Waterguide - SDI
What is the SDI?
SDI is short for silt density index. It is used to express the amount of particles in the water. It is also used to predict the fouling tendency of a membrane.
What does fouling mean exactly?
As described above, fouling is when a membrane becomes soiled. You need to pay special attention with ultrafiltration and microfiltration, as they are heavily affected by the filter cake that forms on the membrane. In turn, the filter cake is based on how soiled the water/water-based solution filtered by the membrane is. This can cause the following problem: the filter cake can block the pores of the membrane and end up filtering particles that are not actually meant to be filtered.
The filter cake is the layer of accumulated dirt on the filter medium.
How do you determine the SDI?
The following formula is used to calculate the SDI:
The designations stand for:
tx: First measurement for 500 ml (in seconds)
t0: Second measurement for 500 ml (in seconds) after T minutes
T: Time between the start of the first measurement and the start of the second measurement (in minutes)
The standardised test is conducted in the following way:
- First, you measure the time that it takes for 500 ml of water to pass through the membrane.
- Wait 15 minutes
- Repeat the measurement
The parameters are:
- Pressure 2 bar
- Water volume 500 ml
- Filter size 0.45 µm
Over time, the amount of particles that can pass through the membrane reduces. The filter cake prevents them from doing so and the membrane becomes blocked.
So the SDI is the percentage change in volumetric flow over time.
If the SDI < 3, the water has a low fouling tendency, which means it can be used directly as feedwater for reverse osmosis. If the SDI > 3, the water requires pre-treatment before it can be used as feedwater for reverse osmosis.
Technical application area of the SDI
The test is used in membrane technology and particularly in reverse osmosis units. It enables you to predict early on how likely the membrane is to become blocked, and to take the necessary steps to prevent this.
The SDI test involves measuring colloidal particles, which are evenly distributed substances in the medium and are between 1 nanometre and 1 micrometre in diameter.
Typical SDI values
Deep well SDI ≤ 3
Surface well > 3 (generally require pre-treatment)