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MEMBRANE PROCESSES 13.5

rect treatment objectives. Source water quality should be compared with product water
quality goals to determine the degree of removal required for various constituents.
Removal may be by membrane process or associated pretreatment and posttreatment
processes.
Historical, current, and expected future water source quality data and product criteria
should be considered.
The following factors should be considered in membrane selection:
• Source water characteristics and availability
• Pretreatment and posttreatment requirements
• Product water quality and quantity requirements and blending options (for example,
split-flow treatment)
• Waste residuals disposal
• Need for bench or pilot testing
• Capital and operation and maintenance (O&M) costs
Table 13.2 provides a starting point for a preliminary determination of which mem-
brane processes, if any, apply to the treatment objectives. Membrane characterizations
conducted during bench and pilot tests may be necessary to determine which process will
provide the desired degree of removal (Bergman and Lozier, 1993).

Membrane Composition
Most membranes used in processes for municipal water treatment are prepared from syn-
thetic organic polymers. Pressure-driven RO and NF processes use either cellulosic or
noncellulosic membranes. Cellulosic types include cellulose acetate, cellulose acetate
blends, and cellulose triacetate. Noncellulosic types include polyamides, polyurea, sul-
fonated polysulfone, sulfonated polyfuran, polypiperazides, polyvinyl alcohol derivatives,
and other composites. MF and UF membranes use polyvinylidene fluoride (PVDF),
polypropylene, polyethersulfone, polysulfone, cellulose acetate derivative, polyacrylni-
trile, and other materials. Inorganic UF membranes, such as ceramic membranes with
an alumina barrier layer, are available but are used to a lesser extent than are organic
membranes.
ED and EDR membranes are essentially ion exchange resins in fiat-sheet form. These
are synthetic polymers consisting of either cross-linked sulfonated copolymers of vinyl
compounds (cation transfer type) or cross-linked copolymers of vinyl monomers with
quaternary ammonium anion exchange groups (anion transfer type).

Membrane Configurations
Most pressure-driven membranes for municipal water treatment are arranged in spiral-
wound and hollow-fiber configurations and, to a lesser extent, tubular and plate-and-frame
configuration.
Flat-sheet membranes for pressure-driven processes are most commonly assembled
into a spiral-wound element (module) in which multiple membrane "leaves," each com-
posed of two membrane sheets separated by a permeate carder, are connected to a cen-
tral permeate collector tube. A feed concentrate spacer is placed between each leaf, and
the leaves and spacers are rolled around the central permeate collection tube (Figure 13.4a).