The MBR Book: Principles and Applications of Membrane

94 The MBR Book
across the fibre bundle, mirroring the increase in the standard deviation of
the fluxes. At some point, both TMP and standard deviation rose rapidly. This
is believed to be due to flow mal-distribution within the bundle leading to local
pore and flow channel occlusion. It was possible to obtain more steady TMP
and standard deviation profiles when the flow regime around the fibres was
more rigorously controlled by applying higher liquid and/or air flows.
2.3.9 Fouling control and amelioration in MBRs
Whilst an understanding of fouling phenomena and mechanisms may be enlightening,
control of fouling and clogging in practice is generally limited to five main
strategies:
1. applying appropriate pretreatment to the feedwater,
2. employing appropriate physical or chemical cleaning protocols,
3. reducing the flux,
4. increasing the aeration,
5. chemically or biochemically modifying the mixed liquor.
All of the above strategies are viable for full-scale operating MBRs, and each are considered
in turn below.
2.3.9.1 Feed pretreatment
It is generally recognised that the successful retrofitting of an ASP or SBR with an
MBR is contingent on upgrading the pretreatment and, specifically, the screening.
Whilst an MBR can effectively displace primary sedimentation, biotreatment and
secondary solid–liquid separation, as well as tertiary effluent polishing, classical
screens of around 6 mm rating are normally insufficient for an MBR. Such relatively
coarse screens increase the risk of clogging of the membrane module retentate flow
channels, especially by hairs in municipal wastewaters, which aggregate and clog
both the membrane interstices and aeration ports. HF membranes have a tendency
for aggregates of hair and other debris to collect at the top of the membrane element.
Hairs may then become entwined with the filaments and are not significantly
removed by backflushing. FS membrane clogging occurs when debris agglomerate
at the channel edges and entrance. If the aeration fails to remove these aggregates,
sludge accumulates above the blockage, increasing the affected excluded area. Fibres
collecting in the aeration system can change the flow pattern and volume of air to
the membranes, reducing the degree of scouring. As a result of the decreased scouring,
membrane fouling is increased. Aerators are thus normally designed to resist
clogging and/or allow periodic flushing with water.
Since HF modules are more susceptible to clogging and the impact is rather more
severe, for such modules screens normally rated at between 0.8 and 1.5 mm are
usually employed. FS modules are slightly more tolerant of clogging, despite being
non-backflushable, and screens of 2–3-mm rating are normally adequate for MBRs
of this membrane configuration.