Two trials were established to investigate the effect of recycled effluent on hydrolysis during anaerobic co-digestion of vegetable and flower waste. Trial I evaluated the effect by regulating the flow rate of recycled effluent, while Trial II regulated the ratio of hydrolytic effluent to methanogenic effluent, which were recycled to hydrolysis reactor. Results showed that the recirculation of methanogenic effluent could enhance the buffer capability and operation stability of hydrolysis reactor. Higher recycled flow rate was favourable for microbial anabolism and further promoted hydrolysis. After 9 days of hydrolysis, the cumulative SCOD in the hydrolytic effluent reached 334, 407, 413, 581 mg/g at recycled flow rates of 0.1, 0.5, 1.0, 2.0 m3/(m3·d), respectively. It was feasible to recycling a mixture of hydrolytic and methanogenic effluent to the hydrolysis reactor. This research showed that partially introducing hydrolytic effluent into the recycled liquid could enhance hydrolysis, while excessive recirculation of hydrolytic effluent will inhibit the hydrolysis. The flow ratio 1:3 of hydrolytic to methanogenic effluent was found to provide the highest hydrolysis efficiency and degradation rate of lignocelluloses-type biomass, among four ratios of 0:1, 1:3, 1:1 and 3:1. Under this regime, after 9 days of hydrolysis, the cumulative TOC and TN in the hydrolytic effluent reached 162 mg/g and 15 mg/g, the removal efficiency of TS, VS, C and cellulose in the solid phase were 60.66%, 62.88%, 58.35% and 49.12%, respectively. The flow ratio affected fermentation pathways, i.e. lower ratio favoured propionic acid fermentation and the generation of lactic acid while higher ratio promoted butyric acid fermentation.

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