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Membrane Filtration Regulations and Determination of Log Removal Value 155
Cryptosporidium is approximately 3 mm, and thus, a conservative surrogate might be the one in
which 99% of particles have a diameter of 1 mm or less.
Generally, it is desirable to use a surrogate that is of the same shape as the target organism. In
the case of Cryptosporidium, an appropriate surrogate would have a spherical shape, although in
some cases a nonspherical surrogate might be considered. If a nonspherical surrogate is used, it is
recommended that the smallest dimension be considered as the effective size since particles can
interact with a membrane barrier at any orientation.
Another consideration is the surface structure of the proposed surrogate. A particle that has a
highly irregular surface structure may be removed more efficiently than a similarly sized particle
that has a smooth surface. While it may be difficult to completely characterize the surface of a
potential surrogate, those with rough surfaces that are known to exhibit a high degree of adherence
may be removed through mechanisms other than size exclusion, and thus may not provide a
conservative estimate of removal efficiency.
The manner in which the surrogate disperses in the challenge test solution has a significant
impact on the effective size and shape of the challenge particulate. Some may agglomerate or
become attached to other particles while in solution which would yield larger effective particle
sizes. For example, organisms such as Staphylococci exist as clumps and Streptococci exist as
chains. In its aggregate form, each of these organisms is too large to be considered conservative
surrogates for Cryptosporidium. Surface structure also impacts the tendency of particles to
agglomerate, and in general, particles with a smooth surface are more likely to be monodispersed
in solution.
2. Particle surface charge: A conservative challenge particulate should have a neutral surface
charge, since charged particles may interact with other particles and surfaces, thus enhancing
removal. The solution pH can also affect the charge of some surrogates and thus should be
considered in the preparation of a test solution. If there is a concern regarding the charge of the
surrogate such that mechanisms of particle retention other than size exclusion may be responsible
for surrogate removal in a MF or UF system, a nonionic surfactant could be used in the challenge
test solution to significantly reduce the impact of charge-related removal mechanisms.
3. Ease of use and measurement: Although factors such as ease of handling and measurement are not
critical in determining the appropriateness of a surrogate, these nevertheless may be important
factors to consider. Handling the surrogate could expose personnel to the challenge particulate,
and thus, the surrogate should be selected to minimize unacceptable risk to the technicians
conducting the test. The material should also be easy to work with, in a dose-accurate way,
since repeated tests may be conducted in which reproducibility is desirable. Surrogates that could
degrade during the test, resulting in an inconsistent challenge concentration, should be avoided.
It is also desirable to use a surrogate that is easy to enumerate through established analytical
techniques. Furthermore, the LT2ESWTR requires that the concentration of challenge particles
determined using a discrete measuring technique such that gross measurements such as turbidity
are not acceptable.
4. Cost: The cost of seeding and analysis may preclude the use of some surrogates. Both the cost of
the surrogate itself and cost of the required analytical techniques should be considered, as well as
any other miscellaneous costs associated with the surrogate.
4.7.3. Surrogates for Cryptosporidium
In the absence of an acceptable surrogate, formalin- or heat-fixed Cryptosporidium
parvum could be used as the challenge particulate for compliance with the requirements
of the LT2ESWTR. However, the rule does permit surrogates for the purpose of challenge
testing, and several different surrogates have been successfully used in studies evaluating