Impact of operating temperature on dissolved organic nitrogen and UV absorbance in thermal hydrolysis pretreated digestion return liquor

Return liquor from thermal hydrolysis process (THP) can significantly add to the nitrogen load of a wastewater treatment plant (WWTP) and introduce UV quenching substances to the wastewater stream. While there are mature technologies in place to handle the inorganic nitrogen produced due to the thermal pretreatment, organic nitrogen remains to be a parameter of concern for utilities employing THP pretreatment. In this study, the impact of operating temperature of THP on dissolved organic nitrogen (DON) and UV absorbance in return liquor was studied at operating temperatures of 130°C, 150°C and 170°C. An increase in operating temperature resulted in an increase in DON, which was primarily contributed by the hydrophilic fraction. An increase in temperature also resulted in increased UV254 absorbance. However, this trend was not linear and the increase was more pronounced when the temperature was increased from 150°C to 170°C.