Biology Reference
In-Depth Information
4.3. CONCENTRATION TECHNIQUES
4.3.1. Ultrafiltration
In ultrafiltration, size-exclusion filters with a pore size in the range of
10-70 kDa are used. Due to the small pore size, all types of microorganisms
including viruses, but also macromolecules such as proteins, are retained in
the filter. Most commonly, the filters consist of hollow fibers made from
materials such as polysulfone, cellulose acetate, or polyacrylonitrile and with
surface areas of several square meter, which enable filtration of large vol-
umes also when turbid surface waters are sampled. As the water sample is
pumped into the fibers, counter pressure forces water and particles/mol-
ecules smaller than the pore size through the membrane (permeate or fil-
trate) while microorganisms are retained inside the fibers (retentate). These
are collected directly, or by elution or back-flushing of the filter, after fil-
tration is complete. The volume of the eluate will differ depending on the
inner volume of filter and elution technique, but usually secondary concen-
tration is required before detection of the microorganisms.
Two different setups are used: (1) tangential flow or recirculating filtration
and (2) dead-end ultrafiltration (DEUF) (
Fig. 4.2
). In tangential flow filtra-
tion, the water sample is continually circulated through the filter until the
volume has been reduced to a few 100 mL, which is collected with or with-
out an elution step. This setup requires a pump which means that the water
sample has to be collected in containers and either brought to the laboratory
Figure 4.2
Image of filtration set up. (a) Tangential flow ultrafiltration where the water
is recirculated through the filter while the tightened screw clamp forces a part of water
to cross the filter and out through the upper port. (b) Dead-end ultrafiltration. The
arrows show the direction of the water flow. (For color version of this figure, the reader
is referred to the online version of this topic.)
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