The MBR Book: Principles and Applications of Membrane

Fundamentals 87
air cannot be used routinely. Air sparging has been shown to be effective in an MBRs
(Lee et al., 2001c) but the duration of air sparging was necessarily brief in this study
(5 s every 10 min), providing little permeability promotion overall. Sparging with
head space gas has been shown to be effective for immersed polymeric (Fawehinmi
et al., 2004; Stuckey and Hu, 2003) and sidestream ceramic (Kayawake et al., 1991)
membranes. As with the aerobic MBRs described above, a maximum permeability
was reached at a certain gas flow rate (Imasaka et al., 1989; Stuckey and Hu, 2003).
In common with all membrane processes (Section 2.1.4.5), increasing crossflow
increases flux in sidestream anMBRs by suppressing the fouling layer concentration
polarisation (Grethlein, 1978; Imasaka et al., 1989; Saw et al., 1986). However, a
plateau has been reported at Reynolds numbers beyond 2000-or-so where no further
increase in permeability takes place (Choo and Lee, 1998; Choo et al., 2000; Elmaleh
and Abdelmoumni, 1997, 1998). For ceramic membranes, where the fouling layer is
minimal, high crossflows have been reported as having a detrimental effect because the
thinning cake layer offers less protection against internal fouling (Choo and Lee, 1998;
Choo et al., 2000; Kang, 1996). Elmaleh and Abdelmoumni (1997) have reported close
to zero fouling for crossflows above 3 m/s in an MT organic membrane module sidestream
anMBR, with flux increasing linearly with shear stress up to this point. Baffles
were shown by these authors to increase flux by promoting shear, the effect being greatest
in the transition region between laminar and turbulent flow. However, the increase
in flux attained by these measures is normally at the expense of a punitive increase in
energy demand (Bourgeous et al., 2001) and non-uniform, and thus sub-optimal, TMP
distribution (Lee et al., 1999). High-shear operation might also be expected to impact
negatively on floc size and biomass bioactivity (Brockmann and Seyfried, 1996; Choo
and Lee, 1998; Ghyoot and Verstraete, 1997) with, at the highest shears, cell lysis taking
place, though it has been concluded by Elmaleh and Abdelmoumni (1997) that
such effects are less severe for anaerobic than aerobic biomass.
Reports of operation at different TMPs indicate that, as with most membrane separation
processes, membrane resistance determines flux at low TMPs, with no impact
of crossflow or MLSS concentration above 2.5 g/L (Beaubien et al., 1996). At higher
TMPs, crossflow (and thus surface shear) becomes important (Beaubien et al., 1996;
Zhang et al., 2004), the flux increasing linearly with CFV (Beaubien et al., 1996), the
slope decreasing with increasing MLSS partly due to viscosity effects. At very high
TMPs, permeate flux has been shown to decrease with increasing TMP due to compaction
of the fouling layer (Elmaleh and Abdelmoumni, 1997). However, this effect
appears to depend on the membrane filter; Saw et al. (1986), filtering anerobic
sludge, observed that at very high TMPs the permeate flux decreased with TMP for an
MF membrane but was constant for an 8–20 kDa MWCO UF membrane. The authors
suggested that this was due to the impact of the membrane substrate on the fouling
layer structure, but a more likely explanation is migration of fines through the cake at
higher TMPs into the more porous MF membrane, causing pore plugging (Beaubien
et al., 1996).
The use of extended intermittent aeration has been reported for nitrification–
denitrification MBR systems (Nagaoka and Nemoto, 2005; Yeom et al., 1999). In this
less common scenario, a single tank was used for both anoxic and aerobic biological
degradation. Filtration was carried out in only the aerobic phase to take advantage