The MBR Book: Principles and Applications of Membrane

(Madoni et al., 1993). The inter-relationships developed between aeration and various
system facets and parameters are thus complex, especially given that, for an iMBR,
aeration is also used for membrane scouring (Fig. 2.18). This complex relationship is
usually accounted for by the � factor. The � factor is the difference in mass transfer
(k La) between clean and process water, and has the most significant impact on aeration
efficiency of all three conversion factors. It is accepted that � factor is a function
of SRT (mean cell retention time), air and liquid flow rate and of tank geometry for a
given wastewater (Rosso and Stenstrom, 2005), and is defined as:
k a
a �
k a
Aeration
characteristics
Bubble size
Flow rate
Aerator
area
L wastewater
L cleanwater
Intensity
Floc size
Viscosity
MLSS
Bulk biomass
characteristics
O 2 transfer
Loading rate
Membrane
operation
Fundamentals 49
(2.24)
Wastewater composition and, in particular, the level of surfactants affect the bubble
size, shape and stability. Surfactants are found in detergents of all kinds, including
washing up liquid, laundry powder and soap. A high concentration of contaminants
builds up on the outside of the bubble, reducing both the diffusion of oxygen
into solution and the surface tension. Reduced surface tension has the beneficial
effect of reducing bubble size, thereby increasing the water-air interfacial area (a).
Fine bubble aeration systems are most negatively affected by surfactants, since bubbles
produced are already small and cannot be further reduced in size by a reduction
in surface tension (Stenstrom and Redmon, 1996). It has been shown from experiments
testing oxygen transfer in waters containing different surfactants that the
ratio of mass transfer from surfactant water to clean water varies between 1.03 and
0.82 (Gillot et al., 2000). However, surfactants have a negative effect on ASP
processes overall due to the promotion of foaming (Section 2.3.6.3).
Studies of the impact of solids concentration on oxygen transfer in biological wastewater
treatment systems have all indicated a decrease in OTR with increasing solids
concentration regardless of the system studied, though the relationship is system- and
feedwater-dependent (Chang et al., 1999; Chatellier and Audic, 2001; Fujie et al., 1992;
Gunder, 2001; Krampe and Krauth, 2003; Lindert et al., 1992; Muller et al., 1995).
TMP
Flux
Permeability
Cleaning
Figure 2.18 Aeration impacts in an iMBR (adapted from Germain, 2004)