In the last decades much effort was put in understanding fouling phenomena on membranes. Many new concepts have been introduced in time, and parallel to this many parameters capable to quantify fouling issues and fouling evolution.
One successful approach was the introduction of the critical flux theory. At first validated for microfiltration, the theory applied to ultrafiltration and nanofiltration, too. The possibility to measure a maximum value of the permeate flux for a given system without incurring in fouling issues was a breakthrough in membrane process design. Nevertheless, the application to the concept remains very limited: in many cases, in particular on systems where fouling is a main issue, critical fluxes were found to be very low, lower than economical feasibility permits to make membrane technology advantageous. Despite these arguments, the knowledge of the critical flux value still remains and must be considered as a good starting point for process design concerning productivity and longevity.
In 2011, a new concept was introduced, that is the threshold flux. In this case, the concept evaluates the maximum permeate flow rate characterized by a low constant rate fouling regime, due to formation of a secondary, selective layer of foulant on the membrane surface. This concept, more than the critical flux, may be a new practical tool for membrane process designers.
In this paper a brief review on critical and threshold flux will be reported and analyzed. In fact, critical and threshold flux concepts share many common aspects which merge perfectly into a new concept that is the boundary flux. The validation will occur mainly by the analysis of previous collected data by the authors, during the treatment of olive mill wastewater. A novel membrane process design method based on the boundary flux will then be presented.