A common concern about the novel CMF design was the integrity of the membrane after being coiled. In all our side-by-side comparison tests between the CMF and the SMF (Straight Membrane Filter), the 15-minute Silt Density Index (SDI 15, which is a widely accepted method for estimating the fouling capacity of water) of the permeate from the tested filters was constantly checked to ensure that the membranes were not compromised.  Despite that, a concern was raised that the significant improvement of the output (flux rate) from the CMF may be due to enlarged (stretched) pore sizes caused by coiling.

 

From a literature search, there was a report stating that fibers could be curved to as much as 20 times smaller than their diameter without causing any issue. That statement can only be used as a reference because some key material information for the fibers used in the study could not be confirmed. The typical curvature for the CMF modules from FSI is about 50 times to 80 times the diameter of the coiled fibers, which is likely to create less strain compared to the reference.

 

Looking at it from a different perspective, a 0.02-micron pore on a 3.5” diameter coil (which is a typical small CMF from FSI), is equivalent to placing a 9.4 ft bar (or a 10-ft 2 x 4 piece of lumber) on the surface of the Earth (Earth circumference is 40.075 million meters). It will lie flat on the Earth's surface and it will not warp, even if pressing down on both ends at the same time. Based on this same idea, the membrane pores of a CMF are not likely to be distorted/enlarged after coiling due to the enormous size ratio between the pores and the coil. This analogy will be true for most membranes in UF/MF applications.

 

In addition, the normalized flux (gfd/P, also called permeability coefficient) comparison between the CMF and SMF based on distilled feed water (which is commonly used for the QC of filters) can be used to verify the integrity of the CMF. The table below shows an example, in which the normalized flux of the CMF was lower than that of the SMF – typically 7 to 10% lower as discussed in the 29 May 2020 blog “Coiled Membrane Filter (CMF) vs. Straight Membrane Filter (SMF) Comparison Test”. This observation is one indicator that the CMF membrane is not compromised.