Biology Reference
In-Depth Information
Description of the Three Main Field Methods for Sampling
Three main field methods for sampling are commonly differentiated: in situ;
on-line; and, off-line15: 15 :
1. In situ techniques require the use of sensors placed in the water body with no
sampling step;
2. On-line methods involve a sampling step, followed generally by pretreat-
ment and measurement; all these steps could be automated; and
3. Off-line methods generally require a conventional spot-sampling step,
followed by an analytical method that is faster and/or less expensive than com-
monly used laboratory methods.
Adapted from Ref.15.
Periodic manual grab sampling is considered a relatively inefficient
methodology for capturing mean concentrations of water subjected to
highly variable loads. 15 The alternative is continuous, or on-line, monitor-
ing. There are some examples of this for chemical monitoring of water
quality, with fewer examples for microbiological testing. 16 One example
is the PREDECT system, developed by a Swedish company. The technol-
ogy is based on laser scanning of the water to detect particles as they pass
by, triggering capture of a sample if pathogens presence are suspected, and
is designed to be located on the pipeline after the water treatment process.
There are two different types of collection regimes. The first and most
commonly used is time controlled collection, where samples are taken at
regular time intervals, e.g. every 24 h. The second approach, which might be
more suitable during heavy rainfall, or flood events, is flow controlled collec-
tion where samples are taken at regular intervals, according to the flow rate.
If, in the case of outbreak investigations, the time of sampling coincides
with the peak of contamination, small volumes up to 1 L may be adequate
for the detection of the causative agent(s). For quality control or risk assess-
ment purposes, larger volumes may be required as many kinds of human
pathogens (in particular, human viruses) exist at different levels in the envi-
ronment where they can retain their infectivity for a long duration and be
transmitted by the water route.
In general, the first approach in this box is not applicable for waterborne
pathogens, especially those present at low concentrations. Therefore, for both
field testing and laboratory analysis, sample processing of the collected sample
is essential to concentrate it to a level where detection can be satisfactorily
performed. Different concentration techniques and how they can be applied
to the various classes of waterborne pathogen are discussed in the next section.
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