## 3 Chemical Properties of Waste Cooking Oils and Purification Process
### Purification of Waste Cooking Oil  
Purification/filtration is a critical step in the valorization of waste cooking oil, as it removes impurities such as food particles, polymers, carbon residues, and other solids that can interfere with downstream processes like biodiesel production. Several filtration techniques are used depending on the scale and quality requirements. The purification of waste cooking oils can be achieved through various methods, depending on the intended application. Common techniques employed in the purification process include filtration, drying, and acid reduction, either through additives or esterification, as shown in Table 4 and also Figure 6. The initial and most critical step is filtration, which can be conducted using vacuum filtration or standard filter paper. For applications such as insulating liquids, where high purity is essential, it is advisable to use low-porosity filter paper to ensure the effective removal of fine particulates. Another effective method for purifying waste cooking oil involves the use of commercial products like Magnesol(r) filter media, which provides an efficient and easy filtration process. This method is advantageous as it effectively removes off-flavors, colors, odors, free fatty acids, and total polar materials, significantly improving the quality of the reclaimed oil [89; 90; 91]. During the dehumidification process, maintaining oil quality is crucial; hence, drying should be performed at moderate temperatures to avoid thermal degradation. This process is often carried out under vacuum conditions, as demonstrated in studies by Refs. [92; 93]. The final stage in the purification process, before transesterification, involves the reduction in free fatty acids (FFAs) to prevent saponification reactions during base-catalyzed transesterification. Among the various methods available, the most commonly employed approach for FFA reduction is acid-catalyzed esterification, typically using concentrated sulfuric acid (H\({}_{2}\)SO\({}_{4}\)) and methanol, as illustrated in Figure 7 [94]. The free fatty acids react with the alcohol in the presence of a catalyst to produce fatty acid methyl esters (FAMEs) and water. It is important to reduce the FFA content to a value less than or equal to 1% to ensure excellent phase separation and to minimize soap formation during the subsequent base-catalyzed transesterification reaction [21; 95]. Other catalysts reported in the literature for FFA reduction include hydrochloric acid (HCl), phosphoric acid (H\({}_{3}\)PO\({}_{4}\)), and coal ash [96; 97; 98]. Following the pretreatment process, the treated oils undergo transesterification to reduce viscosity by removing glycerol, resulting in a more suitable liquid for intended applications. Figure 6: Waste cooking oil collection, treatment, and methyl ester synthesis. The purification of waste cooking oils can effectively utilize conventional vegetable or fat oil refining methods, which include degumming, neutralization, and bleaching. Among these, the total degumming process, commonly referred to as the Dijkstra and Opstal method, has been widely adopted by several researchers [21; 31; 32; 101; 33; 102; 103; 104; 105]. In this method, a citric acid solution is added to preheated oil (at approximately 70 \({}^{\circ}\)C) and stirred for a few minutes (15-30 min) to facilitate the removal of phospholipids and other impurities [8]. Citric acid acts as a chelating agent by binding to metal ions such as magnesium, calcium, and iron in the oil, which destabilizes and removes the non-hydratable phospholipids (NHPs). This process converts the NHPs into hydratable phospholipids (HPs), which are easier to remove, leading to the formation of a gum phase. The gum can then be separated through centrifugation or water washing. The second step is the neutralization of the degummed oil, achieved by adding an NaOH solution (typically 8-9.5%) at around 70 \({}^{\circ}\)C and stirring for 30 min. The soap formed during this reaction is then separated from the oil through centrifugation.