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Abstract The treatment of landfill leachate and sewage is crucial for mitigating the environmental impacts of dissolved organic nitrogen (DON) effluent in aquatic ecosystems. This study used three sequencing batch reactors (SBRs) to treat sewage mixed with landfill leachates of varying organic carbon content. While the SBRs significantly removed dissolved inorganic nitrogen (DIN), the effluents were enriched with landfill leachate-induced DON. These landfill leachate-induced DON effluents (R1, R2, and R3) were then photodegraded under simulated summer sunlight conditions based on Greensboro, NC, USA weather data. The study utilized visible light (400–780 nm, 9340 μW/cm²), UVA (365 nm, 1442 μW/cm²), UVB (285 nm, 76 μW/cm²), UVC (254 nm, 315 μW/cm²), and dark controls. Effluents were mixed with Neuse River Estuary water, serving as a natural algal source, and exposed for 90 days under these light conditions. Samples were analyzed every 10 days for DON degradation and algal growth, with molecular changes assessed using FTICR-MS, FTIR, and EEM-PARAFAC. Results showed substantial DON degradation across all light treatments, with UVA achieving the highest reduction (up to 99.07%), followed by UVC (88.85%), visible light (86.19%), and UVB (75.11%), while no degradation occurred under dark conditions. Initial DON levels of 2.69–2.7 mg/L were reduced to as low as 0.025 mg/L under UVA in R3 effluent. UVC treatment led to increased NO3-N concentrations due to the oxidation of DON to NH4-N and its subsequent conversion to NO3-N, reaching 2.66, 2.59, and 2.63 mg/L in R1, R2, and R3, respectively. UVC inhibited algal growth, resulting in no NH4-N uptake and subsequent oxidation to stable, elevated NO3-N levels in the samples. Algal growth responses varied by light treatment, with visible light and UVB promoting the highest algae growth, minimal algae growth observed under UVA, and no growth under UVC or dark conditions. These findings demonstrate the evidence of rDON degradation during the long-term retention in the receiving water bodies and potential impact on the algal growth.