💧 Water Turbidity (Turbidimetry)
In short
Nephelometric determination of water turbidity using infrared scattered radiation at 90° angle. The test is performed according to PN-EN ISO 7027-1:2016-09; Measurement range (nephelometry): 0.05–400 NTU (FNU). The procedure comprises 8 steps (about 26–26 min in total); it is used for: Drinking water quality control (limit ≤1 NTU, recommended ≤0.3 NTU), Water treatment plant monitoring (coagulation, filtration efficiency), Surface and groundwater testing.
At a glance
- Standard: PN-EN ISO 7027-1:2016-09
- Category: Physicochemistry
- Procedure steps: 8
- Total time of stages: 26 min–26 min
- Measurement range (nephelometry): 0.05–400 NTU (FNU)
- Measurement range (turbidimetry): 40–4000 NTU
- Resolution: 0.01 NTU (low range)
Overview
Water turbidity is a measure of reduction in water transparency caused by the presence of suspended and colloidal particles: clays, silts, microorganisms, organic and inorganic substances. It is expressed in NTU (Nephelometric Turbidity Units) or FNU (Formazin Nephelometric Units) — both units are numerically equivalent.
Turbidity is one of the key drinking water quality parameters — the Polish standard requires ≤1 NTU (recommended ≤0.3 NTU). High turbidity indicates potential microbiological contamination (particles shield pathogens from UV and chlorine disinfection), aesthetic problems, and technological issues.
The ISO 7027 method uses infrared light source (860 nm LED) and 90° detection (nephelometry) — this eliminates the effect of water color on results. For low turbidity waters (drinking water) nephelometry is used, for highly turbid waters (wastewater) — turbidimetry (beam attenuation measurement).
Method principle
The water sample is illuminated with monochromatic infrared radiation source (860 nm LED, according to ISO 7027). A photodiode positioned at 90°±2.5° to the incident beam measures the intensity of light scattered by suspended particles.
Nephelometry (90° angle): scattered radiation intensity is proportional to particle concentration. Used for low turbidity waters (<40 NTU, typically drinking water).
Turbidimetry (attenuation measurement): measures beam weakening passing through sample. Used for high turbidity waters (wastewater, very turbid waters >40 NTU).
Calibration using formazin suspension — primary turbidity standard (1 FNU = 1 NTU).
Applications
- Drinking water quality control (limit ≤1 NTU, recommended ≤0.3 NTU)
- Water treatment plant monitoring (coagulation, filtration efficiency)
- Surface and groundwater testing
- Wastewater treatment process control
- Pool and industrial water monitoring
- Disinfection efficiency verification (turbidity reduces UV and Cl₂ effectiveness)
Key parameters
| Parameter | Value |
|---|---|
| Measurement range (nephelometry) | 0.05–400 NTU (FNU) |
| Measurement range (turbidimetry) | 40–4000 NTU |
| Resolution | 0.01 NTU (low range) |
| Wavelength | 860 nm ± 30 nm (infrared) |
| Detection angle | 90° ± 2.5° (ISO nephelometry) |
| Calibration standard | Formazin (hydrazine + hexamethylenetetramine) |
Standard
- Standard number
- PN-EN ISO 7027-1:2016-09
- Title (PL)
- Jakość wody — Oznaczanie mętności — Część 1: Metody ilościowe
- Title (EN)
- Water quality — Determination of turbidity — Part 1: Quantitative methods
Step-by-step procedure
-
Turbidimeter preparation
Turn on turbidimeter min. 15 minutes before measurement (light source stabilization). Check measurement chamber cleanliness.
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Calibration
Perform calibration with standard set: 0.1 NTU (or <0.1), 20 NTU, 100 NTU, 800 NTU (or per manufacturer). Use StablCal standards or fresh formazin. Wipe cuvettes with silicone oil.
-
Sample preparation
Bring sample to room temperature. Gently mix (invert 3×) — do not shake vigorously (air bubbles increase result). Analyze immediately after sampling.
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Filling cuvette
Fill clean cuvette with sample to indicated level (to manufacturer-specified mark). Close with cap. Wipe outer surface with cloth, apply thin layer of silicone oil.
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Measurement
Insert cuvette into measurement chamber (marker orientation!). Close cover. Wait for reading stabilization (AutoRead function). Record result in NTU/FNU.
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Measurement repetition
Remove cuvette, gently invert 3× (eliminate sedimentation), reinsert. Repeat measurement. Average of 2–3 measurements = final result.
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Quality control
Every 10 samples measure control standard (e.g., 20 NTU). Acceptable deviation: ±10% or ±0.5 NTU (whichever is smaller). Blank: water <0.05 NTU.
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Post-measurement cleaning
Rinse cuvettes with deionized water, leave to dry. Do not use detergents (may leave films). Clean measurement chamber periodically.
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Laboratory turbidimeter (nephelometer) | Hach TU5200, Hach TL2300 ISO, WTW Turb 430 | 8 000–25 000 PLN |
| Measurement cuvettes | Dedicated round glass cuvettes for turbidimeter (scratch-free, perfectly clean) | 50–150 PLN/pc |
| Silicone oil for cuvettes | For eliminating scratches and glass surface defects | 50–100 PLN |
| Cuvette cleaning cloths | Lint-free, microfiber | 20–50 PLN |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Formazin standard 4000 NTU | — | Primary turbidity standard: mixture of hydrazine sulfate and hexamethylenetetramine. Prepared fresh or purchased ready-made. |
| Hydrazine sulfate (NH₂)₂·H₂SO₄ | 10034-93-2 | Formazin standard component. 10 g/L — solution I. WARNING: suspected carcinogen! |
| Hexamethylenetetramine (CH₂)₆N₄ | 100-97-0 | Formazin standard component. 100 g/L — solution II. Mixing I+II produces formazin suspension. |
| Ready calibration standards | — | StablCal (Hach) or AMCO Clear sets — stable polymer standards replacing formazin. Shelf life up to 5 years. |
| Turbidity-free water | — | Deionized water filtered through 0.1 μm filter. Turbidity <0.02 NTU. |
Health and safety (OHS)
- Hydrazine sulfate — suspected carcinogen (cat. 1B). Prepare under fume hood, use nitrile gloves.
- Hexamethylenetetramine — irritating, flammable. Protect from fire.
- Safety goggles and laboratory coat mandatory.
- Ready standards (StablCal, AMCO) — safer alternative to formazin.
- Glass cuvettes — risk of breakage and cuts.
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 7027-1:2016-09 — “Water quality — Determination of turbidity — Part 1: Quantitative methods”.
How does this method work?
The water sample is illuminated with monochromatic infrared radiation source (860 nm LED, according to ISO 7027). A photodiode positioned at 90°±2.
What is the measuring range and accuracy?
Measurement range (nephelometry): 0.05–400 NTU (FNU); Measurement range (turbidimetry): 40–4000 NTU; Resolution: 0.01 NTU (low range); Wavelength: 860 nm ± 30 nm (infrared).
How long does the test take?
The times given for the individual stages add up to approximately 26 min–26 min. The procedure comprises 8 steps.
What equipment is required?
Laboratory turbidimeter (nephelometer), Measurement cuvettes, Silicone oil for cuvettes, Cuvette cleaning cloths. Indicative cost of the core instrument (Laboratory turbidimeter (nephelometer)): 8 000–25 000 PLN.
Where is this test used?
Drinking water quality control (limit ≤1 NTU, recommended ≤0.3 NTU); Water treatment plant monitoring (coagulation, filtration efficiency); Surface and groundwater testing; Wastewater treatment process control; Pool and industrial water monitoring; Disinfection efficiency verification (turbidity reduces UV and Cl₂ effectiveness).
What safety precautions apply?
Hydrazine sulfate — suspected carcinogen (cat. 1B). Prepare under fume hood, use nitrile gloves.; Hexamethylenetetramine — irritating, flammable. Protect from fire.; Safety goggles and laboratory coat mandatory.; Ready standards (StablCal, AMCO) — safer alternative to formazin.; Glass cuvettes — risk of breakage and cuts.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-EN ISO 7027-1.