Harnessing ferric chloride dosing for removal of Maillard reaction-derived recalcitrant dissolved organic nitrogen during dewatering of municipal sludge treated by thermal hydrolysis-enhanced anaerobic digestion

• FeCl 3 dosing monotonically reduced rDON during THP-AD sludge dewatering • XPS revealed a Fe-rDON carboxylate inner-sphere complexation mechanism • Filtrate alkalinity decrease is a key trade-off of the FeCl 3 -driven rDON removal • Enhancement in dewaterability, UV-quenching, and PO 4 removal are added benefits Recalcitrant dissolved organic nitrogen (rDON) formed as a byproduct of thermal hydrolysis pretreatment (THP) resists mainstream biological nutrient removal and thus poses a challenge to water resource recovery facilities (WRRFs) that must achieve stringent total nitrogen standards. This study systematically evaluated the effects of targeted FeCl 3 dosing during post-dewatering of THP-digested sludge as a simple-to-implement, emergency measure to remove filtrate rDON before it returned to the mainstream flow. A combination of FeCl 3 doses ranging from 0.0% to 5.5% (g Fe/g dry solids) and polymer doses ranging from 0.0 to 1.0 × optimal polymer dose (OPD) was assessed for their effects on rDON removal, chemical consumption, filtrate quality, sludge dewaterability, orthophosphate removal, and potential downstream impact. Results revealed that FeCl 3 dosing monotonically reduced filtrate rDON from 223.0 to 92.1 mg/L along with the FeCl 3 increase from 0.0 to 5.5% Fe at 0.5 × OPD (i.e., 50% less polymer). X-ray photoelectron spectroscopy suggested inner-sphere complexation between Fe and rDON carboxylate functional groups as a key removal mechanism. Co-benefits of rDON removal using FeCl 3 during dewatering included improved cake dryness, efficient ortho-P removal, and UV-quenching mitigation. However, the side-effects of this rDON control strategy were filtrate alkalinity consumption and, in turn, pH depression, with the need for alkalinity supplementation to ensure sufficient sidestream deammonification if implemented downstream. Overall, findings from this study bridge a critical knowledge gap and offer a simple, rapid-response tool that WRRFs can deploy when excessive biosolids loading leads to elevated rDON production, providing immediate support to safeguard nitrogen permit compliance.