Bench‐scale evaluation of critical flux and TMP in low‐pressure membrane filtration

The biggest operational obstacle to the application of low-pressure membrane filtration is fouling, i.e., the reduction of flux or the increase in transmembrane pressure (TMP) during operation because of the accumulation of materials within the membrane pores or on the surface of the membrane. Fouling increases rapidly once a critical permeate flux has been exceeded. However, no standard protocols exist for measuring critical flux or TMP as a function of coagulation or changing water quality. Such a protocol, which could provide information analogous to that provided by jar testing in conventional water treatment, is necessary. This article describes bench-scale techniques that can be used to measure critical flux and determine the effects of changing water quality on membrane performance. The bench-scale test is also used to demonstrate the effectiveness of coagulation on the performance of low-pressure membrane systems. The concept of critical flux describes the maximum permeate flux that can be applied without rapid fouling of the membrane. Critical flux has been described in a theoretical fashion, but the testing procedure proposed in this article relies on an operational definition of critical flux: the highest flux for which there was only a small linear increase in TMP with time of filtration. In the experiments presented in this article, fluxes greater than the critical flux resulted in exponentially increasing TMP with filtered volume and were labeled super-critical. The measured value for critical flux decreased with increasing time of filtration (for each step in the critical flux test). An empirical equation was developed to allow prediction of critical flux for longer filtration times. The authors propose that similar bench-scale critical flux measurements should be used by utilities and consultants to assist in the design of low-pressure membrane systems, to modify operation during periods of changing water quality, and to determine the effects of coagulation or other treatments on performance of low-pressure membrane systems. The authors also propose that water utilities and consultants consider adopting bench-scale testing procedures for low-pressure filters, and it is recommended that results are evaluated using plots of average TMP versus flux, with different time steps and with extrapolation of results to longer time steps. This would provide a relatively conservative estimate of critical flux.-KD.