Page 209 - ro membanes

192 9. MEMBRANE FILTRATION
by excessive vacuum, the practical maximum TMP of submerged membrane pretreatment systems is usually limited to 0.7e0.8 bars (10e11 psi).
Depending on the strength of their membranes, pressure systems can operate at TMP of up to 2.5 bars (35 psi). The higher the durability and flexibility of the membrane fibers, the higher the maximum pressure the membrane fibers can handle. Based on this and other criteria the maximum TMP varies by the supplier.
9.2.1.3 Membrane Permeability
Similar to RO membranes, the third critical performance parameter associated with the filtration cycle is the membrane permeability. This parameter is defined as the ratio between the membrane flux and the TMP. Membrane permeability is measured in lmh/bar or gfd/psi (1 gfd/psi 1⁄4 25 lmh/bar). Most MF and UF membrane elements used for saline water pre- treatment operate at membrane permeability of 75e500 lmh/bar (5e20 gfd/psi).
9.2.2 Membrane Backwash
During processing mode, solids filtered out of the source water accumulate on the feed side of the membrane surface. These solids are periodically removed out of the filtration sys- tem by backwashing of the membranes with filtered water or concentrate. Backwash is usu- ally triggered by timer and occurs every 20e120 min for approximately 30e60 s. Backwash could also be initiated when the TMP reaches a certain maximum threshold, beyond which the membrane system cannot perform at target flux and filtered water quality. If the threshold TMP is exceeded, typically the membrane system production capacity (flux) is decreased, the filtered water quality deteriorates, and the membranes could be exposed to irreversible and sometimes permanent fouling. However, most often in practice the pretreat- ment membrane backwash is triggered by preset time. This time is adjusted based on the amount of solids in the source water and its fouling potential.
For saline source waters with high content of turbidity, silt, and dissolved organics, the filtra- tion cycle could be as short as 20e30 min. For source waters with very low organic and solids content such as these collected via intake wells or deep open intakes, the filtration cycle could extend beyond 60 min and backwash may not be needed for periods of several hours.
Membrane backwash is a multistep process, which for most commercially available UF and MF systems applies a combination of filtered water and air in a sequence and at rates designed to maximize the removal of particulates that have accumulated in the membrane system during the processing cycle. Backwash plays a very important role in the normal oper- ation of membrane systems because membranes have significantly smaller volume/capacity available to store solids within the filtration vessels than granular media filters. This smaller solid’s retention capacity of membrane systems is the main reason why membrane modules have to be backwashed 30e50 times more frequently than granular media filter cells (i.e., typically once every 30 min vs. one every 24 h).
Air and water backwash is mainly intended to remove particulates out of the membrane pretreatment system and does not involve the use of any cleaning chemicals. However, over time membrane surface would also accumulate organic deposits and biofilm. This type of membrane fouling is controlled by chemically enhanced backwash (CEB) also referred to as maintenance wash, which is typically practiced once or two times per day.