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178 N.K. Shammas and L.K. Wang
4. Challenge testing using a poor-quality feed water matrix: Although it is recommended that
challenge testing be conducted using clean make-up water for the test solution and unfouled
membranes to provide a conservative estimate of removal efficiency, prior studies conducted with
waters with a greater fouling potential (e.g., untreated surface waters) can potentially be considered
to meet the LT2ESWTR challenge test requirement, since the removal of Cryptosporidiumsized
particles is generally unaffected by the presence of foulants for membrane filtration
processes as defined under the LT2ESWTR. However, recent research suggests that integrity
defects on the order of 200 mm can be obscured by foulants in some cases, improving pathogen
rejection (29, 39). Thus, prior studies conducted with poorer quality water should be considered
for grandfathering on a case-by-case basis.
4.14. Summary of the US EPA Required Challenge Testing
The Cryptosporidium log removal that a membrane product is capable of achieving is
determined via challenge testing. Thus, the objective of challenge testing is to demonstrate
Cryptosporidium removal efficiency. Challenge testing is intended to be a one-time, productspecific
test to establish the maximum Cryptosporidium log removal credit that the product is
eligible to receive as applied for LT2ESWTR compliance (subject to state approval);
however, the demonstrated sensitivity of the site- and system-specific direct integrity test
utilized may limit the maximum log removal credit that can be awarded. Retesting for a
specific membrane product may be required if the manufacturer makes significant changes to
the product. Guidance to assist membrane manufacturers and state regulators with assessing
what types of changes may require retesting is provided in the MFGM (3). A general
overview of challenge testing under the LT2ESWTR is provided in Table 4.8.
Challenge testing involves: (a) seeding the feed water with Cryptosporidium or an acceptable
surrogate (i.e., a “challenge particulate”), (b) determining the maximum feed concentration
using Eq. (1), and (c) measuring the log reduction in the concentration of the challenge
particulate between the feed and filtrate, as shown in Eq. (2).
The rule requirements associated with challenge testing are summarized in Table 4.9.
A detailed explanation of these requirements, along with guidance for compliance, is
provided in the MFGM (3).
Table 4.8
General overview of challenge testing (Source: US EPA)
Description One-time, product-specific test event designed to demonstrate Cryptosporidium
removal ability
Purpose Demonstrate Cryptosporidium removal efficiency of an integral membrane product
and establish the maximum removal credit that product is eligible to receive
Applicability Membrane product
Frequency Once