This paper presents a methodology for evaluating the water purification process efficiency attained by 1) jar tests, which are commonly used to determine the treatability of water, and 2) waterworks performance in different phases of the purification process as well as in its entirety. This methodology provides an important tool for the following: 1) determination of optimised reaction conditions under which water is treatable to its best attainable quality; 2) evaluation of the purification process efficiency attained by waterworks during different periods and under different operational conditions; 3) comparisons of performance efficiency of different waterworks; and 4) comprehensive commissioning of waterworks and identification of bottlenecks if any exist in process and plant design. This paper describes procedures for pre-processing of water samples to be analysed as well as mathematical formulas for processing of the results obtained. A few examples of the practical application of the methodology are included and the potential to obtain the optimization of waterworks’ purification processes is herein illustrated.