How Turbidity Is Measured

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Turbidity in water is caused by colored material and particles that become suspended. Turbidity is often measured relative to water clarity using a Secchi disk, or directly with a turbidimeter or a handheld sensor. 

Turbidity is a difficult parameter to measure, as it is an optical property of water. Measuring the turbidity of water tells us how murky or opaque the water is. Depending on which method you use to measure turbidity, different units have been designed to standardize turbidity results and to allow comparison between them. 

With advances in technology, there are now more modern ways to measure turbidity, providing high-class accuracy results. Whether you use more modern measurements or historical methods, each has its advantages and limitations.

What Is Turbidity?

The analytical technique to measure the turbidity of a liquid is called turbidimetry. Turbidimetry measures the haziness or cloudiness of a liquid, like water.

Let’s put it into perspective. Take drinking water, for example, the water is colorless and clear, therefore the turbidity level is low. But if you were to take water samples from a river with high discharge rates, the water is usually brown and opaque, so the turbidity is high. 

When looking at samples, you also need to consider the total suspended solids (TSS), as these affect the clarity of the water. Turbidity only measures the clarity of a liquid, so also measuring the TSS or total dissolved solids (TDS) will indicate if the clarity of the sample has been influenced by inorganic sediment, bacteria, or other precipitates. 

Despite other factors other than clarity affecting the turbidity of the water, turbidity is a great indicator of water quality. Industries that measure the water quality of drinking water and aquatic environments are just two examples where turbidity comes in handy. 

Why Is Turbidity Important?

Turbidity is an important indicator of suspended sediments in water, which can negatively affect aquatic life. Suspended sediments can block sunlight, stunting the growth of aquatic plants and smothering aquatic organisms.

Measuring turbidity is also used to indicate the presence of pathogens, bacteria, and other contaminants such as lead and mercury which are harmful to both aquatic life and human health. This is why turbidity is measured in drinking water and wastewater systems. 

Turbidity Units

When measuring turbidity, most often, you will see the units NTU or JTU.

  • NTU = Nephelometric Turbidity Units 
  • JTU = Jackson Turbidity Units

However, you may also come across alternative turbidity units, such as:

  • FNU = Formazin Nephelometric Units
  • FTU = Formazin Turbidity Units
  • FAU = Formazin Attenuation Units (common when using colorimeters or spectrophotometers)

As the optical design of the light, aperture, and photodetector differ between turbidity instruments, samples must be calibrated with an NTU standard or other turbidity standard

How Is Turbidity Measured?

The way we measure turbidity has far evolved from Italian astronomer Angelo Secchi, who discovered turbidity in 1865 and later created the Secchi disk, which we still use today. 

Jackson Candle Method

But, let’s go back to the first known measurement of turbidity, and that was the Jackson Candle Method. This turbidimeter consists of a flat-bottomed glass tube that sits above a burning candle. 

A sample (usually water) is poured into the tube until the person measuring can no longer see the candle flame. The final height of the sample corresponds to a certain number of JTUs, which can later be converted into NTU.

2.5 JTU = 1 NTU

Handheld Methods: Portable Spectrophotometers

Handheld spectrometers are a fast and accurate way to measure turbidity in liquids in both the lab and in the field. 

Handheld spectrophotometers allow you to measure the sample by directly immersing the tip into the sample to read the turbidity level. 

They contain an optical system that integrates a photodiode detector with a light-emitting diode, usually LED.

Secchi Disk

Secchi disks are used to measure the clarity of the water, named after Angelo Secchi. The disks are usually quartered black and white or colored, mounted to a line, and then slowly dropped into the water until the disk is no longer visible. The line usually has markers on to measure the depth that the Secchi disk is no longer visible, but if not, a marker is made at the depth the Secchi disk is absent, and then the line is measured.

The depth at which the Secchi disk is no longer visible is called the ‘Secchi depth’. A high Secchi depth indicates high water clarity and low turbidity, and shallow Secchi depths are associated with higher water turbidity. 

Secchi disks are most commonly used to measure water quality in lakes, oceans, and deep rivers, and are often used by oceanographers and other environmental scientists to analyze water quality. 

Although this way of measuring turbidity is very simple and cheap, it is not very accurate as it relies on the person observing to make the judgments, so there is room for human error. 

What Else Affects Secchi Disk Readings?

  • Sunlight conditions
  • Choppy water/wave action
  • The time of day
  • Water pollution
  • Obstructions such as aquatic plants

Types Of Secchi Disks:

  • Solid white Secchi disks are commonly used in marine environments.
  • Black and white Secchi disks are used in lakes.
  • Solid black Secchi disks are used in shallow lakes and rivers. 

Turbidimeters

Modern methods used to measure turbidity also use turbidimeters, but instead of a flame, a fixed light beam, aperture, and detector are used. 

Most modern turbidimeters work the same; a sample is collected and added into a vial. The vial is then placed in the instrument where a fixed light beam passes through the sample to measure how much light is transmitted, and how much light is scattered via photodetectors, which are usually set at 90° to the sample. 

When TSS and TDS are present, the light is scattered, which results in the detection of light from the photodetectors. The more TSS and TDS present, the more light is scattered, which results in greater turbidity measurements. 

For samples with high amounts of TSS and TDS, the difference in the light intensity from the transmission beam is measured to obtain the turbidity result, while light scattering is more suitable for samples with low amounts of TSS and TDS.

To comply with the Environmental Protection Agency (EPA): 180.1, a tungsten type meter or ISO specified design turbidity meter is used. 

The turbidity of a liquid can be measured in four ways using a turbidimeter.

  1. The transmitted light method

This method uses a photodetector at 180°, measuring the weakening of the transmitted light. The transmitted light method is used for high turbid samples with large TSS and TDS. The units used are FAU. 

  1. The scattered light method

Also known as the nephelometric method, this measures the intensity of scattered light at 90°. This method is perfect for measuring samples with low turbidity, and both FNU and NTU can be used. 

  1. The reflection measurement

As the name suggests, this method measures the scattered light intensity directly reflected from the sample. The transmitter and receiver are in line, therefore the light must be reflected at 180°. This method is only used when the water has a very high turbidity level of ~2000 NTU and where measurements are restricted.

  1. The ratio method

Just like how a ratio works, these types of turbidimeters show the relationship between two objects: the light transmission and the ‘stray light’ measurement. It is not often used, because it is not compliant with international standards for measuring turbidity. However, this method is occasionally used because it can minimize stray light and color interferences. 

Some studies have also found that the ratio method is much more accurate than the transmission and scattering method. 

How To Measure Turbidity In Water?

A turbidimeter is used to analyze the turbidity in drinking water. A minimum of two readings are taken to meet calibration standards and improve accuracy. A nephelometer (turbidity meter) is used and either NTU or FTU units can be used. 

Turbidity Levels In Drinking Water

In developed countries, drinking water should have a turbidity level below 1 NTU; 0.2 NTU is what most water companies aim for. 

In developing countries or countries with low resources, the turbidity of drinking water should be below 5 NTU. 

Turbidity Methods

The two types of turbidity methods used are:

ISO 7027 – The Infrared Method

Using a formazin-based metric method, the ISO 7027 standard specifies the key parameters for the optical system of the turbidity sensor when measuring drinking and surface water. 

ISO turbidity meters use a 90° sensor and high emission infrared LED light source. The light detects the amount of light scattered by undissolved particles in the sample, and the microprocessor converts the readings into FTU or FNU units. 

The US Environmental Protection Agency Approved Method (USEPA): 180.1

The USEPA method specifies the key parameters for the optical system of the turbidity sensor when measuring drinking, saline, and surface water. The units used are NTU, and they measure in ranges from 0 to 40 NTU using the nephelometric method. 

Turbidity meters that use EPA-approved methods follow the specific criteria listed by the USEPA 180.1 method and 2130 B standard method – the higher the intensity of scattered light, the higher the turbidity.

Types of Turbidity Sensors

The type of turbidity sensor you select depends on the application it is being used for. However, for quantitative turbidity measurements, all turbidity probes require calibration, following the regulation by ISO 7027:1999, which has now been revised to ISO 7027-2:2019

Nephelometric Turbidity Sensors

Nephelometric turbidity sensors detect how light energy scatters through a liquid. The sensor uses a light detector (positioned at a 90° angle), and an LED lamp. 

Once the lamp is switched on, the beam shines directly into the sample, where the light scatters when it hits a particle in a defined area. When the light scatters inside the sample, reflection occurs. It is the reflection in the sample that a nephelometric turbidity sensor measures by converting the received light intensity into an electrical signal. 

The greater the reflection intensity, the greater the turbidity of the sample, and vice versa. The result is shown on the turbidity sensor’s electronic display in the desired unit. 

Nephelometric sensors are used to measure drinking water or other types of water with low turbidity according to ISO 2027/ EN 27027

Absorption Turbidity Sensors

Absorption turbidity sensors measure how many particles absorb light (light absorbance) in a solution. Similar to nephelometric sensors, they also contain a light detector and an LED lamp, however, the light detector and lamp are placed directly opposite. 

The particles in the solution attenuate the emitted light, which is then converted into an electric signal by the detector, and then into a final turbidity reading. 

Advanced absorption turbidity sensors are equipped with a second lamp detector arrangement, which allows for more reliable turbidity measurements. The second light detector can correct light intensity variations, color changes, and reduces lens fouling. 

Absorption turbidity sensors are used for water that experiences regular fluctuations in turbidity.

Total Solid/Suspended Solid Turbidity Sensors

Total solid/suspended solid turbidity sensors use a backscattered light, containing two light detectors and an LED lamp positioned at 90° and 135° angles. 

The turbidity of the solution and the amount of TSS present are calculated when the detectors receive the amount of scattered light, which is then displayed by the transmitter in the desired unit (usually g/L or %TS).

Any solid particles in the solution will cause the incident light emitted by the LED lamp to effectively scatter. 

Total solid/suspended solid turbidity sensors are used for measuring water with high turbidity.

Portable Vs Benchtop Turbidity Meters

Portable turbidity meters include instruments based on ISO and EPA methods. Portable meters that use the EPA method display the units as NTU, and are also used to measure chlorine, making them useful tools when measuring drinking water. Meters based on the ISO method use FNU units and are commonly used to measure bentonite in wine and haze in beer.  

Benchtop turbidity meters also follow ISO and EPA methods. Meters based on the EPA method are also used to measure the amount of chlorine in a sample. Benchtop meters commonly use NTU units, but EBC and Nephelos units are also used, and ISO methods use FNU, FAU, NTU, and EBC.

Benefits Of A Turbidity Sensor

There are many benefits to using a turbidity sensor:

  • There are so many different turbidity sensors available to accommodate your needs. 
  • They are highly accurate. 
  • They are easy to use. 
  • They can be used in a wide variety of environments, for example, they can be fed down a pipe, or they can be handheld, immersing the probe into a sample. 
  • You can obtain turbidity measurements in different metrics (NTU, FTU, etc.)

Limitations To Measuring Turbidity

Most modern turbidity measurements require sending the sample to a laboratory to finish the measurement process, while this makes the result very accurate, the transportation process to the lab can cause errors. 

These errors come from TDS and TSS. During transport, these solids can aggregate and allow algae or bacteria to grow in the sample, which increases the turbidity – for the scientists, this is what we call contamination! 

This is why handheld turbidimeters are frequently used, as the samples do not need to be transported elsewhere to get a result. Handheld turbidimeters are widely used in wastewater treatment plants and the food and beverage industry. 

Does Temperature Affect Turbidity Measurements?

Turbidity is not directly affected by temperature, however, if you are using a submersible turbidity sensor with an LED light source, you must compensate for the temperature of the sample. This is because changes in temperature can affect the electrical output of the turbidity sensor. 

If the sample being tested experiences fluctuations in temperature, it can alter the turbidity readings, even though the turbidity level would not have changed. 

As temperature increases in a sample, the output decreases, and therefore less light will reach the photodetector, giving you a lower turbidity reading and erroneous result. 

Water temperatures do not fluctuate significantly in most applications, however, it is still important to account for any minor changes in the temperature of the sample. This is particularly important when using turbidity sensors with LED light sources, as these can heat up during use. 

Summary 

Turbidity can be measured in a variety of ways, depending on how accurately you require the results, and the amount of total dissolved solids and total suspended solids present in the sample. 

The most accurate way to measure turbidity is with a turbidimeter, however, industries still use the Secchi disk, Jackson Candle method, and handheld turbidity meters such as portable spectrophotometers because they are easier to operate. 
If you have any questions regarding turbidity/water quality or the water testing kits we have to offer, please feel free to reach out to our world-class team at Atlas Scientific, we are always happy to help!

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