction was optimized at an ambient temperature, using 1.26% KOH and 12% methanol, and stirring for 1 min [40]. Some authors have optimized the reaction by using methanol (alcohol-waste oils molar ratios between 3.6 and 5.4) and 0.2-1% NaOH [146], or methanol (molar ratios in the range of 1:74 to 1:245) and acid catalyst (sulfuric acid) [147]. Al-Widyan and Al-Shyoukh have performed waste palm oil transesterification under various conditions. The best process combination was 2.25 M H2SO4 with 100% excess ethanol in about 3 h of reaction time [148].

Several parameters (e. g., heating conditions, FFA composition, and water content) can influence conversion from waste oils into biodiesel. Mittelbach et al. have found that heating over a long period led to a sig­nificantly higher FFA content, which can reach values up to 10% and have detrimental effects during the transesterification process. Nevertheless, in most cases, simple heating and filtering of solid impu­rities is sufficient for further transesterification [20]. The methyl and ethyl esters of fatty acids obtained by alcoholysis of triglycerides seem to be excellent fuels [5]. Anggraini found that it was also important to keep the water content of used cooking oils as low as possible [149]. Dorado et al. have compared biofuels from waste vegetable oils from sev­eral countries (different FFA composition) including Brazil, Spain, and Germany. The transesterification process was carried out in two steps, using a stoichiometric amount of methanol and the necessary amount of KOH, supplemented with the exact amount of KOH to neutralized acidity. Both reactions were completed in 30 min [41]. Results revealed that to carry the reaction to completion, an FFA value lower than 3% is needed. The two-step transesterification process (without any costly purification step) was found to be an economic method for biofuel pro­duction using waste vegetable oils. To reduce FFA content, a two-step transesterification using 0.2% ferric sulfate and 1% KOH with methanol (mole ratio 10:1) was also developed [150]. Acid-catalyzed pretreatment to esterifiy the FFA before transesterification with an alkaline catalyst was also proposed [151]. This procedure can reduce the acid levels to less than 1%. Some authors have proposed a three-step process in a fixed — bed bioreactor with immobilized Candida antarctica lipase [152]. Brenneis et al. also developed a process involving C. antarctica through alcoholysis of waste fats, with excess of water. The optimum amount of water was found to be 80-10% of the amount of fat [153]. Chen et al. preferred the use of immobilized lipase Novozym-435 in transesterifi­cation of both waste oil and methyl acetate. However, they found that the reaction rate decreased with increasing water content [154].

Engine tests have been performed with biodiesel from different kinds of waste oils. Al-Widyan et al. tested several ester-diesel blends in a direct-injection diesel engine. Results indicated that the biodiesel burned more efficiently with less specific fuel consumption. Furthermore, 50% of the blends produced less CO and fewer unburned hydrocarbons than diesel [155]. Also, Mittelbach and Junek stated that it improves exhaust gas emissions, as compared to esters made from fresh oil [156]. However, despite the exhaust emission reduction, there are some dis­crepancies in terms of NOx emission related to the process and raw material [1, 105, 157]. In general terms, most studies show a slight decrease in brake power output, besides an increase in specific fuel consumption [158, 159]. To solve this problem, Kegl and Hribernik have proposed to modify injection characteristics at different fuel temperatures [160].

Several authors have worked on related topics. Kato et al. have used ozone treatment to reduce the flash point of biodiesel from fish waste oil, resulting in easy combustibility [161]. The immiscibility of canola oil in methanol provides a mass-transfer challenge in the early stages of transesterification. To exploit this situation, Dube et al. developed a two-phase membrane reactor. The reactor was particularly useful in removing unreacted oil [162].