7/30/2019 Filtration + Separation Sept Okt 2012
1/1
Feature44Filtration+SeparationSeptember/October 2012
Sealed (Dead End)capillary membranes
Epoxy
Epoxy
Untreated Water
Sterile
Water Exits
capillary membranes
Bacteria control:
Testing membranefilter cartridges
The standard test method for determiningbacterial retention in
membrane filtersrequires a one-time challenge of 107 bacteriaper
cm2 of the effective membrane surface[ASTM F838-05 Standard Test
Method forDetermining Bacterial Retention of MembraneFilters
Utilized for Liquid Filtration]. In factthis method replicates what
normally occursgradually in a sterilising filtration process.
Themethod used originated from the point of viewthat the bacterial
load of a membrane filteroccurs over time and over the service life
of the
filter. The goal was to determine if the filterretains a total
bacterial load of 107 per cm2 (1010bacteria/filter cartridge) over
its anticipatedservice life of approximately 5000 L.
ASTM F838-05 prescribes Brevundimonasdiminuta as the test
organism. However, asthere is no specific selective culture medium
forthis organism, it is not possible to differentiateBrevundimonas
diminuta from other bacteria.Therefore the tests were carried out
with asubstitute, the Pseudomonas aeruginosa bacteriumfor which
specific culture media exists. Anotherreason that Pseudomonas
aeruginosa was used
is that it has a higher passage percentagethan Legionella
pneumophila [Influence of size,shape and flexibility on bacterial
passagethrough micro pore membrane filters. T.Egli Environmental
Science & Technology, 008,4, 6749-6754.], which is the target
bacteriastrain to be retained. The choice to perform the
esearchers at the Flemish Institute for Technological Research
VITO
NV have undertaken tests on membrane filter cartridges to
determine
their suitability for use in creating Legionella Safe shower
units.
validation with Pseudomonas aeruginosa is wellfounded: the
smallest width ofPseudomonasaeruginosa bacteria is 0.50 m, whereas
thesmallest width ofLegionella pneumophila is 0.55m (versus the
smallest width ofBrevundimonasdiminuta of 0.40 m).
For the dynamic test method two cubitainerswere filled with tap
water filtered overcarbon and micro-filtration cartridges inorder
to ensure sterility of the test water. Thecontainers in which
bacterial suspensions were
stored were placed in parallel. The bacterialsuspensions were
consecutively pumpedthrough Legionella Safe shower filters (-3bar).
Filtration was achieved by means of asuper-efficient hollow fibre
micro-filtrationmembrane, with a maximum pore size of0.15 m, which
has a high affinity for waterby virtue of a unique blend of
polymericcomponents. It only needs low pressure tostart the
filtration process and delivers highflow rates at a low pressure
gradient.
The membrane filters (manufactured by PrimeWater bvba) consist
of a bundle of asymmetric
hollow fibre membranes in a tube, where thespace between the
membranes and the tube ishermetically sealed with an epoxy resin
(seeFigure 1). In the so called Dead end filtrationtechnique (see
Figure ), the water flowsfrom the outside to the inside and the
filteredparticles are caught in the porous membrane
wall, which gradually clogs. A decrease in flowrate to an
unacceptable level is a signal forreplacement of the filter. This
micro filtrationtechnique is primarily applied in LegionellaSafe
shower filters.
From both containers the inlet bacterialsuspension was
continuously sampled todetermine the exact inlet
concentrationofPseudomonas aeruginosa. With intervalsof
approximately 1000 L permeate sampleswere taken from each of the
filters. The
flow rate at the end of the test was reducedto approximately 5
LpM, from 10 LpM atthe start. After a simulated service life of5000
and 4500 L respectively with watergradually contaminated with up to
E+10 CFUPseudomonas aerigunosa bacteria, the bacterialretention of
both filters was calculated by theanalysis of permeate samples at
the beginningand the end of two days of filtration. Itwas concluded
that both filter cartridgeseffectively stop bacteria in the
inflowing waterand demonstrate a log reduction of at least6
(99.9999%). The ASTM F838-05 criteriawere satisfied in the
determination of the
bacterial retention of membrane filters. Contact:
Sabine Kreps
VITO NV Gebouw Prodem
Tel. + 32 14 33 69 42
E-mail:[email protected]
www.vito.be
R
Figure 1: Cross section of hollow fibre micro-filtration
membrane filter. Figure 2: Dead end filtration principle.
mailto:[email protected]:[email protected]://www.vito.be/mailto:[email protected]://www.vito.be/
