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through the RO unit (i.e., the amount of blowdown water produced). Specific guidelines
should be obtained from the RO manufacturer.
. Biological fouling
is a condition that must
prevented. Potentially, the RO unit membrane may be damaged when chlorine or other
oxidants are used. For proper operation, the water must be dechlorinated with a
reducing agent or with activated carbon before it contacts the RO membrane.
Membrane Configuration. There are three basic membrane
configurations: tubular, spiral-wound or scroll, and hollow fiber.
Tubular Configuration. The tubular configuration is simply a porous tube
supporting a membrane. Feedwater is introduced into the tube. Product water
permeates the membrane going to the outside of the tube. The reject water exits from
the far end of the tube.
Spiral Configuration. The spiral configuration is a sheet membrane that
is supported on each side by a porous material that provides flow distribution and rolled
into a spiral or "jelly roll" configuration. The membrane is put in a pressure vessel so
pressure can be maintained on its surface. This pressure forces the water through the
membrane, separating it from the impurities. The membrane is laminated between
porous sheets and sealed on three sides. The laminate is then attached on the fourth
side to a porous tube and rolled around the tube into an element. Feed solution is
forced into the element at one end, and the permeate works its way through the spiral to
the axis tube where it emerges as purified product.
Hollow Configuration. The hollow fiber configuration consists of small
(85-millimeter [3.3-inch] diameter) tubes whose outside wall is semi-permeable. A large
number of these tubes are placed in a shell, similar to a heat exchanger. Water, under
pressure, on the exterior of the tubes permeates the tubes and is collected from the
Sea Water. RO technology is particularly useful when feedwater is high in
dissolved solids or when the source is brackish water or seawater. When used ahead of
a deionizer, the chemical requirements for the deionizers are greatly reduced, resin life
is extended, and a smaller quantity of chemical regenerants is required.
Ultrafiltration. The term "ultrafiltration" describes a pressurized
membrane process in which particulate, colloidal, and high-molecular-weight dissolved
materials are filtered from the water. Ultrafiltration is a process that is similar to RO in
that a semi-permeable membrane is used to remove the filterable solids, except that the
membrane is more porous, thus allowing some water-dissolved minerals to pass
through with the product water. The feedwater flows through the inside of the fibers,
permeates through the membrane, and is removed as product from the shell side. The
filtered solids are continuously removed from the other end of the fiber in a reject stream
that typically contains 5 to 10% of the feedwater dissolved solids.