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Membrane Filtration Regulations and Determination of Log Removal Value 167
Figure 4.4 shows a schematic of a pressure-driven apparatus operated in suspension mode
with continuous seeding and grab sampling. In systems that operate in suspension mode, the
concentration of suspended solids increases on the feed side of the membrane. While it may
not be practical to accurately replicate the solids concentration profile for a membrane system
in which the feed side concentration of suspended solids varies as a function of filtration time,
concentrate recirculation can produce conservative feed side conditions for the purpose of
challenge testing, assuming appropriate operating conditions for recovery and recycle ratio
are selected. Selection of an appropriate recovery for challenge testing can be complicated by
the fact that system recoveries can vary significantly in some cases (particularly for NF/RO
systems). The recycle ratio should be selected such that velocities across the membrane
surface are high enough to keep particles in suspension. The manufacturer can typically
recommend a minimum scour velocity for a cross-flow system.
For systems that utilize concentrate recycling, there are some additional considerations
that are important to be taken into account regarding the feed side system volume and the
location of the feed sample point. In general, larger feed side system volumes require longer
system equilibration times. For example, Fig. 4.4 shows the concentrate return location at the
feed tank rather than directly into the module feed line, thus increasing the effective feed side
system volume significantly. If such an arrangement is necessary (e.g., to provide an air break
in the recirculation system), then the feed tank volume should be minimized. In an apparatus
utilizing concentrate recycling, the feed sample point must be located upstream of the return
point, as shown in Fig. 4.4.
Figure 4.5 shows a typical vacuum-driven test apparatus operated in deposition mode with
continuous seeding and grab sampling. Although the module is immersed in a tank, the feed
water is not agitated, thus allowing particles to deposit on the membrane surface. With this
apparatus, the filtrate sampling point must be located downstream of the vacuum pump.
Figure 4.6 illustrates an apparatus for a vacuum-driven system operated in suspension
mode with continuous seeding and grab sampling. With this apparatus, the feed tank is
mechanically agitated to keep particles in suspension and can be modeled as a continuous
stirred tank reactor (CSTR). As with the vacuum-driven apparatus shown in Fig. 4.5, the
filtrate sampling point must be located downstream of the vacuum pump.
4.9.2. Test Operating Conditions
The design of a challenge test includes specifications for the following operating conditions:
flux, recovery, and hydraulic configuration. The LT2ESWTR requires the challenge test to be
conducted at the maximum design flux and recovery and that the test apparatus be operated
under representative or conservative hydraulic conditions. These requirements dictate the
operating conditions for the test apparatus during challenge testing. Note that under the
LT2ESWTR, recovery is defined as the volumetric percent of feed water that is converted to
filtrate in the treatment process over the course of an uninterrupted operating cycle (i.e.,
excluding losses that occur due to the use of filtrate in backwashing or cleaning operations).
Testing at the maximum recovery is important to ensure that the volumetric concentration
factor (VCF) simulated during challenge testing is representative of (or conservative for) fullscale
system operation. For systems that operate in deposition (i.e., direct flow or “dead-end”)