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 . 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 inoculumand the restwas orange peel andwater.Thereactors
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. Reactors containing only water and inoculum were used as a blank. The experiments were performed in
triplicate.At the end of the digestions, the pH of the digestates wasmeasured. For semicontinuous digestions, the pretreated wastes were chosen based on the results obtained from the batch digestion. The semicontinuous digestions were performed in 2L reactors (AutomaticMethane Potential Test System I, Bioprocess Control, Sweden) with a liquid volume of 1.8 L.The reactors were placed in a water bath at 55∘C. The organic loading rates (OLR) for both untreated and treated orange peels were set at 1 gVS/L/day during the starting up period and gradually increased to 3 gVS/L/day.Thehydraulic retention timewas set at 30 days. Gas production, pH, volatile fatty acids, and buffer capacity ratio were monitored during the digestion process.