## 4 Quality of Biodiesel Produced from Animal Fat Waste  
Biodiesel has to comply with regulations EN14214 from the European Committee of Standardization (ISO) and the ASTM6751-3 from the American Society for Testing and Materials (ASTM). There are relevant benefits in the use of biodiesel produced from animal fats, such as the emission of polycyclic aromatic hydrocarbons being reduced by 75-90% and total unburned hydrocarbon by 90% when using biodiesel produced from animal fats when compared to conventional diesel [10,11]. The emissions of sulfur dioxide and CO are also reduced [12] as well as the particulate matter and nitrous oxides at part loads [18,66]. The properties of biodiesel produced from animal fats are compiled in Table 3. The cetane number reflects the ignition characteristics of the fuel. A high number is associated with better ignition quality [47]. The cetane number of biodiesel produced from animal fats is >50 and higher than that produced from vegetable oils due to their content of saturated fats (higher than 40%) and lower content of carbon and higher content of oxygen in comparison to conventional diesel [14]. The acid value is associated with the content in free fatty acids which, at high concentrations, can cause corrosion of the fuel supply system of the engine [66]. The cold filter plugging point (CFPP) represents the lowest temperature that a volume of liquid fuel will still flow through a given filter in a determined period of time when the fuel components start to gel or crystalize [102]. It is especially important for cold weather conditions. CFPP of biodiesel from animal fats is higher than 2 \({}^{\circ}\)C due to a higher content of saturated fatty acids. Flash point is the temperature at which the biodiesel exposed to a flame will ignite. Biodiesel from animal fats has a flash point higher than 150 \({}^{\circ}\)C and this provides better safety during transport and storage [47]. Biodiesel provides good lubricity that helps for a longer engine life [103]. However, the viscosity is higher than 3.5 mm\({}^{2}\)/s due to the saturated fats that give higher amounts of CH\({}_{2}\) in the fatty acid methyl esters [104] which might create problems in pumping and combustion [11]. Finally, the percentage of free glycerin reflects the amount of glycerol remaining in the final biodiesel and, if the content is high, it could result in coking of the injector and damage to the fuel injection [47].  
The cost of the obtained biodiesel from animal fat waste depends on several parameters such as the cost of the feedstock, the amount of free fatty acids and the type of necessary pretreatment, the type of catalyst, the operational maintenance and the type of purification for the biodiesel [105]. The refining of crude biodiesel obtained from animal waste is more costly than that from vegetable oils. Large amounts of glycerine (glycerol) are produced during transesterification which need to be removed because they influence the quality of the fuel and reduce the engine durability. The purification process (see Figure 1) usually consists of either wet washing based on water, dry washing based on adsorption and ion exchange, or novel methods based on liquid-liquid extraction, deep eutectic solvents or membranes. The purification step will thus remove glycerol as well as other impurities like residual catalyst, unconverted fats, and soap, and provides a better quality of biodiesel fuel [106]. About 1 kg of glycerol is produced for each 10 kg of biodiesel [107]. The recovered glycerol may be used for pharmaceutical, food, personal care biopolymers, or fuel additive applications although the value is low due to the large worldwide production of glycerol [92; 105].