Enhancing nitrogen removal from sludge-derived hydrochar via hydrothermal carbonization fortified with advanced oxidation process pretreatment

In order to prepare low-nitrogen hydrochar, this study used sodium persulfate pretreatment coupled with hydrothermal carbonization (HTC) to destroy the sludge structure to release nitrogen-containing substances by free radical. Meanwhile, the effects of sodium persulfate concentration and free radical activation ways on nitrogen transition were also discussed. After the sludge was pretreated with 40 mmol/L sodium persulfate for 3 h under UV/heat synergistic activation, the maximum nitrogen removal efficiency of the hydrochar can reach 60.05 % at 240 °C hydrothermal temperature, which was 20.91 % higher than that of the original sample (39.14 %). Furthermore, in this condition, the ammonia nitrogen and total nitrogen in the aqueous phase reached the highest of 3008 mg/L and 6316 mg/L, respectively. During pretreatment, the persulfate radical (SO4·-) and hydroxyl radical (·OH) were the key to the destruction of the EPS structure, which realized the transfer of nitrogen from the solid to liquid. In addition, the acidic environment generated by the hydrolysis of sodium persulfate promoted the conversion of stubborn proteins to easily decomposed proteins (such as Nitrile-N) and the inhibition of the Maillard reaction. This study supplements relevant theories for the production of low nitrogen hydrochar by pretreating sludge with sodium persulfate.