The objectives of this paper are to study the effect of discharge of Zarqun drain into Mahmoudia canal on the natural self-purification along the canal stream and to suggest and simulate different scenarios to improve the water quality of the Mahmoudia canal system to safely discharge water of Zarqun drain into it. Natural selfpurification model based on oxygen sag curve introduced by Streeter and Phelps has been applied in two cases; the first is the current situation case, where no drainage water is discharging into the canal because of Edko Irrigation Pumping Station stops lifting drainage water from Zarqun drain into the canal. The second case provides additional safe reuse water through lifting drainage water from Zarqun drain into the canal using Edko Irrigation Pumping Station. The result of this case will determined the required canal length to achieve self-purification. Different scenarios will be designed to simulate different conditions of the water quality system improvement. The best scenario neeb aah stated and detailed and recommendations nave been done.