NAMS 2002 Workshop - ICOM 2008

were collected, the viable bacteria were enumerated. Influence of the
characteristics of the defect upon the microorganism retention, i.e. the log
reduction value (LRV), was analysed in order to assess the impact of a defect
which alters only the skin in comparison with a defect crossing the whole
membrane structure.
Experimental results confirmed the leading part of the selective skin towards
bacterial removal : as long as the selective skin is not altered on its whole
thickness, the altered membrane keeps a retention efficiency equivalent to the
one of an uncompromised membrane (LRV > 7). Nevertheless, the skin is not the
only part of the membrane occuring in the retention mechanisms. For
membranes with a fully punched skin but with an uncompromised macroporous
support, the bacterial tranfer through the defect is highly limited by the support
since a log reduction value of 4 log may be attributed to this part of the
membrane structure. In order to get a better understanding of the retention
mechanisms provided by the macroporous support, a comparison between the
two types of defects altering the whole thickness of the membrane was done.
Here, the log reduction value is around 2 log when the support was punched by
the tip to be compared to 0.3 log in the case of a membrane altered with a defect
of same diameter made by burning the whole support with the femtosecond laser
beam. The observed discrepancy between those two results is analysed as the
swelling of the macroporous structure under the selective skin owing to the
applied transmembrane pressure. This change in material structure leads to the
partial clogging of the punched defect which was confirmed by scanning electron
microscopy observations. Under such conditions, we conclude that the
macroporous support works as quite an efficient fibrous particles collector.
To conclude, a highly compromised membrane (one defect of 200 µm diameter
for an effective area of 13.4 cm 2 ) is likely to keep a non negligible bacterial
removal efficiency thanks to the part taken by the macroporous support in
bacteria retention mechanisms. To complete this study, experiments with smaller
defects are still in progress. However, by gradually decreasing the size of the
defects until the range of the microorganisms size, we will have to cope with
bacterial specific behaviour such as their deformation under mechanical stress.
References
[1] Kobayashi et al. 1998 J Membr Sci vol.140 p.1.
[2] Causserand et al. 2006 Desal vol.199 p.70.
[3] Urase et al. 1996 J Membr Sci vol.115, p.21.
[4] Gitis et al. 2006 J Membr Sci vol.276 p.199.