The MBR Book: Principles and Applications of Membrane

Viscosity (mPa s)
100
10
1
Viscosity correlations are complicated by the non-Newtonian pseudoplastic nature
of the sludge, but it has nonetheless been shown to have a negative influence on the
oxygen transfer coefficient (Badino et al., 2001; García-Ochoa et al., 2000; Jin et al.,
2001; Koide et al., 1992; Özbek and Gayik, 2001). This has variously been attributed
to bubble coalescence and solubility impacts, with larger bubbles forming (Özbek and
Gayik, 2001) and greater resistance to mass transfer recorded (Badino et al., 2001) at
higher viscosity. Air is also less well distributed at higher viscosities, the smaller bubbles
becoming trapped in the reactor (Jin et al., 2001).
Correlations between the �-factor and viscosity (� kg/(m s)) have been presented,
these correlations being more pronounced than those between �-factor and MLSS
concentration (Wagner et al., 2002). Relationships presented take the form:
a � h�x
(2.25)
where x � 0.45 (Günder, 2001) or 0.456 (Krampe and Krauth, 2003) at a shear rate
of 40 s �1 in activated sludge of high MLSS concentrations. The correlation is sheardependent:
increasing shear stress decreases viscosity (Dick and Ewing, 1967; Wagner
et al., 2002). An increase in aeration rate therefore offers the dual benefit to oxygen
transfer in that it increases the amount of available oxygen and also decreases biomass
viscosity by increasing shear stress. Viscosity increases roughly exponentially with
increasing MLSS concentration (Manem and Sanderson, 1996; Rosenberger et al.,
1999, Fig. 2.20), impacting negatively on both oxygen transfer and membrane fouling
(Section 2.3.6.3).
2.2.6 Nutrient removal
Fundamentals 51
0 10 20 30 40 50 60 70 80
MLSS (g/L)
Filamentous structure*
Free suspended cells*
Aggregate data**
Figure 2.20 Viscosity vs. MLSS concentration, *Rosenberger et al. (1999), **data from Stephenson et al.
(2000)
The removal of total nitrogen by biochemical means demands that oxidation of
ammonia to nitrate takes place under aerobic conditions (Equations (2.27–2.29)),
and that nitrate reduction to nitrogen gas takes place under anoxic conditions