Why is it Necessary to Test the Turbidity of Water? How does Turbidity Sensor Test it?

Do you know that clean water is essential for human health, the ecosystem, and the irrigation system? Clear and less turbid water is vital for the well-being of society. Turbidity sensors, therefore, play a critical role in water purification. A turbidity sensor is a device that shines a beam of light on the water and detects the scattered lights at a certain angle. The more scattered light, the more will be the turbidity.

This article will discuss turbidity, why it is essential, and different methods for measuring it. It will also discuss the impact high turbidity has on various areas and regulations involved in water chemistry.

1. The Necessity of Testing Water Turbidity

1.1. Defining Turbidity

Turbidity measures the clarity of water and indicates the presence of suspended particles that block the light passing through it. Turbidity sensors check turbidity by detecting the rays scattered by those particles. Some known particles increase turbidity, including algae, clay, silt, sediments, and organic matter. More turbid water has a cloudier look, as shown in the picture below.

Turbidity is an essential parameter that shows water's degree of purity in different sectors. It affects the health of people and aquatic life. Turbidity sensors measure it in different units. Here are two of them:

• Nephelometric Turbidity Unit:

The NTU (Nephelometric Turbidity Unit) is the most commonly used unit for measuring turbidity. It measures the amount of white light scattered by the suspended particles at an angle of 90 degrees. The higher the NTU, the more turbidity; the lower the value, the better the clarity. If 1 liter of water contains 0.3 mg of suspended particles, this equals 1 NTU.

• Formazin Nephelometric Unit:

FNU (Formazin Nephelometric Unit) measures turbidity in liquid. This type of turbidity sensor uses infrared light with a wavelength of 860 nm instead of white light. The advantage of this is that this type of turbidity sensor is unaffected by watercolor. The normal turbidity sensor range is 0.01 FNU to 4000 FNU. FNU is a reference from the ISO7027 (European) turbidity method. According to WHO, the average amount of drinking water is 0.2 - 1 FNU, which should never increase above 5 FNU.

1.2. Impacts of High Turbidity

High water turbidity affects many aspects, which shows the importance of the turbidity sensor.

• Drinking Water:High turbidity in household water can pose health risks. It contains higher amounts of microorganisms, such as viruses, parasites, and bacteria, which cause diseases. These can cause nausea, diarrhea, and headaches. Also, high turbidity combines with disinfectant, making removing pathogens and other harmful microorganisms inefficient.
• Environmental Impact:Even aquatic plants grow in water; they still need sunlight for photosynthesis. More turbidity in water blocks sunlight, slowing plant growth. This blockage affects the whole aquatic food chain. High water turbidity levels also increase the probability of pollutants disturbing the marine habitat, reducing underwater life, such as the fish population.
• Industrial Impact:High-turbidity water passes through equipment such as heat exchangers and boilers, decreasing efficiency by depositing solid suspended particles. This results in more fuel consumption and cost. High turbidity also affects the material of this equipment, increasing the chance of earlier failure.

1.3. Regulatory Standards and Guidelines

All over the world, different regulations and guidelines standardize turbidity limits and measurement techniques:

Drinking Water Standards: CEN standards are in effect in almost 34 countries. EN27027 is the standard for drinking water, and ISO7027 is another. WHO strictly requires the turbidity of drinking water to be no more than 1 FNU, with 0.3 FNU for water samples from big desalination companies. The EPA is another standard used in the USA.

Industrial Discharge Regulations: The abovementioned regulations have also established limits for industrial water discharge. The regulations are categorized based on the types of waste products produced. For example, GB3838 is the Chinese regulation that lists 69 kinds of water pollutants and 10 other pollutants and their parameters and limits.

2. How to Test Water Turbidity?

2.1. Traditional Methods

• Jackson Candle Method:this was the first known manual turbidity sensor for turbidity measurement. It consists of a tube tube placed on a candle. A water sample is filled on the tube to a level where the candle flame is invisible from upside down. That height is calibrated into JTU and then to NTU (2.5 JTU = 1 NTU).
• Secchi Disk:It consists of a black-and-white disk whose center is connected to a wire with marks. The disk is inserted into the water and kept inserted until it becomes invisible. At that point, the marking is done on a line immersed in water, and the length is measured. That length is called Secchi depth. Secchi Disk is a visual clarity test.

Although the methods mentioned above are simple and cheap, they are inaccurate as they depend on many factors, such as a person's eyesight, light conditions, time of day, etc.

2.2. Turbidity Sensors and Meters

Modern methods of measuring turbidity involve turbidity sensors, which use the interaction of a light beam with suspended particles. These sensors are not only accurate but also long-lasting. There are three types of turbidity sensors.

• Nephelometric Turbidity Sensor:This detector shines a light on the water sample, and the suspended particles scatter the light. The sensor is positioned at a 90-degree angle. The measure of light scattering determines the level of turbidity in the water. Measuring through light is the most accurate method of measuring turbidity to date, which is why it is mainly used for drinking water with low turbidity.
• Absorption Turbidity Sensor:This method measures the amount of light absorbed by the particles in water despite the scattering of light. It is not as accurate as the NTS, but it is used in water that constantly changes its turbidity.
• Total Suspended Solids Turbidity Sensor (TSS):This type of sensor has the same working principle as NTS but requires the detector to be placed on backscattered lights at 90 and 135 angles. Since this captures more scattered light, it is generally used for water with a considerable turbidity value.

3. How Turbidity Sensors Work?

3.1. Light Source and Detectors

The light source is usually an LED of a specific wavelength. For most sensors, infrared light with an 860 nm wavelength is used, as it complies with the ISO 7027 standard. White light is also used, but only for very selected NTU meters, and it complies with the EPR standard. Once the light is scattered from the impurities, a detector should catch it at specific angles. Photodiodes are commonly used for this purpose. Photomultipliers are also used on some meters to detect tiny quantities of light.

3.2. Signal Processing and Measurement

Photodetectors are the components that convert light energy into electrical signals, which are then calibrated into the NTU Output—conversion of Light to Electrical Signal. Turbidity sensors are also calibrated for the required output by preparing a standard water solution whose NTU is already known and then adjusting the value of NTU output to the same as the sample value. Usually, Infrared 860 nm turbidity sensors are advised to be calibrated after every 3 months.

3.3. Factors Affecting Sensor Accuracy

There are many factors affecting the accuracy of the Turbidity sensor:

• Color:The chemicals used in the water treatment may have colors in the product, which indicates a higher turbidity.
• Air Bubbles:Air bubbles scatter light beams and show a higher turbidity value than the actual ones.
• Temperature:Water temperature also changes the suspended particles' refractive index, which again changes the actual value.
• Settlement of Particles:If the suspended particles settle down, they don't participate in the scattering and cannot be detected by a turbidity sensor. Therefore, thorough mixing is essential before the measurement.
• Calibration:Make sure the electric signals produced by the photocell are accurately calibrated for NTU measurement.

4. Parameters to Monitor

4.1. Turbidity as a Primary Indicator

Turbidity is widely used as a primary indicator of water quality. Therefore, many industries are incorporating Turbidity sensors that continuously monitor water turbidity at different points, making it easy to check for any abnormality at any process step. Setting alarms on critical values with a specific offset helps the operator be alert about water quality and take measures, such as changing filters or regenerating membranes.

4.2. Correlating Parameters

Many parameters also need to be monitored along with the Turbidity to have a better picture of water quality.

• Suspended Solids (TSS):TSS shows suspended solids in water, which can be found in a grab sample taken into the laboratory. Since an increase in TSS also increases turbidity, turbidity sensors can help detect the rise in the trend by giving real-time values.
• Potential of Hydrogen (pH):It hinders the actual value of particles in the water and can also be determined for a complete picture of water. Companies often put limits on pH, too, to ensure better quality. Color also changes the turbidity but can be ruled out as many turbidity sensors detect the light scattered at 90°, which can only be done by a particle. The size of particles also affects turbidity, as large particles scatter more light than smaller ones. The shape and refractive index of the particle also affect turbidity.

5. Applications of Turbidity Sensors

5.1. Drinking Water Treatment

Turbidity sensors have an essential role in the quality of water treatment. Desalination plants typically use Turbidity sensors to check water quality at different points during the process. The raw water to be treated is always checked from the start so that the level of filtration and other methods, such as Coagulation or flocculation, can be assessed. If the filters start exhausting before the estimated time, the inlet water has more deteriorated chemistry, which turbidity sensors can check. A sample of post-filter water can also be used to check the capacity of filters so that the filter may discharge wastewater if necessary. Turbidity sensors can fulfill this need by giving real-time turbidity values.

5.2. Wastewater Treatment

In many industries, wastewater discharge monitoring is critical as it affects the rivers and oceans used as raw water sources and aquatic life. For this purpose, regulations like the EU and EPR are established to limit the continuous monitoring of discharge wastewater is necessary to ensure compliance with the regulatory limit. Turbidity sensors can fulfill this need by providing real-time turbidity values.

5.3. Environmental Monitoring

Turbidity meters are also used to check the quality of rivers and lakes and assess the overall water quality trend in the environment. The results help researchers check the water quality change over a certain period. Moreover, it also shows how effectively pollution control measures are being taken worldwide.

5.4. Industrial Processes

- Water quality should also be maintained during the industrial processes as it is not only needed for the process but also supports achieving the quality limit for wastewater discharge. Since Turbidity is the direct measure of water quality, many industries use this as a controlling factor in water chemistry. The regulator also imposed values to limit the Turbidity. Water used in Food & Beverages and Pharmaceuticals should have less than 1 NTU turbidity, whereas water used in chemical Processing has 1-2 NTU.

6. Conclusion

Using turbidity sensors is crucial to check the water quality so that corresponding methods can estimate the treatment or the existing system's effectiveness. As the world goes on, maintaining water cleanness is becoming more of a challenge due to urbanization and industrialization. Local communities and stakeholders should take part in controlling the water purity levels through strict vigilance and regular turbidity testing. Regulatory bodies also need to work on their defined water parameters limit by reviewing the effect of compliance on society and industries. Moreover, an ordinary person should also know the terms turbidity and turbidity sensors. All these factors can free this earth from dirt and pollution and make it a better place to live and grow.

If you are looking for a sensor that confirms international standards and provides accurate results, consider Rika Sensor RK500-07 (SS). It employs an optical method, using an infrared light source at 860 nm to detect light scattered at a 90° angle, ensuring compliance with EN27027 and ISO7027 standards. This approach minimizes interference from watercolor, enhancing measurement accuracy.​ Visit the Rika website for a comprehensive list of high-quality sensors.