Figure 7-2. Revere osmosis principles.
(3) Hollow fine fibers. A large research and
If product water is not supplied, then the osmotic
suction, if the feed water side of the membrane is
development effort made it possible to coat minute
depressurized, will draw air back into the membranes
hollow fibers, smaller in diameter than a human hair,
and dehydrate them. Usually, the required volume of
with semipermeable membrane material. This reverse
product water is supplied by a suck-back tank, which
osmosis membrane configuration is known as the hollow
maintains a minimum volume and back pressure on the
fine-fiber configuration. Packing densities with hollow
reverse osmosis membranes.
fine fibers have exceeded 4,900 square feet of surface
(1) Cellulose acetates. This material suffers
area per cubic foot of volume. See figure 7-7.
d. Membrane materials. There area number of
from slow chemical decomposition through a process
called hydrolysis. The use of acids to prevent scaling
successful reverse osmosis membrane materials.
increases the rate of this form of membrane decay.
Currently, two principal types of membrane materials
Cellulose acetates are also biodegradable and must be
are being used: cellulose acetate and polyaromatic
protected from bacterial attack. One of the important
amide. Both materials are destroyed by dehydration of
advantages of cellulose acetate is its resistance to
the membranes. To avoid dehydration, product water
attack by chlorine.
must be supplied to allow osmotic water to flow back
through the membranes in order to dilute the feed water
to approximate product water concentrations.