Thesis - faculty.ait.ac.th - Asian Institute of Technology

Mixed Liquor Suspended Solids and Dissolved Substances
The effects of the MLSS concentration on the membrane fouling have been reported
by many researchers as membrane resistance varies proportionally in MLSS concentration
(Fane, et al., 1981) and when the MLSS concentration exceeded 40,000 mg/L, the flux is
found that dramatically decrease (Yamamoto, et al., 1989). However, Lubbecke, et al.
(1995) illustrated that MLSS concentrations upto 30,000 mg/L is not directly responsible
for irreversible fouling, and that viscosity and dissolved matter have a more significant
impact on flux decline. The increase in viscosity to yield a substantial suction pressure
increase can causes the failure of MBR system (Ueda, et al., 1996).
The effects of MLSS, dissolved matter, and viscosity on membrane fouling could be
estimated as given by Sato and Ishii (1991) in the following manner:
Where:
0.
926
47
1.
368
0.
326
R = 842 . 7 * ∆P
* ( MLSS)
* ( COD)
* ( µ )
Eq. 2.1
R = Filtration resistance, m -1
∆P = Transmembrane pressure, Pa
µ = Viscosity, Pa.s
MLSS = mixed liquor suspended solid, mg/L
COD = Soluble chemical oxygen demand, mg/L
According to the few researches, the role of mixed liquor in membrane fouling was
due to the presence of suspended solids (SS), colloids, and dissolved matter which
contributed to resistance against filtration by 65, 30, and 5 % respectively (Derfrance, et al.,
2000). Through fractionation of the mixed liquor of activated sludge into floc cell, EPS
and dissolved mater, Chang and Lee (1998) indicated EPS as an important component
contributing to fouling causing resistance in the filtration process. However, these studies
show that individual fouling resistances were not additive due to the sum of the resistances
given by each component was found to be greater than the measured total resistance.
Wisniewski and Grasmick (1998) fractionated the activated sludge suspension into
settleable particles (particle size above 100 µm), supracolloidal-colloidal fraction (nonsettleable
particle with a size ranging from 0.05 to 100 µm), and soluble fraction (obtained
after filtration with 0.05 µm membrane). They revealed that 52% of the total resistance
could be attributed to soluble components.
Particle Size Distribution
Many researchers have sought to establish the influence of particle size on the cake
layer resistance. Generally, the particle size of an activated sludge floc ranges from 1.2 to
600 µm (Jorand, et al., 1995). The break-up of biological flocs, generating fine colloids
and cells which later form a denser cake layer on the membrane is due to the shear force
rising as a result of pumping during cross-flow filtration (Wisniewski and Grasmick, 1998;
Kim, et al., 2001). According to Wisniewski, et al. (2000), after the floc breakup, the
suspension produced consists mainly of particles having a size of around 2 µm causing a
decrease in flux. 97% of the particles in the MBR system have an average diameter smaller
than 10 µm, while the activated sludge contained flocs range from 20 to 200 µm in size
(Cicek, et al., 1999).