The MBR Book: Principles and Applications of Membrane

92 The MBR Book
In a more recent study, the contribution of conditioning fouling to overall resistance
was found to become negligible once filtration takes place (Choi et al., 2005a). By
applying a vacuum (rather than suction) pump coupled with air backflushing, Ma et al.
(2005) were able to reduce colloidal adsorption onto the membrane (Ma et al., 2005).
These studies suggest that colloid adsorption onto new or cleaned membranes coupled
with initial pore blocking may be expected in MBRs (Jiang et al., 2005). The intensity
of this effect depends on membrane pore size distribution, surface chemistry and
especially hydrophobicity (Ognier et al., 2002a). In a test cell equipped with direct
observation through a membrane operating with crossflow and zero flux, flocculant
material was visually observed to temporarily land on the membrane (Zhang et al.,
2006). This was defined as a random interaction process rather than a conventional
cake formation phenomenon. While some flocs were seen to roll and slide across the
membrane, biological aggregates typically detached and left a residual footprint of
smaller flocs or EPS material. Biomass approaching the membrane surface was then
able to attach more easily to the membrane surface to colonise it and contribute to
Stage 2.
2.3.8.2 Stage 2: Slow fouling
Even when operated below the critical flux for the biomass, temporary attachment of
the floc can contribute to the second fouling stage. After Stage 1, the membrane surface
is expected to be mostly covered by SMP, promoting attachment of biomass particulate
and colloidal material. Because of the low critical flux measured for SMP
solutions, further adsorption and deposition of organics on the membrane surface
may also occur during Stage 2. Since adsorption can take place across the whole
surface and not just on the membrane pore, biological flocs may initiate cake formation
without directly affecting the flux in this initial stage. Over time, however, complete
or partial pore blocking takes place. The rate of EPS deposition, and resulting
TMP rise, would then be expected to increase with flux leading to a shorter Stage 2.
Such fouling would prevail even under favourable hydrodynamic conditions providing
adequate surface shear over the membrane surface. However, since uneven
distribution of air and liquid flow is to be expected in iMBRs, correspondingly inhomogeneous
fouling must take place.
2.3.8.3 Stage 3: TMP jump
With regions of the membrane more fouled than others, permeability is significantly
less in those specific locations. As a result, permeation is promoted in less fouled
areas of the membrane, exceeding a critical flux in these localities. Under such conditions,
the fouling rate rapidly increases, roughly exponentially with flux according
to data of Le-Clech et al. (2003b) (Fig. 2.33). The sudden rise in TMP or “jump” is a
consequence of constant flux operation, and several mechanisms can be postulated
for the rapid increase in TMP under a given condition. As with classical filtration
mechanisms (Fig. 2.9), it is likely that more than one mechanism will apply when an
MBR reaches the TMP jump condition, and a number of models can be considered:
(i) Inhomogeneous fouling (area loss) model: This model was proposed to explain the
observed TMP profiles in nominally sub-critical filtration of upflow anaerobic