Viability of recuperative thickening in upgrading thermophilic and mesophilic anaerobic digestion of hydrothermal high-solid sludge.

• Recuperative thickening (RT) upgraded digestion of hydrothermal high-solid sludge. • Comparative processes with RT were operated at 35 ℃ (MD-R) and 55 ℃ (TD-R). • The MD-R and TD-R increased gas production by 22.8% and 11.0%, respectively. • RT increased hydrolysis rates by 11.1% and 7.4% in the MD-R and TD-R, respectively. • The improved numbers of archaea led to upgraded performances in the RT systems. Recuperative thickening (RT) process was introduced to further upgrade anaerobic digestion of hydrothermal high-solid sludge. Continuous mesophilic (MD-R) and thermophilic (TD-R) digestion with RT (MD-R) were operated synchronously, with corresponding single digestion without RT as controls, namely MD and TD. The MD-R and TD-R increased biogas production rates by 22.8% and 11.0%, and achieved 16.6% and 9.7% higher volatile solids reductions, respectively. The improved performance was partly attributed to increased hydrolysis rate, with 11.2% and 7.4% higher for the MD-R and TD-R than the controls, respectively. The RT increased the numbers of total archaea in the mesophilic and thermophilic systems by 844% and 108%, and the numbers of dominant archaea by 50.4% and 38.1%, respectively, which promoted the degradation of organic matter and the production of biogas. Thus, RT is applicable to further upgrade digesting high-solid sludge. [ABSTRACT FROM AUTHOR]