What Is SDI?

In RO membrane applications, we often see SDI requirements specified in the feed water conditions. So, what exactly is SDI?

SDI (Silt Density Index) is one of the most important parameters used to evaluate water quality. It represents the content of particles, colloids, and other substances in water that may cause fouling or blockage in water purification equipment.

In reverse osmosis (RO) water treatment systems, SDI is a key indicator for assessing RO feed water quality and is widely used to verify whether the pretreatment system is producing water that meets RO inlet requirements. The SDI value has a direct and significant impact on the operating stability and service life of RO membranes.

SDI usually refers to SDI₁₅, which is measured over a 15-minute filtration period. According to membrane design manuals, the SDI value must be lower than 5.

Test conditions:

1. Filter membrane diameter: 47 mm
2. Average pore size: 0.45 μm
3. Filtration pressure: 0.21 MPa

Test procedure:

1. Filter the water sample under constant pressure.
2. Record the time required to filter 500 mL of water at the beginning (t₀).
3. Continue filtration for 15 minutes.
4. Record the time required to filter another 500 mL of water (t₁₅).

SDI calculation formula:

During the test, most particles, colloids, and bacteria larger than 0.45 μm are retained on the membrane surface. As these substances accumulate, the water permeability of the membrane decreases, and the time required to filter the same volume of water increases. Therefore:

• t₀ / t₁₅ < 1
• The higher the concentration of suspended solids and colloidal matter, the smaller the value of t₀ / t₁₅
• A smaller t₀ / t₁₅ results in a higher SDI value

The theoretical maximum SDI value is approximately 6.7.

RO membranes have a very high removal efficiency for colloids. As a result, the colloid concentration on the membrane surface inside an RO system continuously increases along the flow direction.

For example, when the system recovery rate reaches 75%, the colloid concentration in the feed water of the last stage membrane elements (in two-stage or multi-stage RO systems) can be approximately four times higher than that of the first stage.

When the SDI value is too high, it indicates a high content of particles, colloids, and other fouling substances in the feed water, which significantly increases the risk of RO membrane fouling and blockage. Therefore, the SDI value must be reduced to an acceptable level.

According to most membrane manufacturers' guidelines, the required RO feed water SDI is less than 5, or preferably less than 4, to ensure stable and long-term membrane operation. SDI is therefore considered one of the key control parameters in RO system design and operation.