An apparatus for treating particulate biodegradable organic waste includes a thermal hydrolysis reactor receiving the particulate biodegradable organic waste for performing thermal hydrolysis at a temperature of about 130.degree. C. at a pressure at or above the saturated water vapor pressure of the organic waste to produce a slurry including solubilized organic material and residual solids. A pH adjustment means for adjusting the pH of the particulate biodegradable organic waste as necessary to provide an acidic pH of greater than about 3.15 before thermo hydrolysis of the particulate biodegradable organic waste is provided. Separating means receive the solubilized organic material and residual solids for separating solubilized organic material from the residual solids. An anaerobic reactor is configured to receive the solubilized organic material.