Denitrification vs. assimilation: shifting metabolic pathways to enable N recovery from low-strength municipal wastewater

Nitrogen (N) recovery from low-strength municipal wastewater is essential for sustainable treatment. This study operated two pilot‑scale modified anaerobic‑anoxic‑oxic reactors to investigate N transformations under different influent qualities. With low N-load wastewater (NH₄+‑N ≤ 20 mg/L), acetate promoted ammonium assimilation mainly via the glutamine synthetase/glutamate synthase pathway, accounting for 50% of influent TN, whereas glycerol favored denitrification. Under higher N load (≥50 mg/L), glycerol enhanced assimilation through the glutamate dehydrogenase pathway, contributing 46% of influent TN. Sludge analysis indicated that assimilation directed N toward protein synthesis (protein content > 30%), thereby improving N recovery potential. Key microbial phyla included Pseudomonadota and Bacteroidota, with genera such as Defluviicoccus harboring arginine‑synthesis genes, implying their potential role in efficient assimilation under low C/N. Coupling ammonium assimilation with denitrification offers a viable strategy for simultaneous N removal and recovery, providing a potential framework for transitioning treatment plants into resource‑recovery facilities.