Silt Density Index (SDI) Calculator

Formula

The formula to calculate the silt density index (SDI) is:

\[ SDI = \left(1 - \frac{t_i}{t_f}\right) \times 100 \div T \]

Where:

\( SDI \) is the silt density index (%/min)
\( t_i \) is the initial time it takes to collect a 500 ml sample
\( t_f \) is the final time it takes to collect a 500 ml sample after time \( T \) has passed
\( T \) is the total time between tests (typically 15 minutes)

What is SDI?

SDI, short for silt density index, is a measure of the fouling capacity of water in reverse osmosis systems. The SDI measures the rate at which a filter is plugged when subjected to constant water pressure. In other words, as water flows through the filter, how fast does the filter clog with solids. This test typically uses a 0.45 micrometer filter and the pressure is set at 30 psi.

Example Calculation

Let's assume the following values:

Initial Time (t_i): 30 s
Final Time (t_f): 60 s
Total Time (T): 15 min

Using the formula to calculate the silt density index:

\[ SDI = \left(1 - \frac{30}{60}\right) \times 100 \div 15 = \left(1 - 0.5\right) \times 100 \div 15 = 0.5 \times 100 \div 15 \approx 3.33 \text{ %/min} \]

The Silt Density Index (SDI) is approximately 3.33 %/min.