Membranes - Principles of Chemical Separations with Environmental Applications - page 255

Pressure Pressure

(a) (b)

Temperature Temperature

(c) (d)

Concentration

Flow rate

(e) (f)

Figure 9.11 The effects of applied pressure, feed temperature, and water recovery
on membrane flux and product water quality of RO membranes [12].

due to membrane compaction combined with an increase in concentration polarization.
Concentration polarization will be described in detail later in the chapter.
RO is well-suited for use in treating secondary wastewater effluents, even though there
will be some organic fouling. Cellulose-acetate membranes are especially useful for this
purpose, since they can reject 90-99% of all salts and 90% of all organic material - all of
this can be accomplished in one unit.
Some pilot plants are studying the feasibility of RO for demineralization of seawater
and brackish water (Table 9.5), but the cost is very high. In seawater demineralization, 2
of the entire cost is put towards replacement of the membranes. Other applications are:
pretreatment of normal municipal water preceding ion exchange to make ultrapure water
for applications such as boiler feed; recovery of valuable or reusable materials from a
waste via the RO reject stream and reduction in the volume of waste, if required; and
water conservation or recovery such as the cooling tower blowdown.
The flux value of the membranes used in RO will decrease over their lifetime as the
pore passages decrease. This is a permanent and irreversible process, and the flux usually
exhibits an exponential decay. However, it is possible to maximize the useful lifetime