Fig. 1

The changes of pH (a, b), gas production rate (c, d), concentrations of VFAs and furan derivatives (e–h), and COD removal (i, j) in two-stage process. The concentration of HTL liquid products was increased from 0 (Phase 1), 1 (Phase 2), 2 (Phase 3), 3 (Phase 4), 4 (Phase 5), 6 (Phase 6) to 8 g COD/L (Phase 7) in sequence to replace the synthetic wastewater. The minus time means the reactors were operated with synthetic wastewater only