An assessment of the Silt Density Index based on RO membrane ...

234 S. G. Yiantsios. A.J. Karabelas /Desalination 151 (2002) 229-238
-y = lOO- 0.56x R= 0,79
y-100-3,84x R=O,99
+ 0,61x R= 0.98
1
I
,_,’
_,’
_,_’
,.
_,.’
_,”
,/
,I
70
0 2 4 6 8 10 12 14
105
t(h)
-y=lOO-0,22x
R=O.89
y= 100-2.02x R=0,99
Fig. 3. Normalized membrane flux data as a function <strong>of</strong>
time before and after fouling species introduction.
(a) Iron concentration: 5 ppm. (b) Iron concentration:
3.5 ppm.
5 ppm, and thus <strong>the</strong> concentration <strong>of</strong> 5 ppm is
considered an upper limit for linearity to hold
under <strong>the</strong> present experimental conditions (i.e.,
water pH, ionic strength and residence time in <strong>the</strong>
system).
It is clear that <strong>the</strong> present experimental results
represent accelerated fouling conditions since
iron concentrations are far above <strong>the</strong> recommended
values for membrane feed waters. Such
[Fe”1(ppm)
3 4
Fig. 4. Normalized membrane flux reduction rate as a
function <strong>of</strong> iron concentration in <strong>the</strong> feed water.
accelerated conditions are a necessity in laboratory
investigations due to time and resource
limitations. However, it is proposed that <strong>the</strong>
linear relationship found between fouling rate
and iron concentration can be extrapolated to
lower and more realistic concentrations, and here
lies <strong>the</strong> usefulness <strong>of</strong> <strong>the</strong> data obtained. For this
to be true two conditions need to be met. First,
iron precipitation at <strong>the</strong> chosen conditions (i.e.,
pH) must be complete down to lower concentrations,
and precipitation kinetics must be much
faster than <strong>the</strong> macroscopic characteristic times
in <strong>the</strong> system, i.e., iron residence time in <strong>the</strong> test
loop. Second, aggregation phenomena <strong>of</strong> <strong>the</strong>
precipitated crystallites, due to Brownian motion
or shear, must have a small effect on fouling,
namely on deposition rates <strong>of</strong> <strong>the</strong> particles on <strong>the</strong>
membrane and on deposit structure.
Regarding precipitation <strong>the</strong>rmodynamics,
according to <strong>the</strong> reactions given in <strong>the</strong> previous
section and <strong>the</strong> respective equilibrium constants
[13], <strong>the</strong> solubility <strong>of</strong> ferric ions at pH 7 is
estimated to be 5.9~10-‘~ moles/L. Therefore,
even at a total concentration <strong>of</strong> 1 ppb, almost all
iron will be in precipitated form. Regarding
kinetics, it may be recalled that <strong>the</strong> limiting step
for iron oxide particle formation in membrane
5