Effect of liquid digestate recirculation on the ethanol-type two-phase semicontinuous anaerobic digestion system of food waste

This study set up four groups for semi-continuous 150-days experiment to explore the effect of liquid digestate recirculation on the food waste ethanol-type anaerobic digestion system. Results showed that this operation improved the maximum organic load rates to 6.0 g-VS.L-1.d(-1), and increased the average alkalinity of me-thanogenic phase under high load condition by 1.3 times. Total volatile fatty acids/total alkalinity (threshold value was approximately 0.5) could be used as an early warning indicator of methanogenic phase instability. Besides, approximately 64.5% of bacterial species in the hydrolysed acidified phase of ethanol-type liquid digestate recirculation group originated from the recirculated liquid digestate, which enriched the diversity of microbial community, thereby improving the hydrolysis acidification efficiency. Therefore, liquid digestate recirculation improved the stability of system in terms of alkalinity and microecology and then increased the maximum organic load rates.