Proper membrane process design can be a difficult task to accomplish in those cases where membrane fouling plays a leading role and triggers even at low operating conditions. The requirement to obtain the permeate flux project values constantly for a certain period of time induces membrane process designers to oversize the plant capacities. Although plant capacity over-design is unavoidable, the correct understanding of the fouling mechanism of the system is key to limit investment costs. In particular for membrane processes applied to wastewater purification purposes, the concept of threshold flux was introduced in substitution of the critical one. The new concept applies to all systems which trigger fouling formation from start of operation, even at very low operating conditions: this is the case in purifying of olive mill wastewater streams by membranes. The new concept shares many properties with the critical one, such as the impossibility of theoretical prediction of its value. As a consequence, threshold fluxes need to be measured. In this paper, an established method for critical flux measurements, that is the pressure cycling method, will be discussed again for suitability for threshold flux measurements. Moreover, the acquired data by this method is used to allow proper membrane process design by limiting the oversize of the plant to a extent compatible to the fouling behavior of the system. (C) 2013 Elsevier B.V. All rights reserved.