WORD HEALTH ORGANIZATION_ Guidelines for
drinking water quality- 4. Water sampling and analysis
Membrane filtration Technique
“The Membrane Filtration (MF) Technique was introduced in the late 1949 as an alternative to the Most
Probable Number (MPN) procedure for microbiological analysis of water samples. The MF Technique was
accepted by the U.S. EPA for microbiological testing of potable water in the 11th edition of Standard
Methods for the Examination of Water and Wastewater. “
“(…) In the membrane filtration method, a minimum volume of 10 ml of the sample (or dilution of the
sample) is introduced aseptically into a sterile or properly disinfected filtration assembly containing a sterile
membrane filter (nominal pore size 0.2 or 0.45mm). A vacuum is applied and the sample is drawn through
the membrane filter. All indicator organisms are retained on or within the filter, which is then transferred to
a suitable selective culture medium in a Petri dish. Following a period of resuscitation, during which the
bacteria become acclimatized to the new conditions, the Petri dish is transferred to an incubator at the
appropriate selective temperature where it is incubated for a suitable time to allow the replication of the
indicator organisms. Visually identifiable colonies
are formed and counted, and the results are expressed in numbers of “colony forming units” (CFU) per 100
ml of original sample. (…)”
Filtração sob vácuo, de um volume adequado de água a
analisar através de uma membrana filtrante (com
porosidade controlada), onde ficarão retidos os
microrganismos indicadores
Colocação da membrana sobre um meio de cultura seletivo
para a deteção de microrganismos indicadores
Incubação da placa à temperatura e tempo adequado de
desenvolvimento do microrganismo alvo
Contagem de colónias formadas na membrana