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Membrane Systems Planning and Design 373
in TMP at the temperature of interest relative to that at a given reference temperature
(typically 25 C for NF/RO systems) is dependent on the specific proprietary membrane
used and can be calculated by means of a TCF, as shown in Eq. (32):
1 1
TCF ¼ exp U þ ; (32Þ
T þ 273 298
where TCF is the temperature correction factor, dimensionless, T is the water temperature,
C, and U is the membrane-specific manufacturer-supplied constant, 1/ K.
Alternatively, many NF/RO membrane manufacturers may supply tables specifying TCF
values over a range of temperatures for a given membrane. Once the appropriate TCF is
known, the required TMP at temperature T of interest can be calculated by dividing the TMP
at the reference (i.e., design) temperature (commonly 25 C for NF/RO systems) by the TCF
value, as shown in Eq. (33):
TMPT ¼ TMP25
; (33Þ
TCF
where TMPT is the trans-membrane pressure (temperature-corrected), psi, TMP25 is the
trans-membrane pressure at 25 C reference temperature, psi, and TCF is the temperature
correction factor, dimensionless.
Note that Eqs. (6) and (32) also allow the calculation of the TMP reduction resulting from
decreased water viscosity at higher feed water temperatures during warmer months of the
year.
5.4. Cross-Connection Control
In the context of membrane filtration systems, cross-connection control measures are
implemented to prevent chemicals from the cleaning process from contaminating the feed
or filtrate streams. States may have particular requirements for cross-connection control,
although in general there are two strategies that are commonly used: a double block and bleed
valving arrangement or a removable spool. These strategies are applicable to both MF/UF and
NF/RO systems; however, because MCF systems typically utilize modules that are disposable
cartridges, these systems are not usually subject to chemical cleaning and thus crossconnection
control measure are generally unnecessary for these systems.
The double block and bleed valve arrangement is the most common method of crossconnection
control for large membrane filtration systems. A schematic illustrating this
method is shown in Fig. 8.5. In summary, two isolation valves (V-1A and V-2A) are placed
in the feed line to isolate it from the cleaning chemicals. These are the block and bleed valves,
respectively. During the cleaning process, valve V-4A and V-6A are opened to bring cleaning
solution(s) into membrane rack (i.e., unit) A. Although valve V-3A is kept closed during this
operation, if it were to leak it would allow chemicals to pass into the common feed manifold,
causing contamination. In order to prevent this potential contamination, valve V-1A is also
closed and valve V-2A is opened. With this configuration, if valve V-3A leaks, the cleaning
solution(s) simply flows through valve V-2A to waste. A similar block and bleed valve