This study examined the relationship between water quality indicators (five-day biochemical oxygen demand (BOD5
) and ammonia nitrogen (NH3
-N)) by modeling them at sewage treatment plants (STPs) and receiving waters. Discharge loads from 2016 to 2018 were input into the QUAL-MEV water quality model, which is used for the total maximum daily loads (TMDLs) in Korea, and the results showed that the simulated BOD5
values decreased according to the reduction in the BOD discharged pollution load, while the observed water quality increased. This was caused by an increase in the total nitrogen discharged pollution load and the NH3
-N effluence ratio from the STPs. Hence, the model was modified to reflect real-world conditions. After inputting a calibration factor calculated based on real data and the measured ratio of each type of nitrogen, the simulation was repeated, achieving results within ±20% of observed values. This model confirmed that BOD5
in the lower part of the river varies according to the change in the NH3
-N ratio of the STP effluent. To better manage BOD5
and to establish achievable water quality targets, a plan that reflects the linkage among water quality indicators, such as the TMDLs used in the United States, should be introduced in Korea.
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