Membrane and Desalination Technologies - TCE Moodle Website

372 N.K. Shammas and L.K. Wang
membrane area over each of the 12 calendar months, along with its corresponding flow, is
applied to Eq. (1) to generate an associated flux. If this flux is less than the maximum
permitted value, then this largest of the 12 calculated values for membrane area represents
the design value for the system. However, if the resulting flux exceeds this threshold, the
design area must be increased to lower the flux to the maximum permitted value.
Note that use of Eq. (30) in this approach requires values for both the TMP and the total
membrane resistance, Rt, both of which should be considered constant for the purposes of
calculating the various values for membrane area. While a reasonable TMP can be easily
identified, appropriate values of Rt are more difficult to determine. The total membrane
resistance represents the sum of the intrinsic resistance of the membrane (which may be
considered a constant and can generally be obtained from the manufacturer) and the resistance
attributable to fouling at any given point during operation, as shown in Eq. (6):
Rt ¼ Rm þ Rf; (6Þ
where R t is the total membrane resistance, psi/gal/ft 2 /d/cp, R m is the intrinsic membrane
resistance, psi/gal/ft 2 /d/cp, and Rf is the resistance of the foulant layer, psi/gal/ft 2 /d/cp.
Because it is difficult to identify and justify a single specific value for the fouling resistance
for use with this approach, the contribution attributable to fouling may be ignored for
practical purposes. Thus, for the purpose of calculating the membrane area, the total resistance
used in Eq. (30) may be approximated by the membrane’s intrinsic resistance. This
approximation may be reasonable for the membrane system at the start of a filtration cycle
when the fouling resistance is minimal. Since the minimum value for membrane
resistance expected over a filtration cycle is used, the minimum TMP anticipated over a
filtration cycle should also be used. This minimum TMP occurs at the beginning of a filtration
cycle before gradually increasing until the system must be backwashed. Use of the intrinsic
membrane resistance and minimum TMP should result in a reasonable and not excessively
conservative estimate of the membrane area requirements. In rare cases in which the
membrane is experiencing significant fouling under conditions of peak demand such that
the flux and/or TMP are approaching their maximum values, the backwash and/or chemical
cleaning frequencies can be increased temporarily to compensate and keep fouling to a
minimum. Alternatively, standby membrane units may be used when necessary.
A MF/UF membrane manufacturer may have an alternate preferred method of determining
the design membrane area for a particular application based on temperature, membrane
material, or other site- or system-specific factors. It is recommended that the utility collaborate
with the state, the membrane manufacturer, and its engineer (if applicable) to select the
most appropriate method for determining the required area. Note that the addition of
membrane area to compensate for low-temperature flow will also help the system to meet
higher flow demands during warm weather without operating at an exceedingly high membrane
flux. Temperature compensation for MCF systems, if necessary, can be determined
using the methodology for MF/UF systems.
Because spiral-wound NF/RO membrane modules are designed to operate over a larger
range of TMPs than MF/UF modules, in NF/RO systems the TMP is simply increased to
maintain constant flux as the temperature of the feed water decreases. The required increase