The MBR Book: Principles and Applications of Membrane

84 The MBR Book
the colloidal material which is mainly responsible for fouling in an anMBR (Choo and
Lee, 1996b, 1998), as has been shown in aerobic MBRs, and the colloid concentration
is higher in anaerobic than aerobic systems. As with all membrane systems, colloidal
matter is transported more slowly back into the bulk solution than coarser
particulate materials due to the lower diffusion rates (Choo and Lee, 1998), which
means they tend to collect at the membrane surface and form a low-permeability
fouling layer. They are also of a size which can plug the membrane pores, particularly
for the larger pores of microfiltration membranes, if able to migrate into the
membrane.
Whilst far less characterisation of foulants has been conducted for anaerobic
MBRs than for aerobic systems, reported trends tend to suggest that it is the colloidal
component of the SMP fraction which is the dominant component in membrane
fouling. The nature and concentration of the SMP in the anaerobic mixed liquor is
dependent on the feedwater, the concentration being a function of the feedwater
COD (Barker and Stuckey, 2001). However, it is generally higher in organic concentration
than the SMP from aerobic systems and less anaerobically than aerobically
biodegradable (Barker et al., 2000). Also, SMP composition changes as a result of permeation
(Stuckey, 2003), implying that some SMP components are adsorbed onto the
membrane. SMP levels also appear to increase with increasing HRTs in conventional
systems (Barker et al., 2000), attributable to more extensive biomass decay to colloidal
and soluble products. Levels may also increase with decreasing loading rate, since
anMBR membrane permeability appears to increase with this parameter (Kayawake
et al., 1991) at low-intermediate loading rate values – 1.5–10 cf. �1.5 kg/(m 3 /day)
(Hernández et al., 2002). The negative impact of lower temperatures (Section 2.2.7.2)
may also be a reflection of slower biodegradation of fouling constituents of the SMP
fraction (Barker et al., 2000).
The use of supplementary dosing with PAC to ameliorate fouling has been extensively
studied in membrane filtration of potable water and in aerobic MBRs, and such studies
have also been conducted on anMBRs (Park et al., 1999). It has been suggested (Choo
and Lee, 1996b) that the addition of an adsorbent or a coagulant can enhance the
permeate flux by agglomerating colloids to form larger particles of lower fouling
propensity. The coarser and more rigid particles additionally improve scouring of the
membrane surface. Dosing of anMBRs with ion-exchange resin has also been studied
(Imasaka et al., 1989), with beneficial effects noted only at very high concentrations
of 5 wt%.
2.3.7 Operation
2.3.7.1 Membrane aeration or gas scouring
Aerobic systems Aeration is arguably the most important parameter in the design
and operation of an MBR. As already stated, aeration is required for biotreatment
(Section 2.2.5), floc agitation and membrane scouring (Dufresne et al., 1997) and
it is not necessarily essential or desirable to employ the same aerator for both duties.
Ostensibly, air is used to lift the mixed liquor through the membrane module
channels. However, the gas bubbles additionally enhance membrane permeation