Cimanuk River is one of the biggest rivers that pass through the Indramayu City, West Java, Indonesia. The river is categorized as an urban river and functioned as the main drainage system. Its water quality decreasing due to the urban runoff mixed with the wastewater discharged from the domestic and nondomestic activities. Modeling is one of the management efforts to overcome the water quality problem of the river. Deoxygenation rate becomes important to be obtained in order to have an

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... modeling result. This research was conducted to investigate the deoxygenation rate of the Cimanuk River based on the laboratory analysis and empirical formula calculation. Water samples were taken from 2 points in the Cimanuk River. DO measurement were conducted each day from the incubated samples for 10 days. Slope Method was then being used to determine the deoxygenation rate. The investigation results show that the range of deoxygenation rate of Cimanuk River is 0.06-0.12 per day. The BOD ultimate based on the Slope Method calculation ranges between 23.33 and 34.83 mg/L. Using the empirical equation, the deoxygenation rate was obtained between 0.422 and 0.462 per day. The low value of deoxygenation rate observed by laboratory analysis can be caused by the slow microorganism's activity in decomposing the organic matter due to the unfavorable environment in the water.