Page 368 - Water and wastewater engineering

REVERSE OSMOSIS AND NANOFILTRATION 9-7

Source water
Membrane (cast on fabric backing)
Source water &
Porous permeate carrier Membrane leaf
flow spacer
Membrane (cast on fabric backing)
Source water & flow spacer
Processed water passes
through the membranes
Source water
on both sides of the porous
permeate carrier.
Source water
Concentrate Adapted from Hydranautics
Water Systems diagram

Permeate The permeate flows through the
water porous material in a spiral path
until it contacts and flows through
Concentrate
the holes in the permeate core tube.
Cutaway view of a spiral membrane module
FIGURE 9-4
Typical spiral-wound RO membrane element.
( Source: U.S. AID, 1980.)

The hollow-fiber element has several hundred thousand fibers with outside diameters on the
order of 0.085 mm suspended in a pressure vessel as shown in Figure 9-5 . Permeate recovery is
about 30 percent for each element.
The spiral-wound configuration is the most common for the production of drinking water
from groundwater and surface water. The hollow-fiber configuration is used extensively for
desalinization of seawater in the Middle East (Taylor and Wiesner, 1999).

Temperature Effects
Temperature affects water viscosity and the membrane material. In general, the permeate flow
increases as the temperature rises and the viscosity decreases. The relationship between membrane
material, temperature, and flux is specific to individual products. Correction factors should be
obtained from manufacturers (AWWA, 1999).

Service Life
Membrane fouling generally occurs by one of the following mechanisms (AWWA, 1999):

• Deposition of silt or other suspended solids.
• Inorganic scale deposits.
• Biological fouling.
• Interaction of organic constituents with the membrane.

Oxidation of the membrane from chlorination prior to ion exchange will significantly reduce
the service life. This is especially true for PA membranes. If prechlorination is essential, CA res-
ins are recommended (MWH, 2005).