An improved flux-step method is presented incorporating cleaning steps by relaxation to determine the critical flux and the critical flux for irreversibility. Experiments are performed with activated sludge fed with real municipal wastewater. The improved flux-step method is compared with a common flux-step method often used in the literature. The critical flux for a polyvinylidenefluoride flat-sheet membrane with a pore size of 0.1 μm is 56 ± 2 L m−2 h−1 according to the common flux-step method and 52 ± 3 L m−2 h−1 according to the improved flux-step method. The marginal difference is explained by the application of larger flux-steps in the improved flux-step method. By applying intermediate relaxation the fouling appears to be almost completely reversible, indicating that cake layer formation is the dominant fouling mechanism. The cake layer remains removable up to a flux of 100 L m−2 h−1. The critical flux for irreversibility is therefore larger than 100 L m−2 h−1. The influence of fouling history is reduced by intermediate relaxation. As a result, the total fouling rate is much lower with the improved flux-step method compared to the common flux-step method, showing almost no fouling rate hysteresis in the improved method. A low amount of irreversible fouling is measured due to the adsorption of macromolecules, pore blocking, and/or gel formation. Applying the improved method ten times consecutively shows the influence of irreversible fouling on the long term. The critical flux decreases by 4 L m−2 h−1 as a consequence of irreversible fouling during the ten runs representing a long filtration run of 150 h at a net flux of 19 L m−2 h−1.