Critical, sustainable and threshold fluxes for membrane filtration with water industry applications

Critical flux theory evolved as a description of the upper bound in the operating envelope for controlled steady state environments such as cross-flow systems. However, in the application of UF membranes in the water industry, dead-end (direct-flow) designs are used. Direct-flow is a pseudo steady state operation with different fouling characteristics to cross-flow, and thus the critical flux concept has limited applicability. After a review of recent usage of the critical flux theory, an alternative concept for providing design guidelines for direct-flow systems namely that of the threshold flux is introduced. The concept of threshold flux can also be applicable to cross-flow systems. In more general terms the threshold flux can be taken to be the flux that divides a low fouling region from a high fouling region. This may be linked both to the critical flux concept and to the concept of a sustainable flux. The sustainable flux is the one at which a modest degree of fouling occurs, providing a compromise between capital expenditure (which is reduced by using high flux) and operating costs (which are reduced by restricting the fouling rate). Whilst the threshold flux can potentially be linked to physical phenomena alone, the sustainable flux also depends upon economic factors and is thus of a different nature to the critical and threshold fluxes. This distinction will be illustrated using some MBR data. Additionally the utility of the concept of a threshold flux will be illustrated using pilot plant data obtained for UF treatment of four sources of water.