Fig. 2

The changes of pH (a), gas production rate (b), VFAs concentrations (c, d), and COD removal rate (e) in single-stage systems. In Phase 1 and 2, the single-stage systems started with synthetic wastewater with a HRT of 48 and 24 h, respectively. In Phase 3, the HTL liquid products were used instead of synthetic wastewater with a HRT of 24 h. In Phase 4, the HRT was decreased to 12 h. The minus time means the reactors were operated with synthetic wastewater only