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SDI test kits: what are they and how do they work?

What is SDI?

Silt Density Index (SDI) is a measure to assess the fouling in water purification systems from Suspended solids and colloidal materials. When it comes to reverse osmosis membranes, suspended solids within the water can cause problems. Then these levels get high, it can cause fouling of the membrane. This will lead to:

  • Increased cost for maintenance
  • Reduced lifetime
  • Increased cost for replacements

What is an SDI kit?

An SDI Test kit is used to calculate the degree of fouling, by exposing a 0.45-micron filter to the feed water under pressure. The Lovibond SDI test kit can be used to calculate the membrane fouling time, backwash frequency and membrane lifespan.

Lovibond SDI test kit SKUĀ - 1192508
Lovibond SDI test kit SKU – 1192508

The kit includes the following:

These can be used to take the measurements to calculate the silt density index.

How to calculate SDI

First, using the SDI kit,  you measure the time taken to collect 100ml of water (ti). You then allow the water to continue to flow and after 5 minutes (measurement T = 5).  After this time, you then leave the water to flow and record how long it takes to collect another 100ml of water (tf). Repeat these last 2 steps after 10 and 15 minutes (to produce the measurements for T = 10 and T = 15).

To calculate the SDI, you input the collected values into the given formula:

SDI formula
SDI formula

T = Total elapsed time in minutes (for example, 5, 10 or 15 minutes).

ti = Initial time it takes to collect 100ml of water (in seconds).

tf = Time it takes to collect the further 100ml of water (in seconds) after time T.

A spiral wound reverse osmosis system requires SDI < 5, while a hollow fiber reverse osmosis system requires SDI < 3.

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