The MBR Book: Principles and Applications of Membrane

72 The MBR Book
Activated sludge
(suspended solids �
colloids � solubles)
Supernatant
(colloids � solubles)
Dead-end filtration
R sup
Centrifugation
Filtration
Solubles
Dead-end filtration Dead-end filtration
R sup � R sol
R t � R sup
Of these, evidence suggests that it is the physical nature, and specifically the size,
of the foulant that has the greatest impact on its fouling propensity. Hence, activated
sludge biomass can be fractionated into three categories: suspended solids, colloids
and solutes. The fractionation methodology critically affects the measurements made.
Typically, the biomass sample is centrifuged. The resulting supernatant is then filtered
with a dead-end membrane cell, with the calculated hydraulic resistance being
attributed to colloidal and soluble matter combined (R col and R sol, respectively, Fig. 2.27).
Another portion of the biomass suspension is then microfiltered at a nominal pore
size of �0.5 �m and the fouling properties of this supernatant (R sol) attributed
solely to the soluble matter. The relative fouling contributions of the suspended and
colloidal matter can then be calculated (Bae and Tak, 2005). The resistance provided
by colloidal matter has also been attributed to the difference between the levels
of TOC present in the filtrate passing through 1.5 �m filtration paper and in the
permeate collected from the MBR membrane (0.04 �m) (Fan et al., 2006).
Fractionation methods may vary slightly for different studies, but results are often
reported in terms of hydraulic resistances for suspended solids, colloids and soluble
matter, the sum of which yields the resistance of the activated sludge. Although an
interesting approach to the study of MBR fouling, fractionation neglects coupling or
synergistic effects which may occur among different biomass components, and also
with operating determinants. Such interactions are numerous and include feedwater
quality (Li et al., 2005b), membrane permeability, particle size and hydrodynamics
conditions (Bae and Tak, 2005) (Fig. 2.26). An attempt to compare results obtained
from different studies is depicted in Fig. 2.28, where relative fouling resistance contributions
have been calculated.
The relative contribution of the biomass supernatant to overall fouling ranges from
17% (Bae and Tak, 2005) to 81% (Itonaga et al., 2004). Such variation is probably
attributable to the different operating conditions and biological state of the suspended
biomass. It appears from these data that fouling by suspended solids is rather
R sol
�R col
R t
�R SS
Figure 2.27 Experimental method for the determination of the relative fouling propensity for the three
physical biomass fractions