Obtain High Quality Hydrochar from Waste Activated Sludge with Low Nitrogen Content Using Acids and Alkali Pretreatment by Enhancing Hydrolysis and Catalyzation

Hydrolysis has been recognized as the rate-limiting step of waste activated sludge hydrothermal carbonization, resulting in a low degree of carbonization with partial unconverted feedstock and a high nitrogen content in hydrochar. In the current study, acids and alkali pretreatment was adopted to accelerate solubilization and hydrolysis of carbohydrates and proteins, catalyze carbonization, and obtain high quality hydrochars with low nitrogen content. The nitrogen content decreased significantly to 1.05% and 0.94% with acids and alkali pretreatment at pH of 1 and 13, respectively, from 1.80% without pretreatment. The fuel properties of hydrochars were improved with acids pretreatment at pH of 1 and 4, but reduced with alkali pretreatment at pH of 13. On the contrary, the thermal decomposition stability of hydrochars were improved with alkali pretreatment at pH of 13, but reduced with acids pretreatment at pH of 1 and 4. Acids pretreatment led to enhanced degradation of organic substances and formation of more small-molecule acidic compounds, which promoted the aromatization reaction and increased the higher heating values and organic matter contents in hydrochars. Organic matters were mostly released to the aqueous phase with alkali pretreatment at pH of 13, resulting in lower organic matter content in hydrochar.