The MBR Book: Principles and Applications of Membrane

48 The MBR Book
high-strength effluents when the oxygen limitation is reached. In an iMBR, additional
aeration is also required for scouring of the membrane (Section 2.3.5).
Changes in airflow have been shown to produce the largest changes in mass
transfer in a coarse bubble aeration system (Ashley et al., 1992), with k La increasing
with gas velocity in an airlift reactor (Lazarova et al., 1997; Masoud et al., 2001).
Nordkvist et al. (2003) proposed that both the liquid and gas velocities impact on
mass transfer, confirmed by experiments based on a jet loop MBR by Kouakou et al.
(2005). However, the authors of this paper also noted a linear relationship between
the mass transfer coefficient and the liquid recirculation velocity. Also, increasing
horizontal velocity has been shown to increase the value of k La in an oxygen ditch in
both pilot (Gillot et al., 2000) and full-scale plants (Deronzier et al., 1996).
2.2.5.3 Correction for temperature and process water
� relates to the effect of temperature on the mass transfer and is corrected by:
T 20
kLa (
k a
T )
� ( )
L ( 20°C)
� �
(2.21)
where T is the temperature (°C) and � a constant. Typical � values are between 1.015
and 1.040 with 1.024 being the ASCE standard (Iranpour et al., 2000) for temperature
correction of viscosity:
T�20
w � 1.024(
)
(2.22)
Salts and particulates in wastewater both impact on the oxygen transfer rate.
Comparative tests on synthetic wastewater and tap water performed by Lazarova et al.
(1997) showed that below 2 g/L salt concentration has little effect on the oxygen
transfer. Kouakou et al. (2005) performed comparative studies between clean water
and wastewater with a salt concentration of 0.48 g/L and found the mass transfer
coefficients did not significantly vary. The effect of such constituents is accounted for
by the � factor which is defined as:
b � C*
C*
wastewater
cleanwater
(2.23)
and is usually around 0.95 for wastewater (EPA, 1989).
Both biomass characteristics and aeration system design impact on oxygen transfer
(Mueller et al., 2002). Biomass is a heterogeneous mixture of particles, microorganisms,
colloids, organic polymers and cations of various sizes and surface properties
which can all impact on oxygen transfer through contact area and surface energy.
Bubble characteristics differ depending on the aerator type and bubble stability, the
latter being influenced by the biomass characteristics and promotion of bubble coalescence.
At the same time, biological and physical characteristics of the mixed liquor
are affected by the shear imparted by the air flow, which can fragment flocs (Abbassi
et al., 1999) and cause the release of chemicals, as well as impacting on biodiversity