written by
Subscribe to our monthly water newsletter
We might send you an email if we have something news worthy, written by our water professionals
What Are Total Suspended Solids?
Total Suspended Solids refer to particles that are physically suspended in water but not dissolved. Unlike dissolved salts that affect conductivity, suspended solids remain as small particles floating in the water column.
Examples include:
Clay and silt
Organic debris
Algae
Bacteria colonies
Biofilm fragments
Industrial particles
TSS is typically measured in:
mg/L (milligrams per liter)
Higher TSS levels usually indicate higher turbidity and lower water clarity, although turbidity and TSS are not exactly the same measurement.
Basic Principle of a TSS Sensor
Most modern TSS sensors work using optical measurement, specifically by detecting how particles scatter light.
The principle is relatively straightforward:
The sensor emits a beam of light (usually infrared or near-infrared).
Suspended particles in the water scatter the light.
A photodetector measures the intensity of the scattered light.
The sensor electronics convert the detected signal into a TSS concentration value.
The more particles present in the water, the more light is scattered, resulting in a higher TSS reading.
Light Scattering Measurement
The most common optical method is called nephelometry, where the detector measures light scattered at 90 degrees from the light source.
The process works like this:
An LED emitter sends light into the water sample.
Suspended particles intercept the light.
Particles scatter the light in multiple directions.
A detector positioned at a 90° angle measures the scattered light intensity.
The sensor’s internal calibration curve converts this optical signal into a TSS value in mg/L.
This technique is very sensitive to small particles and is widely used in environmental and wastewater monitoring.
Infrared Light to Avoid Interference
Many TSS sensors use infrared (IR) light, typically around 850 nm.
Infrared is preferred because:
It reduces interference from water color.
It minimizes influence from sunlight.
It is less affected by algae pigmentation.
This allows more stable readings in natural water systems and industrial environments.
Turbidity vs TSS
Turbidity and TSS are closely related but not identical measurements.
Turbidity
Measures water cloudiness caused by particles.
Expressed in NTU (Nephelometric Turbidity Units).
TSS
Measures the actual mass concentration of suspended solids.
Expressed in mg/L.
A turbidity sensor measures optical scattering directly, while a TSS sensor usually applies calibration curves that correlate scattering intensity with actual solid concentration determined by laboratory filtration methods.
Because particle size and composition vary, TSS sensors must often be calibrated for specific applications.
Types of TSS Sensors
1. Optical TSS Sensors
These are the most widely used sensors.
They contain:
Light emitter (LED or laser)
Photodetector
Optical window
Signal processing electronics
Advantages:
Real-time measurement
No chemical reagents required
Low maintenance
These sensors are common in wastewater treatment plants, aquaculture systems, and industrial water processes.
2. Laser Scattering Sensors
Higher precision sensors sometimes use laser light sources instead of LEDs.
Advantages:
More focused beam
Higher sensitivity
Better particle detection
These are often used in research and high-precision monitoring systems.
Sensor Fouling and Cleaning
Because TSS sensors rely on optical windows, they can be affected by:
Biofilm buildup
Algae growth
Mineral scaling
Sediment deposits
To prevent inaccurate readings, many industrial sensors include:
Automatic wipers
Compressed air cleaning
Anti-fouling coatings
Regular cleaning and calibration are important for reliable measurements.
Typical TSS Ranges
Different applications operate within different TSS ranges:
Application | Typical TSS Range |
|---|---|
Drinking water | < 5 mg/L |
Aquaculture systems | 5 – 80 mg/L |
Rivers and lakes | 1 – 100 mg/L |
Wastewater influent | 100 – 350 mg/L |
Activated sludge | 2,000 – 6,000 mg/L |
Importance in Water Treatment Systems
Monitoring suspended solids is critical because high TSS levels can:
Reduce oxygen transfer
Increase biofilm growth
Clog irrigation systems
Decrease filtration efficiency
Impact aquatic life
In biological systems such as aquaculture or hydroponics, excessive suspended solids can also promote bacterial growth and degrade water quality.
TSS Monitoring in Advanced Water Treatment
In advanced water treatment technologies, such as nanobubble aeration and oxidation systems, TSS monitoring provides insight into system performance.
For example:
Decreasing TSS can indicate improved biofilm breakdown or particle oxidation.
Stable low TSS levels can indicate better microbial balance.
Rising TSS may indicate biofilm detachment or system disturbance.
Because suspended solids strongly influence oxygen transfer and biological activity, TSS sensors are often used together with:
Dissolved Oxygen (DO) sensors
ORP sensors
pH sensors
Conductivity sensors
Together, these measurements provide a comprehensive picture of water chemistry and treatment performance.
