3. Water & Environmental / 7. FAQ's and Knowledgebase / Filtration / Water Analysers / Water purifcation

How do I measure suspended solids? How is the gravimetric method different to other methods?

The measurement of Suspended Solids is often required for discharge consent, and values may be reported to the Environment Agency (EA).

cloudy water from suspended solids
Suspended solids give water a cloudy appearance

There are several methods for testing the suspended solids present in a water sample, here we discuss the differences between the different techniques both in what the method requires and in which methods can be reported in the UK.

The Gravimetric Method

This is the traditional and most accurate method of determination.

It’s also the method described by the Blue Book (also referred to as the 1980 SCA Blue Book) published by the Environment Agency in the UK.

filters for suspended solids analysis

How does the gravimetric method work?

A fixed volume sample of water is filtered under vacuum through a glass fibre filter of a set specification and a known weight – such as these ones. The solids present in the water are kept behind on the filter.

The solids remaining on the filters must then be dried until all residual moisture present is gone. This is done by drying in an oven, cooling in a desiccator, then weighing. This cycle of oven, desiccator and weighing should be repeated until a constant weight is seen – showing that all moisture has evaporated.

The final weight of the solids is then known and can be reported.

 

What equipment is needed?

To work to the full gravimetric method you will need;

 

Can the results be reported?

Yes – this is the method described by the Environment Agency in their SCA Blue Book (method 105 “Suspended settleable and total dissolved solids in waters and effluents 1980” ).

In the current Environment Agency advice on reporting results for discharge consent, using an approved method such as those described in the SCA Blue Books is sugested.

 

Colorimetric Methods

Using a colorimeter to read suspended solids is a newer method which is less accurate but far faster and easier to perform.

Many sites use it as an initial indicative result, then also perform full gravimetric analysis on some of the samples to check how they correlate.

MD100 colorimeter for suspended solids
The MD100 colorimeter for Suspended Solids testing

How does the colorimeter method work?

After the meter is calibrated (usually with deionised water) some of the sample water is put into one of the sample cells. If the sample contains large particles or pieces it should first be blended at high speed for 2 minutes. The sample cell is placed into the meter and read immediately.

The meter uses the the transmission of light to determine the amount of suspended solids present, so gives an immediate result.

The limit of detection is higher than the gravimetric method, at 20 mg/L TSS.

You should also bear in mind that air bubbles can interfere with results – try to ensure no air bubbles are in the sample cell by gently swirling it. Never shake the sample cells as this will introduce bubbles.

 

What equipment is needed?

  • A colorimeter for suspended solids testing
  • Sample cells (usually included with the meter – check with your supplier)
  • Deionised water (for calibration – this may vary over different colorimeter models so check with your supplier)
  • A blender if your samples contain larger particles

 

Can the results be reported?

Possibly – while the Environment Agency recommends methods taken from approved texts such as the Blue Book or ISO standards, they do also state that “Alternative methods, such as in house methods, modified methods or test kits, with prior approval from us. We may also impose extra requirements.” in their guidelines on discharge consent testing.

You should speak to your Environment Agency contact to see if they would approve colorimetric testing for your site, and if any other tests or conditions would be required.

Many sites use colorimetric testing for fast results in ongoing monitoring, but then also perform a gravimetric test on some samples.

 

All of the references to reporting and Environment Agency recomendations are based on the advice in the UK at the time of writing. Always check with your local authority for their preferred test methods and current advice.

For more information on colorimetric or gravimetric testing of suspended solids, just contact us;

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