Optimization and comparison of methane production and residual characteristics in mesophilic anaerobic digestion of sewage sludge by hydrothermal treatment.

Anaerobic digestion is the preferred method for treating sewage sludge because of its ability to reduce sludge volume and produce biogas. However, conventional anaerobic digestion has a long retention time and low degradation rate. In recent years, hydrothermal treatment has been used to improve the hydrolysis of sewage sludge and biogas production. This process tends to focus on maximizing biogas production. However, very little research has been done on anaerobic digestion residues. In this study, batch experiments were conducted to investigate the effect of hydrothermal temperature on methane production and the contents of liquid fraction after anaerobic digestion (centrate). Experimental conditions were designed using a response surface method and central composite model. A quadratic equation was used to interpret the individual and interactive effects of hydrothermal conditions on anaerobic digestion. Given the maximum biogas production and the minimum concentrate concentration, the optimal operating condition was determined by a 186 °C hydrothermal temperature and a reaction time of 106 min. Under these conditions, the following results could be obtained: methane production (200.5 ± 7.7 mL-CH4/gVSadded), TCOD (16,572 ± 348 mg/L), sCOD (1240 ± 65 mg/L), sTN (658.9 ± 8.0 mg/L) and ammonia (525 ± 27 mg/L).