The present study proposed a water quality management system for multiwastewater treatment plants discharging wastewater into the Tigris River in Iraq. The proposed system will be useful to determine the optimum removal efficiency of multiwastewater treatment plants that discharge their wastewater into the Tigris River. Hence, the main objective of this study was to determine the most economical efficiency of each plant that ensures reduction in the biological oxygen demand (BOD) concentration of the wastewater to an acceptable level before discharging in the river. Five wastewater treatment plants were randomly selected from the Tigris River region in the Baghdad city, Iraq. The BOD value of .30 mg/L was selected as a water quality parameter according to the Iraqi standards. For this purpose, the BOD actual data were collected from each plant and then fed into the mathematical model. Furthermore, the data collected of BOD, river flow rate, and plants wastewater discharges were found to significantly vary with the month of the analyses. Therefore, the model results were expressed as a monthly variation of efficiencies. The water quality management system of multi-wastewater treatment plants usually occurs in the form of linear equations. Therefore, the optimization techniques of linear programming (LP) were used in combination with the MATLAB program to solve the mathematical model equations in this study. In many cases, the determined efficiencies were found to be lower than the present efficiencies. Although this was good for the environment, it was not costeffective. In addition, this study revealed that the river flow rates were not an important factor in the dilution of BOD concentrations in the river and hence on the effects on the x determined values of operating efficiencies. It was found that the variation of BOD decomposition value in the river and the operating efficiency value 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. This study presents the modeling method in detail and has a pedagogical value for similar researches.