The MBR Book: Principles and Applications of Membrane

86 The MBR Book
In the case of HFs, effective distribution of air over the whole element cross-section
and length becomes particularly challenging. For MT membrane modules in particular,
provided an air bubble of diameter greater than that of the tube diameter is
introduced into the tube, then air scouring of the entire membrane surface is
assured. This is not necessarily the case for the FS and HF configurations, and HF
systems additionally provide no fixed channel for the air bubble to travel up; this
appears to impact on membrane permeability. On the other hand, experimental
studies and heuristic investigations reveal FS systems to generally demand higher
aeration rates than HF systems to sustain higher membrane permeabilities, and this
is reflected in aeration demand data from pilot-scale studies and full-scale operating
plant (Section 3.3.1.1). Some HF systems are operated with intermittent aeration,
lowering the aeration demand further, and aeration demand may also be lowered by
stacking the membrane modules such that the same volume of air is passed over
twice the membrane area.
A number of authors (Le-Clech et al., 2003c; Liu et al., 2003; Psoch and Schiewer,
2005b; Ueda et al., 1997) have demonstrated that flux increases roughly linearly
with aeration rate up to a threshold value beyond which no further increase in permeability
takes place. It follows that operation is sub-optimal if the aeration rate,
and specifically the approach velocity, exceeds this threshold value. Intense aeration
may also damage the floc structure, reducing floc size and releasing EPS in the bioreactor
(Ji and Zhou, 2006; Park et al., 2005b) in the same way as has been reported
for CFV in sMBRs (Section 2.3.1). Given that aeration lifts the sludge through the
module, a relationship must exist between gas and liquid velocity (U G and U L),
respectively. Determination of U L induced by aeration can be difficult; techniques
such as electromagnetic flow velocimetry (Sofia et al., 2004), particle image
velocimetry (Yeo and Fane, 2004), and constant temperature anemometry
(Le-Clech et al., 2006), have all been used for liquid velocity estimation in iMBRs.
Based on short-term critical flux tests, a direct comparison between immersed and
sMBRs showed that similar fouling behaviour was obtained when the two configurations
were respectively operated at a superficial gas velocity (U G) of 0.07�
0.11 m/s and CFV of 0.25–0.55 m/s (Le-Clech et al., 2005b). An increase of U G in
the iMBR was also found to have more effect in fouling removal than a similar rise of
CFV in the sidestream configuration.
In practice, much development of commercial systems has been focused on
reducing aeration whilst maintaining membrane permeability, since membrane
aeration contributes significantly to energy demand (though not generally as much
as biochemical aeration demand). A key parameter is thus the specific aeration
demand (SAD), either with respect to membrane area (SAD m in Nm 3 air/(h m 2 )) or
permeate volume (SAD p Nm 3 air/m 3 permeate). The latter is a useful unitless indicator
of aeration efficiency, and values for this parameter, which can range between
10 and 100, are now often quoted by the membrane suppliers. Further discussion
of specific aeration demand is provided in Chapter 3 and values from case studies
included in Chapter 5.
Anaerobic and anoxic systems Gas sparging to maintain a high membrane permeability,
as used in immersed aerobic systems, is more problematic in anMBRs since