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Membrane Filtration Regulations and Determination of Log Removal Value 169
processed to generate an effective recovery of at least 90%. For MF/UF systems that operate
with a concentrate waste stream, it is recommended that a recovery of at least 75% be utilized
for challenge testing unless a more representative system recovery can be demonstrated by
the manufacturer; a recovery of 100% would represent the most conservative case. NF/RO
systems, for which the utilization of a concentrate waste stream is standard, should be
operated at a recovery of at least 45% for the purposes of challenge testing, which is
representative of a flow-weighted average recovery for a module in such a system operating
at an overall recovery of greater than 90% for a single stage. If challenge testing is conducted
on a small-scale NF/RO system (such as one using a single module), then concentrate
recycling should be used to increase the recovery to at least 45%.
The membrane area is also typically given in the specifications, and the maximum filtrate
flow can be calculated by multiplying the membrane area and the maximum flux (31–33).
Although the LT2ESWTR stipulates specific requirements for challenge testing flux, the
filtrate flow is also necessary for designing the challenge test solution. Table 4.6 summarizes
some typical specifications for membrane area and maximum flux associated with various
types of membrane modules, as well as the corresponding filtrate flow. For the purposes of
challenge testing, the membrane area exposed to the feed (i.e., as opposed to that to the
filtrate) should be used in all calculations. Note that the values listed in Table 4.6 are
examples only and that particular product specifications will vary with module manufacturer.
A challenge test should be designed to simulate the hydraulic configuration of full-scale
system operation, since it affects the concentration of suspended solids on the feed side of the
membrane and thus the removal efficiency of the process. A membrane filtration system can
be operated in either suspension or deposition mode. While it is relatively straightforward to
simulate a system operating in deposition mode during a challenge test, it may not be possible
to simulate all variations of suspension mode operation using a single-module challenge test
apparatus. For example, it is not practical to simulate a plug flow reactor (PFR) configuration
typical of full-scale RO systems. In such cases, the challenge test apparatus should be
designed and operated as a CSTR, since a CSTR configuration generally results in the highest
concentration of suspended solids on the feed side of the membrane. If the challenge test can
successfully demonstrate the target LRV with a higher concentration of suspended solids on
the feed side of the membrane than expected with the full-scale system, then the use of a
Table 4.6
Typical parameters for various types of modules
Module Example membrane
area [ft 2 (m 2 )]
Example maximum flux
[gal/ft 2 /d (L/m 2 /h)]
Filtrate flow
[gpm (L/min)]
Cartridge filter 5 (0.46) 1,364 (2,316) 4.8 (18.2)
Spiral-wound 4" diameter 75 (7.0) 17.8 (30.2) 0.9 (3.4)
8" diameter 350 (32.5) 17.8 (30.2) 4.3 (16.3)
Hollow-fiber Outside-in 350 (32.5) 53.5 (90.8) 13.1 (49.6)
Inside-out 1,400 (130) 107 (182) 103 (390)