Digestion processes were performed in batch and semicontinuous reactors. The determination of biogas potential
of the orange peels in batch digestion was carried out according to a previous study [19]. In the experiment to select the solvent, the digestion was conducted with different concentration of volatile solids (VS) ranging from 0.5 to 2%. In the optimization study, two percent of VS of the untreated and pretreated peels were placed in 120mL glass bottle. The
total volume of the mixture was 30mL including 20mL of inoculum and the rest was orange peel and water.The reactors
were then flushed with amixed gas containing 80%ofN2 and 20% of CO2 for 2min. The reactors were incubated at 55∘C
for 33 days.