Membrane and Desalination Technologies - TCE Moodle Website

152 N.K. Shammas and L.K. Wang
4.5. Selection of Modules for Challenge Testing
The intent of challenge testing under the LT2ESWTR is to characterize the removal
efficiency of a specific membrane product without requiring challenge testing for all production
modules. In addition, the rule does not specify a particular number of modules that are
required to undergo challenge testing to demonstrate Cryptosporidium removal efficiency.
However, because it is important that manufacturing variability in the product line be
considered in the development of an appropriate challenge test, the number of modules
subject to challenge testing, as well as the particular modules chosen, should be carefully
selected on a rational and scientific basis. Although manufacturers or independent testing
organizations may develop any number of different procedures for module selection, two
common approaches are discussed as illustrative examples (3):
1. Selection of modules based on previously collected QC data for the product line.
2. Random sampling of membrane modules from several manufactured lots according to a statistical
sample design.
Use of the first approach listed above is predicated on the existence of significant QC data
for the product line accumulated over time by the manufacturer. Since manufacturers typically
conduct some kind of NDPT on all modules produced to ensure quality and characterize the
variability of a product line independent of the link established between nondestructive
performance testing and challenge testing under the LT2ESWTR, such data should generally
be available. Because the modules subject to challenge testing will be subsequently recharacterized
with the NDPT to establish an acceptable QCRV required for all modules to be
eligible for Cryptosporidium removal credit under the LT2ESWTR, it may be most advantageous
for a manufacturer to select modules for challenge testing that are near the lower end of
the statistical distribution of acceptable (i.e., under the manufacturer’s in-house QC procedures)
NDPT results based on historical data. If these tested modules yield a QCRV that is
consistent with the resolution requirement of the rule, then it is likely that the majority of
production modules will also meet the established QCRV and thus be eligible for Cryptosporidium
removal credit. Using this approach, the number of modules selected for challenge
testing is generally at the discretion of the manufacturer or independent testing organization.
If historical QC data for the product line is not available from the manufacturer, the second
approach listed above may represent an appropriate option. This method involves the
evaluation of a statistically significant random sample of modules from a number of production
lots. Because the modules at the lower end of the QC data are not artificially selected (as
with the first approach described above), it is likely that this method will result in a higher
QCRV, resulting in a somewhat higher rejection rate of modules eligible for Cryptosporidium
removal credit under LT2ESWTR. This number of modules selected for challenge testing
using this approach will likely be dictated by the particular statistical sampling technique
used. Either of these two approaches or other rationale approach developed by the manufacturer
or independent testing organization could be utilized to select modules for challenge
testing. Regardless of the method used, it is suggested that at least five membrane modules
from different manufactured lots be evaluated during a challenge test.