Membrane and Desalination Technologies - TCE Moodle Website

158 N.K. Shammas and L.K. Wang
monodispersed in solution might constitute a conservative surrogate for Cryptosporidium that
would not require direct verification.
3. Molecular markers: The suitability of molecular markers as surrogates for Cryptosporidium should
be considered on a case-by-case basis. While the justification for using microorganisms and inert
particles as surrogates for Cryptosporidium is more straightforward given that all are particulates,
molecular markers are dissolved substances that are fundamentally different from particulate
contaminants. As such, the removal mechanisms for molecular markers may be different than for
those associated with discrete particles in many cases. However, semipermeable membranes that
are capable of achieving very high removal efficiencies for dissolved substances may be capable of
achieving similar removal of particulates such as Cryptosporidium. In addition, porous membranes
with very fine pore sizes may be able to remove large macromolecules via mechanisms similar to
those that filter discrete particles. Thus, the use of molecular challenge particulates is permitted for
the purposes of challenge testing under the LT2ESWTR if the molecular marker used is determined
to be conservative for Cryptosporidium and is discretely quantifiable.
A variety of molecular markers have been historically used to characterize the pore size or
removal capabilities of membrane processes. For example, macromolecular protein compounds
are used to determine the molecular weight cut-off (MWCO) for many UF membranes.
In addition, fluorescent dyes such as Rhodamine WT and FDC Red #40 are used to
characterize NF and RO membranes. These substances have high spectrophotometric absorbance
characteristics that allow measurement and detection at the mg/L level (29). However,
these low molecular weight ( 500 Da) solutions could only be used with RO and less
permeable NF membranes. If molecular markers are considered for challenge testing, it is
desirable to use compounds that are more similar to discrete particles such as macromolecular
proteins. It is also recommended that a mass balance be conducted on the feed, filtrate, and
concentrate streams prior to challenge testing, to assess the efficacy of using a particular
molecular marker.
With some molecular markers, it may be difficult to demonstrate removal in excess of 3 log
unless sufficiently sensitive instrumentation is used. For challenge tests conducted with
molecular markers, the feed and filtrate concentrations are typically quantified in terms of
mass per unit volume. If the analytical method is specific for the molecular marker used in the
test, use of a mass-based concentration is acceptable since the mass of a known substance can
be related to moles, which is a discrete quantification. As is the case with any challenge
particulate, gross measurements cannot be used for the purpose of quantification. This
requirement would preclude the use of analytical techniques such as TOC monitoring and
conductivity monitoring in most cases (3).
4.8. Challenge Test Solutions
Generation of an appropriate challenge test solution is an essential component of an
effective test program. The purpose of the challenge test solution is to deliver the challenge
particulate to the module of interest under the established test conditions. The design of the
challenge test solution includes:
1. Establishing acceptable water quality of the solution.
2. Determining volume requirements.