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9.2 THE MEMBRANE FILTRATION PROCESS 191
only a portion of the source flow (typically 90%e95%) passes through the membranes, whereas the remaining flow (reject) travels along the feed side of the membranes and its movement along the membrane surface generates shearing velocity that evacuates the solids removed from the saline water out of the membrane.
Usually in a cross-flow mode, a portion of the reject stream is recirculated back to the feed system. Cross-flow pattern in such elements is similar to that in RO membrane elements. The key benefit of such flow pattern is that membranes can be operated continuously. The main problem with cross-flow elements, however, is that they have relatively lower packing den- sity that limits their productivity and requires significant energy expenditure to maintain flow tangential to the membrane surface. Therefore, the newest and most commonly used UF and MF membrane elements available on the market are designed to operate in a direct-flow configuration. Direct-flow membranes, however, cannot be operated continu- ously because the solids in the source water rejected by the membranes are accumulated on the membrane surface and have to be removed periodically via intermittent backwash.
The two most important membrane performance parameters associated with the filtration cycle of membrane pretreatment systems are membrane flux and trans-membrane pressure (TMP).
9.2.1.1 Membrane Flux
The membrane flux is the volume of filtered water produced by a unit of membrane area. This parameter is most commonly measured in liters per square meter per hour (L/m2 h) and also often referred to as Lmh or lmh, and gallons per day per square foot (gfd). The two flux measures relate as follows: 1.0 gfd 1⁄4 1.705 lmh. Typically, pretreatment systems of desalina- tion plants are designed for flux of 40e80 lmh (24e47 gfd; see Table 9.1).
Accumulation of solids on the surface of the membranes and in the membrane pores (membrane fouling rate) increases with the increase of the membrane flux. To maintain a reasonably long membrane cycle (30 min or more), the operational flux has to be selected such that the fouling rate on the membranes is reasonably low and within the time of one filtration cycle, the pressure loss created by the solids accumulated on the membranes stays below the maximum pressure that the membrane feed pumps are designed to deliver. Such flux is also referred to as a sustainable flux. Usually, the higher the solids content and fouling potential of the source water, the lower the sustainable flux for the same filtration cycle length.
9.2.1.2 Trans-membrane Pressure
TMP is the difference between the feed pressure and the filtrate pressure of the pretreat- ment system. This pressure drives the flow through the membranes and, therefore, it is directly related to the membrane flux. The TMP also has an impact on membrane fouling and filtration cycle length.
For most UF and MF membrane systems used for saline water pretreatment, the TMP is usually reported in bars or pounds per square inch (psi). Sometimes, TMP is reported in kilopascals (kPa) with 1 bar 1⁄4 100 kPa. Typically, pretreatment systems operate at TMP be- tween 0.2 and 1.0 bar (2.8e14.2 psi). Pressure-driven systems can operate at TMPs higher than 1 bar, whereas the theoretical maximum operating vacuum of the vacuum driven (sub- merged systems) is 1 bar. However, due to the potential for collapsing of the membrane fibers