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9.4 FILTER TYPES AND CONFIGURATIONS 197
Tubular membranes have the following advantages: (1) large-channel diameters allow them to treat waters of higher solid content compared to hollow-fiber membranes operating in an inside-out mode (this advantage is not significant if an outside-in hollow-fiber mem- brane is compared with inside-out tubular membrane); (2) these membranes can be operated at approximately two times higher cross-flow velocity, which is beneficial in terms of biofouling control.
9.3.3 Service Facilities and Equipment
All membrane pretreatment systems have three types of service support facilities and equipment: (1) backwash system, (2) CIP system, and (3) cleaning chemical feed system. The backwash system typically includes filtered water storage tank, backwash pumps, and (depending on the membrane type) air compressors for air backwash. Some membrane pre- treatment systems (i.e., Pentair’s, Seaguard) use only water backwash.
The CIP system for the membrane pretreatment facility has very similar configuration to that of the RO CIP system. Sometimes, the same CIP system is used for both pretreatment and RO membrane cleaning, which is not desirable, especially for larger desalination plants because of the added operational complexity. The chemical feed system usually includes acid, base, sodium hypochlorite, and sodium bisulfite storage and feed systems to service the CEB and CIP membrane cleaning activities. In addition, some of the membrane pretreatment sys- tems are designed for enhanced performance by addition of conditioning chemicals to the feed water (coagulants, flocculants, powdered activated carbon, and acid). However, most sa- line water pretreatment systems are designed to operate without source water conditioning.
9.4 FILTER TYPES AND CONFIGURATIONS
Depending on the type of the driving filtration force, membrane pretreatment filters are divided in two categoriesdpressure driven (pressurized) and vacuum driven (submerged).
9.4.1 Pressurized Membrane Systems
Pressurized UF and MF systems consist of membrane elements installed in pressure ves- sels, which are grouped in racks (trains), similar to these of RO systems (see Fig. 9.3). At pre- sent, practically all key membrane suppliers offer pressurized UF and MF systems. Depending on the direction of the feed flow through the membranes, the pressure-driven sys- tems are divided into outside-in [also referred to as pressure-driven outside (PDO) feed] sys- tems and inside-out [or pressure-driven inside (PDI) feed]. In PDO systems, the feed source water is distributed around the filter fibers and after passing through the membranes, the filtered water is collected through the fiber lumen. In PDI systems, the source water is fed into the filter lumens and is collected on the outside of the fibers.
In general, PDO systems are more difficult to clean only with water backwash and require air backwash to achieve the same level of productivity recovery as PDI. In addition, PDO either operate at higher feed pressures for the same design flux as PDI systems or at lower fluxes at the same design TMP.