Guide to turbidity measurement

Turbidity is essentially a measure of how cloudy a liquid is. This cloudiness is often used to quantify the amount of particles disrupting light passing through a liquid. For example, muddy water has a lot of small soil particles, which disrupts light going through it and makes it very cloudy and turbid, while spring water tends not to have many particles in and so is clear and not very turbid. Check out our blog post for further reading. 

Turbidity is one of the most commonly used measures to indicate levels of contamination in water. Turbid water could contain silt, sand, mud or algae all of which can contain dangerous bacteria and pathogens, individually these may be difficult to measure. Turbidity acts as an indicator of water quality, not all contributors to turbidity are harmful but it does indicate a failure of water treatment in drinking water

There are no SI units for turbidity, the most commonly used unit is based on a measurement of a specific compound in water. There are a number of different units in use for different industries and measurement types depending on instruments used and methodology. There are different units depending on if you are using a nephelometer, turbidimeter (NTU) or Jackson turbidimeter (JTU) and many of them use formazin as a standard. Some of them can be related in certain areas of the scale.

There are three typically used methods for turbidity measurement:

• Secchi Disk
• Turbidity tube
• Using a turbidity meter

Each method has is pros and cons and different suitabilities depending on your water type and sampling methods. 

To choose between methods you will need to understand what kind of water are you trying to measure, what range of turbidity are you looking to measure and how can you get the sample.

There are a number of simple methods available. Firstly take a sample of water representative of the body of water you need to measure, try to ensure you don’t create significant quantities of bubbles. You can then measure turbidity by slowly filling a turbidity tube with the sample liquid, until you can no longer see the bottom. This optical inspection will give you an idea of how turbid your solution is and the methodology is very similar to that of using a Jackson candle turbidimeter.

A sechhi disk is a large black and white disk used to measure turbidity in large bodies of water, such as a lake or the sea. It functions similarly to a turbidity tube and uses optical inspection. Spinning the disk as it is lowered once you cannot see the colour changes then you note the depth of the rope through the notches.  

Using a turbidity tube is a simple procedure. The basic principle is that your sample liquid is added to a clear tube until the cross on the bottom of the tube is no longer visible. The depth of the sample is then measured to the closest centimetre and this reading can be approximately converted to NTU using the scale on the side of the turbidity tube.

You need to make sure the sample is representative of the whole water body you are trying to measure. It should be collected in a clean sample vessel such as a 1L HDPE bottle. This collection should be safety done with a sampler other type of dipping device. 

A turbidity meter works by passing a beam of light through a sample and detecting how much the intensity of the beam is reduced by with a light sensor. The two main styles of detectors are:

• Turbidimeters, which detects light transmitted straight through the solution (180°) to a light sensor.
• Nephelometers, which detect the intensity of the scattered light with a sensor placed at 90° to the initial light beam.

The turbidity units NTU (Nephelometric Turbidity Unit), FNU (Formazin Nephelometric Units), FTU (Formazin Turbidity Units), and FAU (Formazin Attenuation Units) are all based on calibrations using the same formazin primary standards. Some meters will have a built in standards in order to increase ease of use, but most others will require formazin or a modern alternative standard in order to take readings.

Nephlometers and turbidimeters detect light at either 90° or 180° however light is generally reflected across a much larger range of angles. Some meters state that they are “ratio” turbidity meters, meaning that to compensate for this range of different angles by creating an average light reading. This creates a more accurate reading than those that state “non-ratio”. You can find out more about how turbidity meters work here. 

Generally speaking, a turbidity reading below 5 NTU appears clear, while a reading of 55 NTU will start to look cloudy and a reading over 500 NTU will appear completely opaque. For drinking water in the UK the turbidity at the tap should be below 4 NTU, and prior to distribution it must be <1NTU.

To measure repeatably in the range 0.5-4 NTU you need to use a high quality meter designed for the purpose. Readings need to be taken with care and meters will need to be calibrated regularly for accurate results. Generally the meters will have a measuring range of up to around 1100 NTU, examples include the following products