Effects of alkaline thermal hydrolysis on the formation of refractory compounds and energy balance of anaerobic digestion of cattle manure

Abstract

Anaerobic digestion (AD) coupled with thermal hydrolysis pretreatment (THP) is appealing because it enhances the energy conversion rate of organic wastes. Here, cattle manure, the most abundant organic waste, was thermally hydrolyzed as a pretreatment of AD. In order to increase the process efficiency, a NaOH solution was introduced. Increasing the amount of NaOH addition during the THP enhances lignin removal and solubilization of cattle manure. The thermally hydrolyzed cattle manure (at 160 °C with 2 % (dry wt.) NaOH addition) showed a significantly higher biochemical methane potential of 227.0 ± 11.0 mL-CH4/g-volatile solid compared to that of intact cattle manure (182.2 ± 2.5 mL-CH4/g-volatile solid). Refractory compounds (humic acid-like substances and furfural) were identified at THP temperatures above 180 °C without NaOH addition, and it was enhanced by increasing NaOH addition. Energy balance analysis showed that additional 161.4 ± 39.3 MJ of energy can be gained per ton of cattle manure by applying THP (at 160 °C without NaOH addition). This study provides insights into the changes in the physicochemical and biochemical properties of cattle manure caused by alkaline THP.