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Water microfilters

Water microfilters

Microfilters work by physically straining out bacteria, protozoa and helminth eggs and larvae with one or more filter elements as water passes through the device.

Most filter elements are ceramic, glass fibre or composite depth filters that trap microorganisms in a maze of tiny passageways and dead-ends. You can usually clean ceramic filter elements by abrading the surface with a scrub pad after it begins to clog with silt, vegetative debris and organisms. Generally, non-ceramic filters cannot be cleaned by scrubbing but some can be back-flushed. Non-cleanable primary or back-up filter elements made of paper, metal, or glass fibre have to be discarded when they clog.

Filter Pore Sizes

Filter pore size, usually expressed in micrometres (microns), ranges from 6 microns down to 0.2 microns. Manufacturer literature often lists either ‘nominal’ or ‘absolute’ pore sizes.

Absolute pore size is the maximum size of any particle that can pass through a filter tested with microscopic beads. This means that a filter with an absolute pore size of one micron will allow nothing larger than one micron to pass through it.

Nominal pore size refers to a reduction level for particles, usually above 90%. This means that a filter with a nominal pore size of one micron will allow up to 10% of particles larger than one micron in diameter to pass through the filter.

It is important that you assess and compare water filters only on their absolute pore size and not their nominal pore size.

In addition, while pore sizes are tested with rigid microscopic beads, microorganisms are flexible, so you should leave a margin of error when selecting a filter for a particular task. For example, if you are selecting a water filter for Cryptosporidium oocysts, which are as small as three microns it is preferable to choose a filter with an absolute pore size in the one or two micron range, or smaller, to ensure that no oocysts are forced through the filter under the pressure of pumping.

While microfilters are very effective against protozoa, helminth eggs and larvae, and even tiny bacteria, no filter element has pores small enough to physically remove viruses unless the viruses are attached to larger particles or clumped together.

The smallest microfilter pore size currently available is in the 0.2 micron range while viruses can be as small as 27 nm (0.027 microns), which is only 1/7 the diameter of the pores.

Microfilter Absolute Pore Sizes for Types of Pathogens
Pathogen Organism Size in microns Maximum Absolute Pore size in microns
Viruses 0.027–0.05 Too small to filter with hand-held devices
Bacteria 0.2–5 0.2
Protozoa 2–20 1
Worm eggs and larvae 20–150 5–6