Thermal and thermo-chemical pre-treatment of four waste residues and the effect on acetic acid production and methane synthesis

In this study four diverse solid waste substrates (coal, Kraft pulp solids, chicken feathers and chicken processing waste) were thermally pre-treated (70, 140 and 200 °C), under an inert (nitrogen) or oxidative (oxygen) atmosphere, and then anaerobically digested. Membrane inlet mass spectrometry during the thermal and thermo-chemical reactions was successfully used to establish oxygen and carbon dioxide gas fluxes and product formation (acetic acid). There was significant solids hydrolysis pre-treatment at 200 °C under an oxidative atmosphere, as indicated by a decrease in the volatile suspended solids and an increase in dissolved organic carbon. Greater concentrations of volatile fatty acids were produced under oxidative conditions at higher temperatures. The methane yield more than tripled for feathers after pre-treatment at 140 °C (under both atmospheres), but decreased after oxidative pre-treatment at 200 °C, due to the destruction of available carbon by the thermo-chemical reaction. Methane yield more than doubled for the Kraft pulp solids with the 200 °C pre-treatment under oxidative conditions. This study illustrated the power of wet oxidation for solids destruction and its potential to improve methane yields generated during anaerobic digestion.