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Membrane Filtration Regulations and Determination of Log Removal Value 165
a manufacturer may maintain a special test apparatus to check individual modules as a
component of its QA/QC program. Such an apparatus may be suitable for conducting
challenge testing and typically includes equipment such as pumps, valves, instrumentation,
and controls necessary to evaluate full-scale modules. This same type of equipment would be
used in the design of systems for testing small-scale modules.
Both the seeding and sampling methods selected for challenge testing, as well as the
hydraulic configuration of the system, affect the design of the test apparatus. Batch seeding
requires a feed tank and mixing equipment, while continuous seeding requires a stock
solution reservoir, chemical metering pump, and in-line mixers. Sampling requirements
may dictate the location and design of sample taps in the system. In addition, the test
apparatus should be designed to mimic the hydraulic configuration of the full-scale system
as much as practical; however, the test apparatus may alternatively utilize a more conservative
recovery than the full-scale system. If 100% recovery (i.e., the most conservative
scenario) is used, a cross-flow system must operate without a bleed stream such that all of
the concentrate is recirculated, and a deposition mode system must filter the entire test
solution volume. Note that a full-scale cross-flow system could not operate at 100% recovery
on a sustained basis, since the feed would become increasingly concentrated. However,
operation at 100% recovery is feasible for a short-term challenge test in the interest of
generating conservative results. The test apparatus should allow the membrane module to
undergo direct integrity testing both before and after the challenge test. Figure 4.2 through
Fig. 4.6 are schematic representations of typical apparatuses for challenge testing under
various conditions. Note that ancillary equipment and operational processes (e.g., backwash,
chemical cleaning, and integrity testing) are not shown.
Figure 4.2 illustrates a pressure-driven apparatus operating in deposition mode with batch
seeding and composite sampling. This type of system may be well suited for a MCF or other
membrane module with limited surface area. With this apparatus, the test solution is prepared
as a batch and a composite filtrate sample would be generated, yielding a single data pair
(i.e., a feed sample and a composite filtrate sample) for the purposes of calculating the log
removal efficiency for the challenge particulate.
Fig. 4.2. Schematic of a typical pressure-driven system in deposition mode with batch seeding and
composite sampling.