Similar to reactor C, reactor G treated solely with the RS substrate displayed a temporary decrease in biogas production in the first 3–5 days after a rapid increase on the first day, which could be due to the degradation of soluble carbohydrates contained in RS. Subsequently, the methanogenic stage could recover gradually as the pH was neutralized by adding NaHCO3, and the peak biogas production of 23.9 mL/(g VS d) was obtained on day 13, at a value of only about a half of that obtained from co-digestion.