Membrane and Desalination Technologies - TCE Moodle Website

110 L. Song and K.Guan Tay
• A few hours
• Low cost
• More alternatives
New
characterization
method Pilot test
Fouling
index
where j is the foulant deposition rate onto the membrane surface (g/m 2 s), v is the permeate
flux (m 3 /m 2 s or m/s), and cf0 is the foulant concentration (mg/L) in the bulk flow. The foulant
amount M (g/m 2 ) accumulated over time t is calculated by
M ¼
Simulation
ð t
0
j dt ¼ cf0
ð t
0
v dt: (3Þ
Resistance of the fouling layer at any point of time is related to the amount of accumulated
foulants with
Rf ¼ rsM ¼ rscf0
ð t
0
v dt ¼ kf
ð t
0
v dt; (4Þ
where Rf is the resistance of the fouling layer (Pa s/m), rs is the specific resistance of the
fouling layer (Pa s m/g), and kf (= rscf0) is a new parameter (Pa s/m 2 ) introduced and is termed
as the fouling potential of the feed water. Rearranging Eq. (4) gives (15, 16)
kf ¼ Rf
Ð t
0
Fouling in
Full-scale
System
• A few months
• Expensive
• Limited coverage
Fig. 3.3. Comparison of the new characterization method and the pilot test.
: (5Þ
v dt
Because the integration term is the volume of permeate collected per unit area of membrane
over time period t, Eq. (5) shows that the fouling potential of the feed water is defined
as the resistance of the fouling layer caused by a unit volume of permeate collected per unit
area of membrane.
In the above derivation, the fouling potential is calculated as the product of the specific
cake resistance (rs) and bulk foulant concentration (cf0). However, the calculation proves to
be difficult with real feed water, primarily because it is really impossible to establish the
specific cake resistance of all foulants in the feed water. Fortunately, the fouling potential of
feed water can be easily determined with a simple fouling test using a small piece of RO
membrane. The total hydraulic resistances at the beginning (R0) and the end (Rt) of the
fouling test are determined and the difference between these two hydraulic resistances gives
the resistance of the fouling layer: