NAMS 2002 Workshop - ICOM 2008

Ultra- and Microfiltration III - Membranes – 3
Friday July 18, 3:30 PM-4:00 PM, Honolulu/Kahuku
Membrane Characterisation : Assessment of the Bacterial Removal
Efficiency
N. LeBleu (Speaker), Université de Toulouse, Toulouse, France
C. Causserand, Université de Toulouse, Toulouse, France
C. Roques, Université de Toulouse, Toulouse, France
P. Aimar, Université de Toulouse, Toulouse, France - aimar@chimie.ups-tlse.fr
The retention of microorganisms is one of the praised advantages of filtration
membranes used for bioprocesses, or water and waste water treatments.
Nevertheless, the removal efficiency of membranes or modules may be reduced
by the presence of a small number of abnormally large pores. The qualification of
such membranes, and the question of their integrity, which can be carried out by
various types of testing methods, have to be relevant and sensitive. However
several works show that membrane characterisation and integrity monitoring
based on tracers rejection and air tests are not sensitive enough to detect such
imperfections [1-3]. They provide at best information on defects larger than 3 µm
[4]. As a consequence, they are not adapted to predict the removal efficiency for
smaller microorganisms such as bacteria [5].
In this context, we have worked on a new approach for membrane
characterization, based on the specific behaviour of bacteria during filtration that
we revealed in a former experimental study [6]. It appears that the bacterial
transport through a porous membrane structure can be assisted by its
deformation and this phenomenon is governed by the structural characteristics of
the cell wall, namely the peptidoglycan layer. The more this layer is thin and
elastic, the more the bacteria will deform, and will likely pass through pores
smaller than its own size. As a consequence, rejection mechanisms based on
steric effects are inadequate to assess the membrane rejection capacity since
bacteria of equal size can exhibit different behaviours in filtration depending on
their deformability. The present study is divided in two parts : the description of
the characterisation methodology and its application to commercial membranes.
Challenge tests were performed on flat-sheet polycarbonate track-etched
microfiltration membranes of different nominal pore sizes (0.05 - 0.2 - 0.4 - 0.8 -
1.2 µm). This type of membrane was chosen as model due to their well-defined
pore geometry which allows breaking the bacterial transfer into elementary
mechanisms. For the following part of the study, we assume that the bacteria
transport trough one these pores is equivalent to the one through a small defect
or an abnormally large pore of an ultrafiltration membrane. Five bacterial strains
of various morphological and structural characteristics were selected in function