Effect of liquid digestate recirculation on biogas production and enzyme activities for anaerobic digestion of corn straw.

To accelerate the degradation of substrate, 50% liquid digestate recirculation (LDR) was used in the anaerobic digestion (AD) of corn straw. The effects of recirculation on the enzyme activities and biogas production were investigated by comparing with control reactor (Reactor). During the AD process, the fermentation system with 50% LDR was more stable. The average biogas and methane production in Reactor were 7,891 mL·d and 347 mL CH·g VS·d respectively. The total volatile fatty acids (TVFAs) concentration in the two reactors both increased at first and then decreased with time. The LDR made the VFAs accumulation significant, especially propionic acid accumulation in 4 ∼ 16 days. The maximum peak value of cellulase, xylanase, dehydrogenase and coenzyme F activities in Reactor were 0.51 mg·g·h, 0.29 mg·g·h, 4.88 mL·g·h and 6.69 μmol·L, respectively, which were higher than that in Reactor. With or without recirculation, the concentration of TVFAs was positively correlated with cellulase, xylanase and dehydrogenase activities, while was negatively correlated with coenzyme F activity. Besides, a very significant correlation existed between hydrolase and dehydrogenase activities and daily biogas production in Reactor. And the peaks of cellulase, xylanase and dehydrogenase activities appeared ahead of the peak of daily biogas production with the LDR.