Comparative Study of High-Temperature Anaerobic Digestion of Municipal Sludge Under Acid/Alkali and Low-Temperature Hydrothermal Synergistic Pretreatment

The efficiency of anaerobic digestion of sludge can be significantly improved through a combination of acid or alkali hydrolysis with low-temperature hydrothermal (LTH) pretreatment. The impact of various agents in conjunction with LTH treatment on sludge properties and high-temperature anaerobic digestion (HTAD) systems was comprehensively examined in this study. Comparative analyses reveal that NaOH-LTH pretreatment surpasses HCl-LTH pretreatment in enhancing sludge organic matter solubilization and enhancing HTAD system. Contrary to expectations, the use of acetic acid (HAc) pretreatment does not further enhance organic matter solubilization in sludge. Instead, it inhibits gas production efficiency and diminishes the removal efficiency of total chemical oxygen demand (TCOD) during HTAD. Among the pretreatments, NaOH (pH 11, 24 h)-LTH (90 °C, 30 min) co-treatment emerges as the optimal condition. This configuration results in a 12.5-fold increase in sludge soluble chemical oxygen demand (SCOD) compared to untreated samples, a 34.1% improvement in methane yield in the HTAD system, and a remarkable TCOD removal efficiency of 36.8%. Notably, this combined pretreatment induces significant alterations in the microbial community structure of the sludge HTAD system. Following NaOH-LTH pretreatment, the total relative abundance of methanogenic archaea increases from 80.2 to 92.3% compared to untreated systems.