Enhancement of biogas production from anaerobic digestion of waste activated sludge by hydrothermal pre-treatment

• Hydrothermal pre-treatment (HTP) is an effective technique for achieving organic material disintegration.
• HTP increased solubilization COD up to 37%.
• The optimum HTP temperature was found to be around 220 °C.

This study is an assessment of hydrothermal pre-treatment of waste activated sludge for enhancement of biogas production by anaerobic digestion. This assessment was carried out in order to ascertain the optimal hydrothermal pre-treatment temperature. Anaerobic digestion efficacy was investigated by biochemical methane potential (BMP) tests and subsequent statistical analyses. Hydrothermal pre-treatment was found to improve solubilization of waste activated sludge and increase the response surface, resulting in increased biogas production. BMP tests revealed that hydrothermal pre-treatment of waste activated sludge increased COD solubilization (30–37%) and soluble COD (SCOD) levels; in addition, it reduced volatile solids (VS), and improved final methane yields. The optimum temperature for hydrothermal pre-treatment using subcritical water was found to be ∼180–210 °C. Methane yields of 130.2 (180 °C pre-treatment) and 126.6 (210 °C pre-treatment) ml CH4/g-VS were measured following hydrothermal pre-treatment. In pre-treated waste activated sludge samples, methane composition was found to be 63.4 and 58.8% (180 and 210 °C pre-treatment, respectively) and VS removal was measured to be ∼60 and 58% (180 and 210 °C pre-treatment, respectively). These findings show that in order to achieve high conversion efficiency, an accurately designed pre-treatment step must be included in the overall anaerobic digestion process for wastewater treatment.