Membrane and Desalination Technologies - TCE Moodle Website

370 N.K. Shammas and L.K. Wang
oxidants may cause the precipitation of iron or manganese salts (either unintentionally or by
design as a pretreatment process), which could accelerate membrane fouling.
5.3. Temperature Compensation
Like other water quality parameters such as turbidity and TDS (for NF/RO systems), the
temperature of the feed water also affects the flux of a membrane filtration system. At lower
temperatures water becomes increasingly viscous; thus, lower temperatures reduce the flux
across the membrane at constant TMP or require an increase in pressure to maintain constant
flux. The means of compensating for this phenomenon varies with the type of membrane
filtration system used. General viscosity-based means of compensating for temperature
fluctuations for both MF/UF and NF/RO systems are described below, although membrane
manufacturers may have a preferred product-specific approach.
MF/UF membrane systems usually operate within a relatively narrow range of TMPs,
which may limit increasing the TMP in order to maintain constant flux as the water
temperature decreases. Because the membrane modules can be damaged if the TMP exceeds
an upper limit, as specified by the manufacturer, it may not be possible to operate the system
at a TMP that is sufficient to meet the required treated water production during colder months
if demand remains high. As a result, additional treatment capacity (i.e., membrane area or
number of membrane modules) is incorporated into the design of the system such that the
water treatment production requirements can be satisfied throughout the year.
For the microporous MF/UF membranes, the relationship between flux, TMP, and water
viscosity is given by Eq. (7):
JT ¼ TMP
; (7Þ
RtðmTÞ where
JT = flux, gal/ft 2 /d
TMP = trans-membrane pressure, psi
Rt = total membrane resistance, psi/gal/ft 2 /d/cp
mT = viscosity of water, cp
If the system is operated at constant flux, then increases in viscosity require proportional
increases in operating TMP (assuming constant membrane resistance). However, once the
TMP approaches the rated maximum for the membranes, further increases in viscosity
necessitate a reduction in flux. Thus, in order to maintain the required filtered water production
flow (so as to satisfy customer demand), the membrane area must increase in proportion
to the flux decrease, as was shown in Eq. (1):
J ¼ Qp
; (1Þ
Am
where J is the flux, gal/ft 2 /d, Qp is the filtrate flow, gpd, and Am is the membrane surface area, ft 2 .