Analytical Measurement · Turbidity · NTU · FTU · Water Quality · Light Scattering
Turbidity is a measure of how much suspended particles in a liquid scatter and absorb light. It is one of the most widely used water quality parameters in drinking water, wastewater, food and beverage, and pharmaceutical industries. This guide covers the physics of light scattering, the NTU and FTU unit systems, four measurement methods, sensor selection, calibration and a live NTU unit converter.
Turbidity Measurements: Introduction
Turbidity is the optical property of a liquid that causes light to be scattered and absorbed by suspended particles rather than transmitted in a straight line. The particles responsible include clay, silt, algae, bacteria, colloidal organic matter, industrial effluents, and microscopic organisms. A highly turbid liquid appears cloudy or murky; a low-turbidity liquid appears clear.
Turbidity is not a direct measure of suspended solids concentration. It measures how much those particles scatter light, which depends on their size, shape, refractive index and colour as well as their quantity. Two liquids with the same suspended solids concentration can have very different turbidity values if the particle size or type differs. However, in a stable process with consistent particle characteristics, turbidity correlates very well with suspended solids and is a faster, simpler and cheaper online measurement than gravimetric suspended solids analysis.
Turbidity is important across many industries. In drinking water treatment, high turbidity indicates the presence of harmful microorganisms or inadequate filtration, and regulatory limits apply. In pharmaceutical manufacturing, turbidity monitors product clarity and detects contamination. In food and beverage production, turbidity controls product appearance. In wastewater treatment, turbidity verifies that effluent meets discharge standards. In process plants, online turbidity transmitters connected via 4-20 mA loops to DCS systems provide continuous monitoring without manual sampling.
The Physics of Turbidity: How Particles Scatter Light
When a beam of light enters a suspension of particles, three things happen simultaneously: some light passes straight through (transmission), some is absorbed by the particles, and some is scattered in different directions. Turbidity measurement instruments detect one or more of these components. The type of scattering depends on the ratio of particle size to the wavelength of light.
Intensity scattered proportional to 1 / lambda^4
Scattering is symmetrical: equal amounts forward and backward
Applies to very fine colloids and macromolecules
Mie scattering (particles similar in size to wavelength, d approximately lambda):
Forward scattering strongly dominates over back-scattering
Applies to most particles in process water (1-100 micrometres)
Less wavelength-dependent than Rayleigh
Geometric scattering (particles much larger than wavelength, d much greater than lambda):
Particles act like mirrors, reflecting and refracting light
Applies to large suspended solids (sand, floc, fibres)
Most turbidity in drinking water and process streams is dominated by Mie scattering because typical particle sizes (bacteria: 1-10 um, clay: 0.1-10 um) are similar in size to visible light wavelengths (400-700 nm). This is why turbidity meters using infrared LED light sources (850-900 nm) give different readings than those using visible white light at the same particle concentration.
Figure 1: Four possible detector positions in a turbidity measurement cell. The 90-degree detector (used in ISO 7027 nephelometric measurement) measures only scattered light at right angles to the beam, which maximises sensitivity at low turbidity. Forward scatter detectors (25-70 degrees) are more linear at high turbidity. Back-scatter detectors work well in very turbid or opaque samples.
Turbidity Units Explained: NTU, FTU, FNU, EBC and More
One of the most confusing aspects of turbidity is the number of different unit systems in use. The units are defined by the light source, detector angle and calibration standard used by the instrument. Understanding which unit a sensor uses is essential for comparing results between instruments and for meeting regulatory requirements.
| Unit | Full name | Light source | Detector angle | Calibration standard | Used in |
|---|---|---|---|---|---|
| NTU | Nephelometric Turbidity Unit | Tungsten lamp (white, 400-600 nm) | 90 degrees | Formazin primary standard | US EPA drinking water regulations (Method 180.1). Most common unit in North America. |
| FTU | Formazin Turbidity Unit | Any (not specified) | Not specified | Formazin | Generic term when method is unspecified. Often used interchangeably with NTU (not always correctly). |
| FNU | Formazin Nephelometric Unit | Infrared LED (860 nm) | 90 degrees | Formazin | ISO 7027 international standard. European drinking water directive. WHO guidelines. Preferred for process and wastewater. |
| FAU | Formazin Attenuation Unit | Infrared LED (860 nm) | 0 degrees (transmission) | Formazin | ISO 7027. Used for high-turbidity samples where 90-degree detection is non-linear. |
| EBC | European Brewery Convention | Tungsten lamp (white) | 90 degrees | Formazin | Beer haze and clarity measurement in brewing industry. 1 EBC approximately equals 0.25 NTU. |
| JTU | Jackson Turbidity Unit | Candle flame (historical) | 0 degrees (visual extinction) | Diatomaceous earth suspension | Historical unit. No longer used in modern instruments. 1 JTU approximately equals 1 NTU at low values. |
Four Methods for Measuring Turbidity
The standard method per ISO 7027 and US EPA 180.1. A detector is placed at exactly 90 degrees to the incident beam. Only scattered light reaches the detector; the transmitted beam is blocked. This gives very high sensitivity at low turbidity (below 40 NTU/FNU) because even small amounts of scatter are detected against a dark background. Used in: drinking water treatment, pharmaceutical water, clean process streams. Limitation: at very high turbidity (above 100 NTU), the sample absorbs and rescatters the primary scattered light (inner filter effect), causing non-linearity.
Uses multiple detectors: 90-degree plus forward scatter (typically 30-45 degrees). The ratio of forward to 90-degree scatter is used to extend the linear measurement range to much higher turbidity (up to 10,000 NTU) while maintaining high sensitivity at low values. The ratio also compensates for colour interference from coloured samples. Used in: wastewater treatment effluent, process streams with variable colour, combined water-quality monitoring. Standard per ISO 7027-1:2016 with multi-angle measurement option.
Detector placed at 90-180 degrees to the beam (back-scatter angle). Works in very high turbidity samples (above 1,000 NTU, up to 100% solids) where the sample is so opaque that forward and 90-degree light cannot penetrate. Used in: sludge monitoring, slurry concentration, pulp and paper stock concentration, paint and coating processes. A probe-style back-scatter sensor is inserted directly into the pipeline, common in 4-20 mA inline instrument loops. Not accurate at low turbidity where 90-degree measurement is preferred.
Measures the attenuation of light transmitted straight through the sample (0-degree detector). As turbidity increases, less light reaches the detector. Simple, wide range, but less sensitive than nephelometry at low turbidity. Used in: high-turbidity industrial samples, activated sludge monitoring, colour measurement combined with turbidity. The Beer-Lambert law applies: A = log(I0 / I), where A is absorbance, I0 is the incident beam intensity and I is the transmitted intensity. Also called FAU (Formazin Attenuation Unit) measurement per ISO 7027.
Turbidity Measurement Physics: Beer-Lambert Law and Calibration
Where:
A = absorbance (dimensionless, or in AU: absorbance units)
I0 = incident light intensity (W/m² or lux)
I = transmitted light intensity after passing through sample
epsilon = molar absorption coefficient (specific to particle type)
c = concentration of scattering particles
L = path length through the sample (metres)
Transmission percentage from absorbance: Transmission (%) = I / I0 x 100 = 10^(-A) x 100
Examples: A = 0.1: T = 10^(-0.1) x 100 = 79.4% transmitted, 20.6% scattered/absorbed
A = 0.3: T = 10^(-0.3) x 100 = 50.1% transmitted
A = 1.0: T = 10^(-1.0) x 100 = 10.0% transmitted (very turbid)
A = 2.0: T = 10^(-2.0) x 100 = 1.0% transmitted (opaque sample)
For nephelometric (90-degree) turbidity, the Beer-Lambert law does not apply directly because the detector measures scattered (not transmitted) light. Instead, calibration is performed using formazin primary standards of known turbidity (in NTU or FNU), and the instrument linearises the scatter signal to match the formazin calibration curve.
Dissolve 100.0 g hexamethylenetetramine (HMT) in 1000 mL distilled water.
Mix 5.0 mL hydrazine sulphate + 5.0 mL HMT solution.
Allow to react at 25°C for 24 hours.
Dilute to 1000 mL.
Result = 4000 NTU primary formazin standard
Dilute to prepare secondary standards:
100 NTU: dilute 25.0 mL of 4000 NTU stock to 1000 mL
10 NTU: dilute 2.5 mL of 4000 NTU stock to 1000 mL
1 NTU: dilute 0.25 mL of 4000 NTU stock to 1000 mL
USEPA Method 180.1 requires fresh formazin preparation for each calibration. Many modern calibration kits use sealed secondary standards (StablCal, Gelex) for practical laboratory and field calibration. These do not expire as rapidly as freshly prepared formazin but must be referenced to primary standards.
Turbidity Regulatory Limits and Quality Standards
| Application | Regulatory body / standard | Turbidity limit | Unit | Consequence of exceedance |
|---|---|---|---|---|
| Drinking water (WHO) | WHO Guidelines for Drinking Water Quality | Less than 1 | NTU | Above 1 NTU: increased risk of pathogen breakthrough during disinfection. Above 4 NTU: elevated microbial risk. |
| Drinking water (EU Directive) | EU Drinking Water Directive 2020/2184 | Less than 1 | NTU / FNU | Member states must take action. Distribution risk increases with turbidity. |
| Drinking water (US EPA) | US EPA Surface Water Treatment Rule | Less than 0.3 (95% of samples) and never above 1 | NTU | Regulatory violation. Plant must notify authorities and take corrective action. |
| Wastewater discharge (EU) | EU Urban Wastewater Directive | Typically 5-10 as SS (mg/L), turbidity used as surrogate | NTU (as indicator) | Non-compliance with discharge permit. Regulatory enforcement action. |
| Swimming pool water | WHO / national standards | Less than 0.5 | NTU | Pool must be closed. Disinfectant effectiveness is compromised. |
| Pharmaceutical water (USP) | USP Purified Water and WFI | Less than 0.5 (visual clarity required) | NTU | Batch rejection. Product recall risk. |
| Beer haze | European Brewery Convention | Product dependent (typically less than 0.5 EBC bright beer) | EBC | Product below specification. Filtration process review required. |
Turbidity Measurement Calculator
Use this calculator to convert between turbidity units, calculate absorbance from transmission percentage, or determine whether a measured turbidity value meets regulatory limits. The measurement uncertainty of your turbidity sensor must also be considered when comparing to regulatory limits.
Turbidity Sensor Selection Guide: Which Technology for Which Application
| Application | Turbidity range | Recommended method | Standard | Key requirement |
|---|---|---|---|---|
| Drinking water filter effluent | 0 to 5 NTU | 90-degree nephelometry, infrared or white light | ISO 7027 / EPA 180.1 | Resolution to 0.01 NTU. Regulatory traceability. Online continuous monitoring with alarm output. |
| Wastewater effluent | 0 to 500 NTU | Ratio turbidimetry (90-degree + forward scatter) | ISO 7027 | Wide range to handle variable effluent quality. Compensates for colour variation in effluent. |
| Activated sludge / sludge blanket | 100 to 100,000 NTU | Back-scatter (inline probe) | Application-specific | High-range probe resistant to coating and abrasion. Self-cleaning wiper or ultrasonic cleaning. |
| Drinking water raw intake | 0 to 500 NTU | Ratio turbidimetry | ISO 7027 | Must handle high turbidity during storm events without saturation. |
| Beer / beverage clarity | 0 to 10 EBC | 90-degree nephelometry | EBC method | Sanitary construction (3A or EHEDG), CIP-compatible, temperature compensation. |
| Pharmaceutical WFI / purified water | 0 to 1 NTU | 90-degree nephelometry, white light | USP / Ph.Eur. | 21 CFR Part 11 data logging, FDA validation documentation, traceable calibration records. |
Quick FAQs: Turbidity Measurement
- pH Sensor Working Principle: Another Key Analytical Measurement in Water Quality
- 4-20 mA Current Loop: How Turbidity Transmitter Signals Connect to DCS and SCADA
- Measurement Uncertainty in Calibration: How to Quantify Turbidity Sensor Accuracy
- Cold Loop and Hot Loop Testing: Commissioning Turbidity Analyser Loops
- Signal-to-Noise Ratio: Minimising Noise in Low-Level Turbidity Measurements
External References
- ISO 7027-1:2016: Water Quality: Turbidity Measurement by Nephelometry
- US EPA Method 180.1: Determination of Turbidity in Drinking Water
- WHO Guidelines for Drinking-Water Quality: Turbidity Chapter
- Hach: Turbidity Measurement Technology and Sensor Selection Guide
What we learn today
- Turbidity measures how much suspended particles scatter light, not how many particles are present. The 90-degree nephelometric method (ISO 7027 gives FNU, EPA 180.1 gives NTU) is the standard for drinking water. FNU uses infrared 860 nm light; NTU uses white tungsten light. They give similar but not identical values. Always specify which standard applies when reporting to regulatory bodies.
- Four measurement methods: 90-degree nephelometry (best for low turbidity below 40 NTU, used in drinking water), ratio turbidimetry (90-degree plus forward scatter, extends range to 10,000 NTU), back-scatter probes (for very high turbidity sludge above 1,000 NTU), and transmission/absorbance (Beer-Lambert law: A = log10(I0/I), T = 10 to the power of minus A, as a percentage). Select the method based on the turbidity range and required accuracy.
- Key regulatory limits: WHO and EU drinking water require turbidity below 1 NTU. US EPA SWTR requires 95% of samples below 0.3 NTU and no sample above 1 NTU. Swimming pools: below 0.5 NTU. Pharma USP purified water: below 0.5 NTU. Calibrate turbidity sensors with formazin primary standards or certified secondary standards (StablCal, Gelex) traceable to formazin.