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156 N.K. Shammas and L.K. Wang
physical removal of Cryptosporidium (21–23). There are three general classifications of
surrogates (3):
1. Alternate microorganisms.
2. Inert particles.
3. Molecular markers.
It is important to note that not all of these classes of surrogates are appropriate for each
type of membrane filtration system, and it is critical that these be compatible for the purposes
of challenge testing. Generally, particulate surrogates such as alternate microorganisms and
inert particles are appropriate for MF, UF, and MCF systems, while molecular markers would
not be removed by these types of membranes. It may be necessary to use molecular makers
with NF and RO membrane systems that can remove dissolved substances and that are not
designed to accommodate large particulate concentrations. Some of the potential advantages
and disadvantages associated with each class are summarized in Table 4.3.
1. Alternate microorganisms: Numerous organisms that have a history of use in filter evaluation
studies are smaller than 1 mm (when monodispersed in solution), and these could be considered
conservative surrogates for Cryptosporidium. A number of these organisms and an appropriate
enumeration method are listed in Table 4.4, including both bacteria and viruses. Table 4.4 also
includes common surrogates for Giardia and enteric viruses. Serratia marcescens and Pseudomonas
diminuta have been widely used as surrogates within the membrane filtration industry, and
the use of MS2 bacteriophage has generally been accepted as a surrogate for enteric viruses, since
it is similar in size and shape to the poliovirus and hepatitis virus.
Although Bacillus subtilis has been used as a surrogate for Cryptosporidium for testing the
removal efficiencies of conventional treatment processes, it is not necessarily a suitable Cryptosporidium
surrogate for challenge testing membrane filtration devices. Because there is limited
data currently available regarding the use of B. subtilis in membrane challenge studies, a rigorous
characterization of this organism would be necessary to determine whether it could be used as a
Cryptosporidium surrogate for the purposes of challenge testing under the LT2ESWTR. Based on
Table 4.3
Comparative summary of Cryptosporidium and potential surrogates
Challenge particulate Size range Advantages Disadvantages
Cryptosporidium
parvum
Alternate
microorganisms
3–5 mm
0.01–1 mm
l No verification of
surrogate
required
l Low cost
l Easy to measure
l Accepted use
Inert particles