Membrane and Desalination Technologies - TCE Moodle Website

360 N.K. Shammas and L.K. Wang
and brackish waters indicating that there is approximately 1 psi of osmotic pressure for every
100 mg/L of TDS. In many cases the filtrate TDS concentration (TDSp) is small and can be
neglected.
While the flux associated with MF, UF, and MCF systems is typically referenced to a
temperature of 20 C for the purposes of assessing operational performance, it is common to
reference the flux associated with NF and RO systems to 25 C (298 K). Accordingly, the
appropriate TCF for NF and RO systems is shown in Eq. (15):
TCF ¼ exp U
1
T þ 273
1
298
; (15Þ
where TCF is the temperature correction factor, dimensionless, T is the water temperature,
C, U is the membrane-specific manufacturer-supplied constant, 1/ K.
Once the TCF has been determined, the flux normalized to 25 C can be calculated
according to Eq. (16):
J25 ¼ JTðTCFÞ; (16Þ
where J25 is the normalized flux at 25 C, gal/ft 2 /d, JT is the actual flux at temperature T,gal/ft 2 /d,
TCF is the temperature correction factor, dimensionless.
As with MF, UF, and MCF systems, it is important to note that the normalized flux (J25) for
NF and RO systems does not represent an actual operating condition. This term simply
represents what the flux would be at 25 C for the purposes of comparing membrane
performance independent of temperature-related affects. Similarly, it is also common to
normalize the flux for pressure in order to identify changes in productivity that are attributable
to fouling, as shown in Eq. (17):
M ¼ J25
; (17Þ
NDP
where M is the temperature- and pressure-normalized flux, gal/ft 2 /d/psi, J25 is the normalized
flux at 25 C, gal/ft 2 /d, and NDP is the net driving pressure, psi.
4.4. Hydraulic Configurations
The term hydraulic configuration is used to describe the manner in which the feed water
and associated suspended solids are processed by a membrane filtration system. Although
there are a number of different hydraulic configurations in which the various membrane
filtration systems can operate, each of these configurations can be categorized into one of two
basic modes of operation:
1. Deposition mode
2. Suspension mode
The hydraulic configuration of a system is determined from operational conditions such as
backwash, concentrate flow, and recycle flow, where applicable
One of the most important implications of a system’s hydraulic configuration is its impact
on the degree to which suspended solids are concentrated on the feed side of the membrane.