(177f) On the Understanding of Critical, Limiting, Threshold and Sustainable Flux Concepts Especially as Applied to Water Treatment

Over the last 15 years, copious amounts of membrane filtration data have been assessed using the concept of critical flux. This concept has been used in a number of different ways often without explicit redefinition and without reference to particular components in the feed. Now with regard to membrane filtration processes, fouling is the dominant limitation and so particular care should be taken with the vocabulary used as this will help to develop an improved fundamental understanding of membrane fouling. The paper will briefly distinguish between critical and limiting fluxes which will be non-contentious and then concentrate upon three questions: (i) What do we understand by a Sustainable Flux? (ii) Is it more useful to distinguish between low and high fouling rates rather than no fouling and fouling? (i.e. Threshold Flux versus Critical Flux?) (iii) Why, during flux stepping experiments, does the rate of TMP increase often decrease, even though flux has been kept constant?

Especially for membrane users dealing with industrial process streams, the concept of a threshold flux (which has evolved from critical flux thinking) is of a great utility. Above the threshold flux (dependent on hydrodynamics, feed conditions and process time), the rate of fouling soon becomes economically and environmentally unsustainable. The relationship between the Threshold Flux, which will be carefully defined, and the Sustainable Flux, as conventionally defined, will be given using data from membrane pilots treating water. With better fouling control, there is reduced chemical consumption, and lower overall costs through both improved operation and greater membrane lifetimes.

The paper will also consider the third question (if time permits) as it will help in our understanding of the fouling process and might influence future strategies of operation. The filtration behaviour of algae will be used as an example.