What is SDI (Silt Density Index?)
Suspended solids and colloidal materials in feed water are one of the biggest
problems in reverse osmosis systems. Even though most systems have some
pretreatment including 5 micron prefilters, these fine particles are responsible
for fouling of reverse osmosis membranes.
In order to have some measure of the degree of this fouling problem, a concept
called Silt Density Index is used. Here a 0.45 micron filter is exposed to the
feed water under pressure and filtration rates are calculated. A detailed
description of the test is available here: Test
for Determining Silt Density Index
An SDI of less than 5 is considered acceptable for the reverse osmosis systems.
This means that at values of SDI of less than 5, the membranes should foul at a
very low rate. Even though the concept works most of the time, there are
exceptions when a lower SDI (less than 3) is desirable due to the nature of the
suspended solids in that feed water.
Once a SDI is calculated, how is that value used to determine RO
SDI of <5: No prefiltration is necessary.
SDI of 5-10: A media (sand-type) filter is required.
SDI of >10: A 2-stage media filtration is necessary -
possibly with the aid of coagulants or settling tanks.
Is SDI the same as Turbidity?
SDI is a measurement of the
fouling potential of suspended solids. Turbidity is a measurement of
the amount of suspended solids. They are not the same and there is no
direct correlation between the two. In practical terms, however, the
membranes show very little fouling when the feed water has a turbidity
of less than 1 NTU. Correspondingly the membranes show very low
fouling at a feed SDI of less than 5.
Is there any direct relation between SDI and
turbidity? will proper flocculation bring down SDI to desired levels?
which method you would recommend to bring down SDI?
No. However, from membrane
fouling standpoint, an SDI of less than 5, is equivalent to a Turbidity
of less than 1 NTU.
In the SDI test, are the t2, t3 (etc) tests made by passing water
through the membrane for 5, 10 and 15 minutes each -- in other words, do
you keep the water running through the whole test. This implies that we
need large quantities of the test water. True?
True. You need fairly large
quantity of water. That is why the SDI test is generally done at the
source of water.
What causes an SDI value to be slightly higher
than 5 following the pretreatment system? What can be done to lower the
SDI and be able to maintain it at the lower rate?
A high SDI can be caused by a
fouling membrane. You can reduce the SDI by injecting a coagulant that
is compatible with the membranes, before the media filter. You may also
inject a dispersant to keep particles from fouling the membranes.
Is it possible to obtain a SDI value if the
pressure is only 4.5 psi? And are there any calculations for
extrapolation of the results?
No. You will need a pump to
pressurize this water to the test conditions.
Does the function of water temperature affect your
SDI readings? Is there a specific correlation?
As long as the temperature
remains constant during the test [+/-1 deg C] there is no effect. Only
if the temperature varies during the test will the data be distorted.
Since the SDI procedure requires a constant 30 psi
pressure, is there a correction that can be done to the result if the
pressure is not 30 psi (id 20 psi)? What does %P30 stand for?
There is no correction factor
or correlation for running the SDI test at pressures other than 30 psi.
However, the SDI value at 20 psi, should still be useful. %P30 is a term
used for plugging factor at 30 psi. It equals SDI multiplied by the
duration of the test [standard 15 minutes].