Many people around the world rely on drinking water that has been treated by water treatment plants, which process raw water obtained from rivers or other freshwater sources in the region. This study aims to apply a simplified methodology to evaluate the efficiency of water treatment plants and to ensure that the treated water complies with water quality standards. In this study, the Weighted Arithmetic Water Quality Index (WAWQI) was used as one of the simplest methods for assessing water quality in order to evaluate the efficiency and quality of the treated water from the Al-Tayyarah Water Treatment Plant, Hilla City, Iraq, and to compare it with the raw water. Samples were collected from the river next to the plant, the Hilla River, and from the treated water from January to November 2019. Eleven parameters were measured (PH, Turbidity, Temperature, EC, T.H, Ca, Mg, SO4, TDS, Na, and K). The results showed that the quality of raw water throughout the study period was "unsuitable for human use” except for April, which had "poor" water quality. For the treated water, it was suitable for drinking water and the water quality ranged from "excellent" water quality in March to "good" water quality in July. Based on the WAWQI results for raw and treated water, the plant demonstrated good efficiency in removing pollutants, and the average efficiency of the plant throughout the study period was 82.7%. Therefore, it can be said that the Al-Tayyarah Water Treatment Plant has revealed excellent performance in treating water, making it suitable for drinking purposes. Furthermore, the use of the WAWQI has proven its efficiency and usability as a simple strategy for evaluating the performance of water treatment plants.