Statistical evaluation of nitrogenous oxygen demand as a dominant driver of oxygen dynamics in wastewater treatment plant effluent-impacted watershed

Nitrogenous oxygen demand (NOD) can substantially distort measurements of biochemical oxygen demand (BOD 5 ), yet its seasonal variability and statistical significance in effluent-receiving rivers remain poorly understood. This study evaluated NOD dynamics in the Yeongsangang and Seomjingang River basins (Korea), using monitoring data from 10 wastewater treatment plants and adjacent river reaches. Water quality parameters (BOD 5 , carbonaceous biochemical oxygen demand CBOD, total organic carbon, total nitrogen T−N, ammonia-nitrogen NH 3 −N, nitrite-nitrogen NO 2 −N, nitrate-nitrogen NO 3 −N, and dissolved oxygen) and physicochemical drivers (temperature, flow rate, conductivity, and pH) were measured across four seasons. Linear regression analysis indicated that in effluent-responsive (RP) sites, NOD accounted for approximately 60% of the variance in BOD 5 , often comparable to CBOD, while in effluent-resistant (RT) sites, NOD contributions were minimal. Analysis of variance (ANOVA) confirmed significant differences between RP and RT sites ( p < 0.05), validating effluent sensitivity as a key determinant of oxygen demand. Pearson correlation and scatterplot analyses were used to identify NH 3 −N and NO 2 −N as strong positive predictors of NOD, while NO 3 −N and T−N were inversely related, reflecting reduced oxygen demand when nitrification was complete. Seasonal factor and redundancy analyses revealed that NOD was amplified in summer by ammonia oxidation and nitrite accumulation, while in winter, oxidized nitrogen species became dominant, with temperature emerging as the most consistent explanatory variable. Overall, the results demonstrate that NOD is a dominant and seasonally dynamic driver of oxygen consumption in effluent-impacted rivers. These findings underscore the need to distinguish between CBOD and NOD in monitoring and regulating to more accurately assess the impacts of effluent and to guide nitrogen management strategies. • Nitrogenous oxygen demand (NOD) identified as a dominant driver of oxygen demand in rivers. • Effluent responsive sites show strong NOD contributions, resistant sites show weak effects. • Seasonal shifts: NH 3 –N, NO 2 –N in summer; NO 3 –N, T–N in winter. • Temperature is the most consistent regulator of NOD variability. • Findings support carbonaceous BOD–NOD separation in monitoring frameworks.