Figure 2: Optimization of combined pretreatment on rice straw with differentbiological periods. (a), Daily methane production of combined pretreatedrice straw for three different periods [A+B5], [A+B10], and [A+B15] (b),Cumulative methane production of combined pretreated rice straw for threedifferent pretreatment period [A+B5], [A+B10], and [A+B15]; [A+B5] is ricestraw pretreated with 1.5M NaOH followed by biological pretreatment for 5days; [A+B10] is rice straw pretreated with 1.5M NaOH followed by biologicalpretreatment for 10 days; [A+B15] is rice straw pretreated with 1.5M NaOHfollowed by biological pretreatment for 15 days.