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170 N.K. Shammas and L.K. Wang
CSTR in place of a PFR configuration would represent more conservative, and thus acceptable
challenge test conditions. A single RO module can be operated as a CSTR by including a
concentrate recycling loop, as shown in Fig. 4.4.
4.10. Sampling
Although the LT2ESWTR does not stipulate any particular requirements for sampling, it is
an important component of a challenge test. An effective sampling program is dependent on
the development of detailed and thoroughly documented sampling plan, as well as the
selection of appropriate sampling methods and locations. These critical aspects of sampling,
along with the monitoring of operational parameters during the execution of the challenge
test, are discussed in the following subsections (3).
4.10.1. Sampling Methods
The two most common approaches for sampling are the grab and composite methods. Grab
sampling involves the collection of one or more aliquots from the feed or filtrate stream,
while composite sampling involves collection of the entire process stream for processing and
subsequent analysis. The concentration of challenge particulates in the feed solution is
typically characterized through grab sampling, while the filtrate stream may be sampled
using either grab or composite sampling. If grab sampling is used for both the feed and filtrate
streams, the number of feed and filtrate samples does not need to be equivalent, and samples
can be collected on different schedules during the challenge test. In many cases, it may be
advantageous to collect more filtrate samples than feed samples, since filtrate concentrations
are expected to be very low, and an error of just a few particles in a filtrate sample can have a
significant impact on the demonstrated removal efficiency. Moreover, if batch seeding is
used, the feed concentration should not vary significantly over the course of challenge testing,
assuming appropriate feed stock mixing. However, if continuous seeding is used, paired
sampling may be preferred for simplicity of data reduction.
Grab sampling typically involves the collection of a predetermined volume of water in an
appropriate collection vessel at predetermined times, as documented in the sampling plan. In
some cases, a composite sample is collected directly into a sampling vessel; however, it is
more common to capture the challenge particulate during composite sampling. A composite
sample is usually compiled by passing the entire filtrate stream through an absolute filter
capable of 100% capture of the challenge particulate. The challenge particulate would then be
enumerated either directly from the filter media or removed from the filter for subsequent
analysis. Grab samples typically do not require this type of processing in the field, and any
extraction or concentration steps for grab samples are typically conducted in the laboratory.
It is important that good sampling practices be employed during challenge testing, such as
flushing taps prior to sample collection (if applicable) and isolating filtrate sampling locations
from feed sampling locations to prevent cross-contamination. Furthermore, appropriate
QA/QC measures should be implemented during sampling, such as collection of duplicate
samples and blanks.