Crude plant oils typically contain FFAs < 3.0 % and gums in the range of 0.05-0.5 % [21]. The only pretreatment needed for such oil feedstocks is to remove the gums us­ing either conventional degumming methods or using enzymes [22]. Higher FFAs oil feedstocks, such as recovered yellow grease, are no longer viable for the production of biodiesel by the conventional alkaline process. Such feedstocks containing up to 9 % FFAs can be pretreated by different methods such as the SRS (see Sect. 18.6.1) continuous flow acid by SRS Engineering, removal of FFAs by distillation or adsorp­tion, or treated with enzymes (Table 18.1) [ 27]. Crude corn oil or yellow grease with FFAs >9% requires acidic pretreatment, enzymes or Ca(OH)2 (Table 18.1). High FFAs oil which is disposed in the drains “brown grease” is a potentially problem­atic waste stream and it clogs installations in waste-water treatment plants and thus, it adds to the cost of treating effluent [23, 24]. Such potential feedstock contains typically 30% and up to 100% FFAs, requires an enzymatic or acid-based esterification methods (Table 18.1).

Biodiesel producers always have to decide on what is the cutoff of the FFAs level in the feedstock and decide how to move forward with biodiesel production. Some producers work with feedstock streams containing FFAs lower than 1 % (virgin oil) where any known technology for making biodiesel can be applied (Table 18.1).

When the feedstock contains up to 3.5 % FFAs, the soap formation during the alkaline reaction will be challenging to work with. One way to deal with it, is blending such oil (FFAs <3.5 %) with a lower FFAs feedstocks (<1 %) to obtain oil feedstocks of FFAs less than 2 %. Also, feedstocks containing FFAs levels below 3 % can be pretreated by adsorbents which extract FFAs from the oil out into the matrix. Spent adsorbents can normally be disposed off in a landfill. Another approach would be to convert the FFAs to their potassium salts and be removed by either water — wash process or centrifugation. When oil feedstocks contain higher than 3.5 % FFAs, pretreatment methods should be applied for the removal of FFAs, simply because the alkaline biodiesel production process converts the FFAs to soaps which eventually result in complicating of the downstream processing of the final products [25].

Not all pretreatment methods are suitable for every biodiesel production plant. Several factors will have to be considered such as the process type (alkaline, acidic, heterogeneous, enzymatic, etc.), the type of existing equipment in the plant, the long-term availability of feedstocks used with variable contents of FFAs, and how the pretreatment technology ties into those systems.