The relationship between turbidity and suspended solids

Puzzle-solving class: Are "turbidity" and "suspended matter" the same thing?

In water quality analysis, turbidity and suspended solids are two brotherly concepts that appear frequently and are easily confused. They both describe "something" in a body of water, but from very different perspectives and dimensions. Customers often ask: "I measured turbidity, do I not need to measure suspended solids?" or "Why do my online SS sensor readings not match the laboratory data?" Today, we will thoroughly clarify the differences, connections and key application points between the two.

In water quality analysis, turbidity and suspended solids are two brotherly concepts that appear frequently and are easily confused. They both describe "something" in a body of water, but from very different perspectives and dimensions. Customers often ask: "I measured turbidity, do I not need to measure suspended solids?" or "Why do my online SS sensor readings not match the laboratory data?" Today, we will thoroughly clarify the differences, connections and key application points between the two.

1. The core difference: one measures the "optical effect" and the other measures the "actual weight"

This is fundamental to understanding the difference between the two. We can understand through a vivid metaphor:

• Turbidity: Just like you are in a smoky room, shining a flashlight, and judging the concentration of the smoke based on the clarity of the beam and the size of the halo. It doesn't care about the specific composition and total weight of the smoke particles, only how much impact they have on the light.
• Suspended matter: It is like using a very fine filter to filter the air in this room, collecting all the particles, and then putting them on a balance to weigh their actual total weight.

Based on this metaphor, we can summarize the precise differences shown in the following table:

Features

Turbidity

suspended solids

definition

Measurement of light scattering and absorption by suspended particles

The mass of all solid matter in water that cannot pass through a specific filter membrane

essence

Optical properties, which are a strength indicator

Physical mass , is a weight concentration

unit

NTU, FNU, etc.

mg/L (mg/L)

Measurement method

Optical sensor (scattered light method)

Laboratory gravimetric method (filtration, drying, weighing)

Measurement characteristics

Fast, continuous, online

Time-consuming, discrete, offline (requires manual sampling)

Influencing factors

Particle size, shape, color, refractive index

The actual mass of the particles has nothing to do with the physical and chemical properties of the particles

2. Advancement of Online Measurement: The Principle and Key “Password” of SS Online Sensor

In order to achieve continuous monitoring of suspended solids, online SS sensors came into being. It is worth noting that the core measurement principle of most online SS sensors is very close to that of turbidity sensors, which are also based on light scattering.

So, how does it derive the mass concentration (mg/L)?

The key lies in a core parameter - density correction factor.

• Working principle: The online SS sensor first measures the optical scattering signal of the water sample (essentially a "turbidity" value).
• Core Conversion: It then needs to convert this optical signal into mass concentration. This conversion process relies on a preset density correction factor (sometimes called the K-factor or conversion factor). You can understand it as the "Translation Dictionary" necessary to translate "optical intensity" into "mass weight". Simplified understanding of the formula: SS (mg/L) ≈ measured scattered light signal × density correction factor
• Simplified understanding of the formula: SS (mg/L) ≈ measured scattered light signal × density correction factor
• Why does density need to be set on site? Factory default: When the sensor leaves the factory, it is preset with an average density value based on a typical water sample (such as municipal sewage). This is a "general" starting point. Necessity of on-site calibration: As mentioned earlier, the suspended solids characteristics of different water bodies vary widely. The density of mineral sediment is much higher than that of organic floc or algae. If the default density is used to measure water bodies with completely different characteristics, there will inevitably be significant deviations in the SS values ​​obtained. Therefore, in order to obtain accurate readings, the user must back-end and correct the density value inside the sensor by comparing it with the results of the laboratory gravimetric method (national standard method) based on the actual conditions of the water body on site.
• Factory default: When the sensor leaves the factory, it is preset with an average density value based on a typical water sample (such as municipal sewage). This is a "general" starting point.
• Necessity of on-site calibration: As mentioned earlier, the suspended solids characteristics of different water bodies vary widely. The density of mineral sediment is much higher than that of organic floc or algae. If the default density is used to measure water bodies with completely different characteristics, there will inevitably be significant deviations in the SS values ​​obtained.
• Therefore, in order to obtain accurate readings, the user must back-end and correct the density value inside the sensor by comparing it with the results of the laboratory gravimetric method (national standard method) based on the actual conditions of the water body on site.

3. Why is there no fixed conversion formula for measurement results?

A common misunderstanding is looking for a conversion formula between turbidity (NTU) and suspended solids (mg/L). This is unscientific, and the reason stems from the above-mentioned essential difference:

• Effect of particle properties: Small amounts of fine clay particles (small particle size) can produce high turbidity, but the total weight (SS) may be very low. A large number of coarse sand grains (large particle size) weigh a lot (SS is very high), but their ability to scatter light is weak, so the turbidity may not be high. The color and material (refractive index) of the particles also affect the relationship between optical signal and quality.
• Small amounts of fine clay particles (small particle size) can produce high turbidity, but the total weight (SS) may be very low.
• A large number of coarse sand grains (large particle size) weigh a lot (SS is very high), but their ability to scatter light is weak, so the turbidity may not be high.
• The color and material (refractive index) of the particles also affect the relationship between optical signal and quality.

Conclusion: For water samples from a specific source with relatively stable composition, by setting the correct density factor, the online SS sensor can provide accurate measurements. However, there is no universal conversion formula between turbidity (NTU) and suspended solids (mg/L).

4. How to select and operate in practical applications?

By understanding their differences and the characteristics of online SS sensors, you can make wise choices and operations:

• Choose turbidity monitoring when you need: Real-time process control and water quality early warning: such as water plant coagulation dosing, filter backwash judgment, or monitoring rapid changes in water quality. Evaluate disinfection effectiveness: Ensure low turbidity for microbial safety.
• Real-time process control and water quality early warning: such as water plant coagulation dosing, filter backwash judgment, or monitoring rapid changes in water quality.
• Evaluate disinfection effectiveness: Ensure low turbidity for microbial safety.
• Choose and use online SS monitoring when you need: Obtain suspended solids mass concentrations close to laboratory accuracy. Key steps: In the early stages of use, you must collect water samples and send them to the laboratory to determine the SS value. Calibrate and enter the density correction factor unique to your on-site water body accordingly. It should also be checked regularly thereafter.
• Obtain suspended matter mass concentration close to laboratory accuracy.
• Key steps: In the early stages of use, you must collect water samples and send them to the laboratory to determine the SS value. Calibrate and enter the density correction factor unique to your on-site water body accordingly. It should also be checked regularly thereafter.

Conclusion

In summary, turbidity is the "visual representation" of water, and suspended solids are the "physical burden" of water. The online SS sensor successfully "translates" the optical signal into mass concentration by introducing the key link of density correction.

YuanGan Technology understands the precise needs of different application scenarios. While our online sensors provide stable and reliable raw optical data, they are also equipped with a friendly parameter setting interface to guide you through key on-site calibration steps. We not only provide accurate "rulers" and "scales", but also provide complete instructions for use to ensure that the data you obtain is both fast and accurate.

—— YuanGan Technology, the water quality sensing expert around you