Silt Density Index (SDI) Testing & Analysis

SDI testing

Expert in-field SDI testing and analysis.

WTS is a specialist water and environmental services company offering expert in-field Silt Density Index analysis and SDI testing services for operators of reverse osmosis (RO) and other membrane filtration systems.

Our in-field and laboratory based water analysis services help organisations using reverse osmosis and other membrane filtration processes to optimise the effectiveness of their systems by reducing costs, enhancing efficiencies, and maintaining water quality and system performance.

If you use a reverse osmosis (RO) membrane system for purifying water in your business, as part of an industrial process or even at home, then you’ll already be aware of the importance of the Silt Density Index (SDI) and the need for regular SDI testing and analysis of the water entering your RO membranes. Such analysis procedures will allow you to understand and monitor the membrane fouling potential of the source water and help determine what steps are needed to keep your systems operating at optimum efficiency.

Expert silt density index testing & in-field analysis

Our specialist water analysis and in-field water testing solutions include:

  • silt density index testing

    Specialist in-field SDI testing and analysis to determine the fouling potential of source waters.

  • Laboratory Water Testing & Analysis

    Expert laboratory based analysis of water for an extensive range of parameters.

  • Monitoring & Validation

    Latest in-field water analysis equipment to monitor and validate programme performance.

  • Treatment programme Development

    Development and specification of specialist water treatment and membrane cleaning programmes to optimise system performance.

  • membrane autopsy

    Specialist membrane evaluation and autopsy.

Full national UK coverage

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 regional teams of specially trained engineers, water treatment service specialists and technicians we can offer professional, cost effective in-field SDI testing and analysis, water sampling, membrane autopsy and specialist laboratory services across the UK and Ireland.

Contact WTS today to learn how our professional Silt Density Index testing and analysis solutions can help improve  the performance of your reverse osmosis and membrane filtration processes. To speak with one of our water testing experts call us on 0330 223 31 31 or simply use the button below to get our full contact details or submit our contact form.

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 (ti ). 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- ti / t5)100

PF10 = (1- ti / t10)100

PF15 = (1- ti / 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 ti 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.

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Further reading…

More information on the Silt Density Index… here →

Silt Density Index SDI Testing & Analysis for Reverse Osmosis Membrane Fouling
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Service Type
Silt Density Index SDI Testing & Analysis for Reverse Osmosis Membrane Fouling
Provider Name
Water Treatment Services,
14 Commercial Street,Manchester,United Kingdom-M15 4PX,
Telephone No.0330 223 31 31
United Kingdom & Ireland
Specialist Silt Density Index SDI testing and analysis to determine reverse osmosis membrane fouling potential.