When waste oil of low cost was used as the raw material, alkali-catalyzed process II required a pretreatment
unit to reduce the content of free fatty acids. The cost associated with this pretreatment unit, including
the cost for extra solvent, more than balanced the credit of using waste oil. This led to a reduced economic
feasibility for process II. The acid-catalyzed system was insensitive to free fatty acids and no pretreatment unit
was required. Accordingly, the acid-catalyzed processes to produce biodiesel from waste cooking oil (processes
III and IV) had lower total manufacturing costs than the alkali-catalyzed process, so that the acid-catalyzed process
would cost less to operate. Glycerine was a valuable by-product, which could add an appreciable credit to