The CH4 content, CH4 yield, and cumulative CH4 production in the four digesters with different substrates and conditions are illustrated in Fig. 1. Among these digesters, the CH4 content of the biogas increased gradually to 60–70% after 8–10 days, except for digester PPW-FR-3 (having the highest TS content of 9.1% and VS content of 5.0%), which realized a biogas content of 65% on day 14 (Fig. 1A)