What is Silt Density Index SDI Definition

The Silt Density Index (SDI) is an analytical procedure which measures the fouling potential of a sample stream, it is basically the parameter most used to determine how much pretreatment is required in designing Reverse Osmosis Systems. In reality, it is a very rough indicator of fouling potential. SDI is based upon the principal that when a filter gets plugged, less water will pass through at the same pressure. Less water passes through because of the resistance to flow caused by particles plugging the spaces in the filtering medium.

Silt Density Index SDI for Clean vs Dirty/Fouled Filter

A sample stream of the raw water is filtered through a new 0.45 micron membrane filter for fifteen minutes. The amount of time required to pass 500 ml through the filter is measured initially, (when the membrane filter is clean), and again after fifteen minutes worth of suspended material have been filtered. The reduction in flow rate (longer time to pass 500 ml at the end of the 15 minute period) is a crude measurement of the fouling potential of the stream.

SDI filter pads after testing turbidity in seawater

Most spiral wound RO membrane element manufacturers require that an RO System feed water has a fifteen minute SDI value (SDI15) of less than 5 (one manufacturer requires less than 4). An SDI15 of 5 is obtained when it takes four times longer to pass 500 ml after fifteen minutes worth of suspended solids have been deposited on the membrane than it did during the initial 500 ml. Most hollow fiber element manufacturers require that the SDI15 be less than 3.

When a membrane element supplier requires that an RO System feed water SDI15 be less than 5, this means that the feed water SDI15 must be less than 5 as it enters the Reverse Osmosis System The problem is that the particles may grow. In the case of living particles, they grow and reproduce like any organism. They add mass by accumulating nutrients from the environment. They add numbers by reproduction. Nonliving particles may grow in an Reverse Osmosis System as well. They grow by physical/chemical means called coagulation or agglomeration.

The purpose of our pretreatment equipment is to remove the relatively large particulate matter from the feed stream so it does not plug the Reverse Osmosis Systen and to limit the number of living particles entering a unit. The purpose of dispersant addition is to then keep the smaller, especially nonliving particles from agglomerating (growing larger) and remaining in the RO System.

Silt Density Index SDI and Reverse Osmosis System

Since SDI is a measurement of both living, dead, and nonliving particulate matter greater than 0.45 micron, it is a useful tool to determine the fouling potential of a feed water and how much pretreatment equipment will be required. Some general guidelines on what it will take to maintain an RO unit’s feed water SDI at less than 4 – 5 are:

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