Biology Reference
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4.4.3. Membrane filtration
For tap water and other low turbidity waters, membrane filtration can be
used for secondary concentration of bacteria and parasites. In this context,
the term membrane filtration is used to describe filtration of samples through
flat thin-sheet membranes ( Fig. 4.6 ). These membranes are often polymeric
with pore sizes of 0.22 or 0.45 µm for bacteria and <2 µm for parasites, and
all material larger than the pre-size will be retained on the filter.
For bacteria, the filter is processed according to standard methods for
detection of bacterial pathogens in water, either placed on selective medium
for quantitative detection or in enrichment broth qualitative detection.
Parasites that are filtered onto membranes can then be subjected to direct
microscopic examination.
Warkiani et al. highlighted some problems with traditional membrane
filtration filters, including tortuous pore path, low pore density, over-
lapped pores, and high coefficient of variation which all lead to relatively
low sample throughput and cell recovery rate. 68 The solution proposed in
2011 by these authors is micro-fabricated filters to obtain membranes con-
taining pores of uniform size and shape ( Fig. 4.7 ). The advantages of this
approach are claimed to be the ability to use a larger (but controlled) pore
size increasing throughput, the narrow pore size distribution ensuring pas-
sage of all smaller unwanted particles, and a straight pore path preventing
Figure 4.6 Image of flat-sheet membrane process the setup used for membrane filtra-
tion using a vacuum source or water suction. The funnel to the left is used for larger
volumes (liters) and has a 90 mm filter on which the microorganisms are retained. The
smaller funnel is used with 47 mm filters. Other types of filter housings for larger filters
are available. (For color version of this figure, the reader is referred to the online version
of this topic.)
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