A computational fluid dynamics (CFD) model was developed for sludge mixing-settling in a laboratory scale sludge tank based on the Takacs exponential function. A dynamic experimental program was also undertaken at different rotational speeds of mixers and the concentration of sludge to validate the sludge sump model. The deviation between the simulated interface height and the experimental data was 2%. The validated CFD model was then employed to develop a novel operating methodology for operation of a full-scale sludge tank. The proposed model includes two consecutive periods of sludge mixing and settling, resulting in reducing the mixers' working time. In the current operating mode, the energy consumption of mixers has been reduced by 91.67%. Based on the two-phase, 3-D modeling results of the sludge tank, the post-construction sludge tank is simultaneously able to act as a secondary clarifier, as well as a storage tank. As a key finding of the study, the treatment capacity of the filter press in downstream has accelerated, so that the working hours and energy consumption of the filter press has been improved by 55%. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.