In the optimization study for pretreatment using hexane, four factors of leaching with two levels including temperature
(20 and 40∘C), time (10 and 300min), orange peel waste and hexane ratio (1 : 2 and 1 : 12), and the citrus waste size
(homogenized and chopped) were investigated.The pretreated orange waste was then digested to select the best condition of pretreatment. The results are summarized in Table 1. According to the statistical analysis, waste size was the only factor that was significant for the methane yield.