Dissolved organic nitrogen (DON) is an integral part of the dissolved organic carbon (DOC) reservoir, encompassing substances with nitrogen content. Within aquatic ecosystems such as lakes and rivers, DON molecules originate from photosynthetic entities like algae and plants, as well as the discharge of nitrogenous waste from mammals. Additional pathways introducing organic nitrogen into water include soil leaching, sewage discharge, and air deposition. Given the predominantly biological origin of most DON molecules, the pool comprehensively encompasses nitrogenous compounds present in living organisms. Notably, prevalent components of freshwater DON comprise proteins, unbound amino acids, amino sugars derived from cell walls, and nucleic acids sourced from RNA and DNA. Additionally, waste products like urea and methylamines are commonly detected in these environments. It is noteworthy that prevailing wastewater treatment methods predominantly target the removal of dissolved inorganic nitrogen (DIN) compounds, often overlooking non-reactive DON. Consequently, untreated DON significantly contributes to the overall nitrogen load. In wastewater treatment facilities (WWTPs) employing biological nitrogen removal (BNR) processes, DON constitutes the majority of the nitrogen present in the effluent. Although dissolved combined amino acids (DCAA) and dissolved free amino acids (DFAA) collectively contribute less than 4% and 1%, respectively, to the DON in the effluent, the main challenge lies in simultaneously maximizing the removal of DIN and the recovery of DON.