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 was measured. 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.