This paper introduces an improved technique to model electrostatic precipitators (ESPs) loaded by dust on one fine computational grid. In this regard, and due to its superiority in terms of computational time on fine domains, the Full Multi-grid method (FMG) and Finite Difference Method (FDM) are combined and implemented to simulate dust-loaded ESP and trace the optimum conditions. These optimum conditions include two main objectives. Firstly, the Poisson and continuity equations are solved globally, with the most tolerable values of round-off and truncation errors being reached. The results of the proposed technique show a great agreement with the experimental results over the previously implemented numerical techniques with errors less than 0.6 % and 1.78 % for clean air and dust, respectively. Secondly, for the first time, an optimum value of the effective mobility of ions in dust-loaded ESP is found at each value of applied voltage and not considered to be constant at all applied voltages, as the previously implemented numerical techniques assumed. Thence, the effective value of mobility of ions is found without additional experiments. Moreover, the more precise applied voltage on discharge wires and current density at grounded plate characteristics are predicted based on the traced optimum conditions. As a result, this provides the suggested approach with the advantage of being able to precisely estimate the influence of changing various design factors on the performance of ESP accurately without the necessity of high-priced experiments.