Three districts in the Karbala governorate are served by the Karbala-Al-Muwahid water treatment plant (WTP): the Aoun district, the Al-Husseiniyah district, and portions of the city center. Monthly laboratory tests were collected for this study in order to evaluate the plant’s performance over the 2022–2023 period. Turbidity, hardness, alkalinity, pH, T.D.S., and other characteristics were assessed These parameters were chosen as they have the most influence on water quality. For the same study period, monthly data on influent and effluent flow rates were also collected. The plant’s operational performance was simulated and calibrated using the GPS-X software. Based on varying discharge levels, four scenarios were developed that included the maximum, minimum, and average discharges. In a different scenario, a filter was added during the forty-ninth phase of rise. The outcomes of these scenarios complied with Iraqi regulations and helped the plant save energy. In the case of the forty-first visit, water productivity was increased by adding a second filter to speed up the filtration process. The results showed that the facility was operating at an appropriate level and that the water quality was suitable for consumption in Iraq.