This work presented the design steps and calculation for each units of the water treatment plant (WTP), due to it’s important role domestically and drinking purpose. It also illustrated and designed the procedures of the water processing units by estimating water demand and designing the unit process. The objectives of this work were to estimate the water demand for a certain comminuity and to present design steps and calculations for the required units of a WTP.The design of the WTP units was applied to Greater-Zab River water for the selected location in Erbil City-Iraq. The quality and quantity of the Greater-Zab River water at various times were statistically analysed and presented.The units of the treatment processes involved intake, coagulation, flocculation, sedimentation, adsorption (optional), filtration, disinfection, storage, and pumping.