Semi-continuous dry anaerobic digestion of rice straw pretreated with swine manure digested effluent in one vertical cascade digester with different solid contents

Energy input for substrate stirring is generally demanded in dry anaerobic digestion. Semi-continuous dry anaerobic digestion of rice straw pretreated with swine manure digested effluent (named as ammoniated rice straw) in a vertical cascade digester with varied solid contents (20 %, 25 %, and 30 %) was investigated for energy and room saving. The analysis of sCOD, VFA, ALK, and TAN which refer to the stability of anaeobic digestion showed that the digestion in the TS = 25 % system was the stablest, while the acidogenesis decreased on days 120–180 in the TS = 20 % digester, and the hydrolysis and acidogenesis both decreased on days 120–180 in the TS = 30 % digester. The correlation analyses of sCOD, VFA, ALK, and TAN showed a positive correlation between factors in the TS = 25 % digester which was favourable for a long-term operation, while negative or weak positive correlations between the above factors in the digesters of TS = 20 % and TS = 30 % were noted. Moreover, the average daily methane yield in the digesters ranged from 1.69 to 2.15 mL/gVS_added, among which the average daily methane yields in the digesters of TS = 20 % and TS = 25 % were 27.21 % and 24.85 % higher than that in the TS = 30 % digester, respectively. The results indicated that a semi-continuous stable biogas production from rice straw-swine manure digestate could be achieved at a 25 % solid content in a vertical cascade anaerobic digester (SVCAD). Also, the microbial community structure analysis in the TS = 20 % digester revealed a decrease in community diversity in the course of digestion time. Bacteroidota became the dominant bacteria (exceeding 50 % in relative abundance) in stage II at the phylum level, and DMER64 and LNR A2-18 became the predominant genera at the genus level. This work firstly reported the anaerobic digestion of ammoniated rice straw in a SVCAD, and drew a conclusion on the appropriate total solid for the long-term running in a SVCAD. It is a contribution for the development of efficient biofuel production from piggery waste and crop residues.