Impact of operating conditions on thermal hydrolysis pre-treated 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 when recycled. While there are mature technologies in place to handle the inorganic nitrogen produced due to the thermal pretreatment, organic nitrogen remains a parameter of concern for utilities employing THP pretreatment. The impact of operating conditions of the THP on dissolved organic nitrogen (DON) and UV absorbance in return liquor was investigated. Operating conditions studied were (1) operating temperature (2) solids retention time (SRT) in the anaerobic digester (3) THP flash pressure (4) the effect of co-digestion of sewage sludge with food waste and, (5) polymer conditioning. Operating temperature and polymer dose had the most significant impact on DON and UV quenching. It was found that 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. Increasing flash pressure from 25 psi to 45 psi did not have a significant impact on the return liquor. However, increasing the flash pressure to 75 psi increased the DON and UV254 absorbing compounds. Co-digesting the sludge with food waste resulted in a slight increase in DON and a decrease in dissolved organic carbon (DOC) and UV quenching compounds. Increasing the SRT from 10 days to 15 days resulted in a slight decrease in DON but did not have any impact on UV254 absorbance. Overall, it can be concluded that optimizing operating conditions of thermal hydrolysis process can result in decreased DON and UV quenching compounds in the recycle stream.