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Membrane Filtration Regulations and Determination of Log Removal Value 159
3. Determining the challenge particulate concentration.
4. Selecting a seeding method.
4.8.1. Test Solution Water Quality
While the LT2ESWTR does not stipulate any constraints on the design of the challenge
test solution, it is desirable to conduct the test in a manner that would be considered valid
under any anticipated conditions to which the product undergoing testing might be applied in
the field. Thus, the water quality of the test solution should be taken into consideration (3, 30).
In designing the test solution, it is important to consider that the primary objective of
challenge testing is to evaluate the removal efficiency of the challenge particulate during
filtration. While water quality may not have a significant impact on the removal efficiency,
particulate matter in the feed water can enhance removal of smaller contaminants. Thus, it is
generally accepted that a test solution matrix comprised of high-quality water provides the
most conservative estimate of removal efficiency. Note that challenge testing is not intended
to yield meaningful information regarding membrane productivity and fouling potential, and
thus, the use of a high-quality water matrix should not be considered inhibitory to testing
these measures of membrane performance that are unrelated to the primary objective of
challenge testing – to evaluate the removal efficiency of the challenge particulate. Productivity
and fouling are addressed using site-specific pilot testing and not on a product-specific
basis.
Some particular considerations regarding water quality characteristics and their implications
for challenge testing are as follows:
1. High-quality water or “particle–free water” with a low concentration of suspended solids (e.g.,
membrane filtrate) should be used as the matrix for the challenge solution, minimizing the
potential for formation of a fouling layer during the challenge test that would enhance removal
of the challenge particulate.
2. No oxidants, disinfectants, or other pretreatment chemicals should be added to the test solution
unless necessitated by process requirements (e.g., acid addition and/or scale inhibitor which may
be necessary with NF/RO processes).
3. If the challenge particulate is a molecular marker, the water quality of the matrix for the test
solution should not interfere with the introduction, dispersion, or measurement of the marker.
Thus, the impact of water quality parameters, such as pH and ionic strength, on the chemical
characteristics and speciation of molecular markers should be considered in the design of the test
solution. This is particularly critical for NF or RO membrane modules, which can concentrate
organic and inorganic solutes, potentially interfering with some molecular markers.
4. If a microbial challenge particulate is used, it may be necessary to add buffers or other materials to
maintain the viability of the organisms. Any additives used must not interfere with any aspect
of the test or result in a change in the concentration of the challenge particulate over the duration
of the test. In addition, because water quality parameters such as pH and ionic strength can affect
microbial aggregation, these solution characteristics should also be considered.
5. It is recommended that the challenge test solution be characterized with respect to basic water
quality parameters, such as pH, turbidity, temperature, total dissolved solids (TDS), TOC, and any
other water quality parameters that are critical to the test or interpretation of the results.