This paper inspects the performance of two separation units in one of the experimental desalination plants. In the first step, the efficiency of the de-aerator system will be specified, and then the capability of improving the rate of dissolved gas removal by using professional types of packing will be evaluated. In the second step, the examination of the sedimentation tank performance will be tested, by using the jar test, and then the best chemical additives concentration will be specified. This work is done at the water desalination plant at Basrah University, in southern Iraq. The plant has a capacity of 9 m(3)/h and uses Koch membranes, model 8822-HR. The results show that the rate of dissolved gas removal can be improved when the area of contact between the gas and liquid streams is increased by using the professional types of packing. Also, the accuracy of chemical dosing for polyelectrolyte and ferric chloride in the sedimentation section improves the performance of the pretreatment section and reduces the chemical additives consumption. The backwash processes for sand filters and the period of replacement for fine filters has been improved after regulating the methods of separations.