- Tubular membrane with crossflow / Flat sheet membrane in plate and frame module
The clean water permeability (or resistance) of MF and UF membrane is not a major factor affecting operating flux assuming it is well above the permeability required for the process. It is mainly because membrane resistance is only a small portion of the total filtration resistance in the equation shown here in practical situation. For example, cake layer resistance dominates the filtration resistance in tubular membrane process as shown in Fig. 1. Thus membrane permeability hardly affects both flux and operating pressure as discussed here in detail.
In addition, flux is self-limiting due to the increased particle deposition when flux is increased as illustrated in Fig. 2. The cake layer formed on membrane surface is easier to collapse at high flux to form denser cake layer, which eventually brings the flux back to the original. As a consequence, membranes with higher permeability do not necessarily have higher flux in field condition. Following are the explanations for each case.
Fig. 1. Breakdown of filtration resistance in tubular membrane process assuming inlet pressure 5.5 bar, outlet pressure 0.5 bar, average flux 100 LMH, and membrane flux 1,000 LMH at 1 bar.
Fig. 2. Self-limiting behavior of flux at a constant flow rate and channel configuration. Flux can be increased by increasing TMP or by increasing membrane permeability hypothetically.
- Immersed (or submerged) membrane
The self-limiting behavior of flux explained above applies for immersed membranes. Therefore, membrane permeability increase does not result in higher flux in theory if it is already above the level required for process water filtration.
In hollow fiber membrane, some degree of imbalanced filtration along the fiber is inevitable because TMP is not identical in all the segments of the fiber due to the internal pressure loss. The imbalanced filtration expedites the permeability loss near the fiber exit by increasing particle deposition. The loss of flux near the fiber exit must be compensated by the increased flux upstream. The propagation of membrane fouling occurs until TMP, flux, and the extent of cake layer formation are balanced among all segments of the membrane fiber. Increased permeability only makes TMP, flux, and extent of cake layer formation profiles along the fiber more dramatic.
Unlike MF and RO, membrane permeability is an important factor affecting the energy efficiency of RO membrane. In normal operating condition, a large portion of the driving force (TMP) is used to overcome membrane resistance. For example, if flux at 15 bar was 35 LMH with a new membrane, the operating flux with feed water can be 25 LMH at the same TMP in surface water filtration. In this case the resistance caused by membrane itself takes ~70% of the total resistance. Therefore, by increasing membrane permeability, same flux can be obtained at lower TMP. However, due to the self-limiting nature of flux, higher membrane permeability does not necessarily mean higher flux, if all other factors remain same.
On the contrary, membrane resistance takes only a small portion of the total resistance in seawater desalination. Majority of the TMP is required to overcome osmotic pressure. Therefore, the savings expected by more permeable membrane is not as large as in surface water filtration.
© Seong Hoon Yoon