The MBR Book: Principles and Applications of Membrane

Fundamentals 69
fouling (i.e. membrane pore plugging) by soluble and colloidal material is less significant
than cake layer fouling (Choo and Lee, 1996a; Kang et al., 2002; Lee et al., 2001c)
on organic membranes. However, for ceramic membranes, which provide higher
fluxes and for which cake layers are much thinner (especially at high crossflows), the
bulk of the fouling has been attributed to internal fouling by struvite. This has been
concluded from SEM studies coupled with a magnesium mass balance (Yoon et al.,
1999), and from observed impacts of ammonia level (Choo et al., 2000). As with aerobic
processes, hydrophobicity suppresses fouling on anMBR polymeric membranes
to some extent, and membrane charge may also be important in determining fouling
(Kang et al., 2002) unless the ionic strength is high enough to compress the double
layer and thus nullify charge repulsion (Fane et al., 1983).
Membrane pore size effects also follow similar patterns to those of aerobic systems
in that large pores provide greater initial fluxes but more rapid subsequent flux
decay (Choo and Lee, 1996b; He et al., 1999, 2005; Imasaka et al., 1989; Saw et al.,
1986), which has been attributed to either internal or surface pore plugging.
However, the optimum pore size appears to depend on the liquor characteristics.
Elmaleh and Abdelmoumni (1997), investigating the impact of pore size on the
anMBR steady-state permeate flux, recorded highest steady-state fluxes at a pore
size of �0.45 �m for an anaerobic mixed liquor, compared with 0.15 �m for a
mixed microbial population of methanogens.
The long-term impact of UF membrane pore size on hydraulic performance has
been assessed by He and co-workers for an anaerobic MBR (He et al., 2005). The lowest
MWCO-rated membrane tested (20 kDa) yielded the largest permeability loss
within the first 15 min of filtration when compared to 30, 50 and 70 kDa membranes.
However, when operated for an extended time (over 100 days) with regular hydraulic
and chemical cleaning, the largest MWCO membrane (70 kDa) experienced the
greatest fouling rate, as 94% of its original permeability was lost, compared to only
a 70% performance decrease for the other three membranes. The 30 and 50 kDa
membranes thus provided the best overall hydraulic performance, possibly indicating
an optimum membrane pore size for a given application. These results also reveal
the impact of test duration. Similar temporal trends where long-term fouling was
exacerbated by larger pores has been demonstrated for microfiltration membranes
ranging from 1.5 to 5 �m pore size for aerobic MBRs (Chang et al., 2001b).
Although both sidestream and immersed process configurations have been studied
for anMBR applications, as with aerobic systems sidestream anMBRs have the
longest history. They generally operate at CFVs and TMPs of 1–5 m/s and 2–7 bar,
respectively to provide reasonable fluxes. Much lower pressures (0.2–1 bar) arise in
immersed systems, though these are still higher than in corresponding aerobic
iMBRs. CFVs in immersed systems have been reported as being less than 0.6 m/s
(Bérubé and Lei, 2004). Stuckey and Hu (2003) reported that slightly higher permeate
fluxes could be maintained for an HF compared with an FS membrane element
in an immersed anMBR.
2.3.5.2 Chemical parameters
Since hydrophobic interactions take place between solutes, microbial cells of the EPS
and the membrane material, membrane fouling is expected to be more severe with