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The present study aims to manage and determine the most economical efficiency of five wastewater treatment plants discharging wastewater into the Tigris River in Iraq. The management system was based on ensuring the five-day biological oxygen demand concentration in the river is <30 mg/L according to the Iraqi standards. In many cases, the determined optimized efficiencies were found to be lower than the present working efficiencies. Although this was good for the environment, it was not cost-effective. This study revealed that the variation of river flow rates was not an important factor that effects on the results obtained. It was found that the variation of organic decomposition value in the river and the minimum efficiency limit of the first upstream plant greatly affected the operating efficiency of the downstream plants. Furthermore, no constant rank was recorded for the effects of the natural decomposition on the operating efficiency of each plant. Three points were highlighted from this study: •The optimization methods were used to determine the most economical efficiency of multi wastewater treatment plants.•The effects of the BOD decomposition value, the river flow, and the minimum efficiency limit were also investigated.•This study presents the linear modeling method in detail and has a scientific impact for similar studies.