Silt Density Index (SDI) Testing & Analysis

What is reverse osmosis & how is it used?

Reverse osmosis, often abbreviated to RO is simply a scientific term that is used to describe a technique used to filter water, wastewater and other liquids. In this type of filtration system, water flows through a specially constructed membrane which can filter out very fine materials including bacteria, salts and other particles that may have contaminated the water. It’s a process which is most commonly used for desalinating seawater and making it safe to drink, filtering wastewater or effluent to make it suitable for reuse, but is also used in other specialist industries which require very pure water as part of their processes.

The membranes that are typically used in RO, desalination and Ultra-Pure processes allow water to flow through several different layers, all designed to filter out particles of differing sizes. These membranes are designed to only allow water to flow in one direction, and are usually made from a high performance plastic material. In a lot of ways the materials used in the membrane are similar to human skin, which allows sweat to flow out of our body, but does not let water pass through in the other direction.

The downside to the reverse osmosis process is that as well as removing potentially harmful bacteria, salts and other contaminants from the water, other more beneficial minerals can also be removed too.

Reverse osmosis and desalination can be an expensive, energy intensive processes. Desalination is typically only used on a large scale for drinking water where there is no other alternative. In the countries of Northern Europe where rainfall is rarely an issue, reverse osmosis water treatment is mainly used in specialist industrial processes, and for ultra-pure and medical water production.

Membranes, silt & analysis of fouling potential

Silt Density Index testing is a way of measuring the quantities of particles, bacteria and other contaminants (effluent) that can become trapped in a membrane. This is often referred to as the “fouling potential” or “fouling capacity” of the suspended solids, or in other words, how quickly a membrane is going to become plugged given the level of contaminants in the water.

Source water which is found to have a high Silt Density Index, and so more likely to cause more rapid fouling might have to go through some form of pre-treatment process before entering a reverse osmosis membrane system. Alternatively, if an operator is aware of the increased membrane fouling potential of their water, they may implement alternative solutions to the issue which could include changing the membranes more regularly, more frequent cleaning or switching to a different type of material for the membrane itself.

Silt density index testing calculations

There is a standard way of measuring the Silt Density Index of source water which is summarised here:

Using a 0.45 micrometre filter, source or feed water is forced through a membrane at a constant pressure of 30 psi (2.07 bar).

First, measure the time taken to collect a 500ml sample of water (t_i ). Continue timing.

Maintain this pressure and take additional timings required to collect three further 500ml samples, typically after 5 (t₅), 10 (t₁₀) and 15 (t₁₅) minutes have elapsed, this is to allow silt to build up.

How to calculate SDI

The plugging factor (PF) can be calculated at the 5, 10 and 15 minute intervals as follows:

PF₅ = (1- t_i / t₅)100

PF₁₀ = (1- t_i / t₁₀)100

PF₁₅ = (1- t_i / t₁₅)100

The SDI test value is then determined as:

 SDI = PF/ t

The lowest SDI value, usually at 15 minutes, is the value that is reported.

Example calculation

Your timings indicate a t_i of 1 minute and t₁₅ of 4 minutes.

The plugging factor is calculated as follows:

PF= (1-1/4)100 = 75

SDI = 75/15 = 5

This indicates a fourfold reduction in flow or a 75 % plugged test filter.

SDI measurements at 5 and 10 minutes are often useful for source waters with higher SDI values where they can give a better indication of the rate at which the RO membrane is fouling.

Interpreting SDI test results

Our specialist SDI testing equipment measures the decrease in water flowing through the membrane over a specific test period.

Decreased flow is caused by particles (suspended solids) clogging or plugging the membrane and preventing the water to pass through.

SDI test results are expressed as a number.

The Silt Density Index isn’t about identifying what particles are present in the water and what sizes the particles are, it’s a way of measuring the total quantity of colloidal and particulate matter in a water source and so its fouling potential.

Interpreting silt density index values

If the testing results show a Silt Density Index value of less than one, this is extremely pure water which can flow through a reverse osmosis system for several years before membranes start to become fouled with silt.

Typically, water with a Silt Density Index of 3 or less indicates a very high level of purity, and that membranes will be able to perform for several months without needing to be cleaned or changed.

An index value of between 3 and 5 means that the water source, although not free from particles and silt, is suitable for use in a reverse osmosis system where the membranes are cleaned or changed regularly.

If the SDI result is 6 or higher, then the source water will require some form of pre-treatment to remove some of the colloidal and particulate matter before being passed through a reverse osmosis system. This could involve an additional filtration stage, UV treatment to remove bacteria or using higher quality water taken from a less contaminated source.

Sampling source water over time

It is important to remember that the SDI of a water source may change over time, especially with sea water desalination, but also with water taken from other sources. Depending on the specific application and the requirement for water purity in high volumes, testing to establish the silt density index might be performed weekly.

Specialist SDI test equipment is needed to force the source water at high pressure through the membrane and measure the results. Take advice from the experts at WTS about how best to test your water according your specific needs, and how often the testing process should be repeated.

Expert SDI testing & water analysis services

Contact WTS today to find out more about our professional SDI testing, laboratory and in-field sampling solutions and how they can help improve your reverse osmosis and membrane filtration processes.

As a leading provider of water tests, sampling and laboratory analysis procedures we can ensure you get the best possible services whenever you need them.

With offices in London serving the South and South East England, Manchester (North West), Birmingham (Midlands), Bristol (South East England and Wales), Leeds (North and North East) and Glasgow (Scotland), supported by teams of specially trained engineers, in-field site service specialists and technicians we can offer professional, cost effective SDI testing and support solutions across the UK and Ireland.

Contact us now for more information or for your FREE, no obligation quote.