In stage 1, the pH in the CSTR reactor was controlled between 4.0 ± 0.1 by the pH controller lasting for 24 days. As shown in Fig. 1, the total COD concentration of fermentation products increased gradually from start-up to 7th day and then maintained relatively stable around 23.5 ± 2.5 gCOD/L till the end of stage 1.