2.2. Methods. Pretreatment of the wastes by leaching was conducted in Erlenmeyer flasks. Four different solvents
including hexane, diethyl ether, dichloromethane, and ethyl acetate were used. These chemicals are toxic and/or
inflammable and should be treated properly. In the experiment to select the solvent, each solvent was added to the
orange peel with orange peel waste and hexane ratio of 1 : 4. Themixture of solvent and orange peelwas shaken vigorously for 10 minutes followed by incubation for 20 minute at room temperature. In the optimization of pretreatment study, hexane was used as a solvent. Two levels of four parameters including temperature (20∘C and 40∘C), time (10 min and 300min), orange peel wastes and hexane ratio (1 : 2 and 1 : 12), and the wastes size (homogenized or chopped) were selected. Forty grams of thewasteswas dissolvedwith a certain amount of hexane in the flasks, followed by shaking vigorously for a determined period of time. After the settlement, the extracts were removed from residuals by vacuum filtration. The residual, pretreated orange waste was then washed three times with water in order to remove remaining hexane. Finally, the pretreated waste was digested to produce biogas.