💧 Turbidity of Water (Nephelometry and Turbidimetry)

Physicochemistry PN-EN ISO 7027-1

Overview

STATUS (as of 28.10.2026). PN-EN ISO 7027-1:2016-09 — English version, published 27.09.2016, implements EN ISO 7027-1:2016 and ISO 7027-1:2016 [IDT]; it replaced PN-EN ISO 7027:2003. The standard is current — no withdrawal note.

SCOPE (clause 1). The standard gives two quantitative methods using turbidimeters or nephelometers: a) nephelometry — measurement of diffuse radiation, for water of low turbidity, e.g. drinking water; results in nephelometric turbidity units (NTU), typically from below 0.05 to 400 NTU, and depending on the instrument design also higher; b) turbidimetry — measurement of the attenuation of a radiant flux, more applicable to highly turbid waters, e.g. waste waters; results in formazin attenuation units (FAU), typically from 40 to 4000 FAU. The units NTU and FNU (formazin nephelometric unit) are numerically equivalent. PN-EN ISO 7027-1:2016, together with part 2 (semi-quantitative methods), replaced ISO 7027:1999.

TERM AND LIMITATIONS (3.1, 5.1). Turbidity is the reduction of transparency of a liquid caused by the presence of undissolved matter. The intensity of diffuse radiation depends on the wavelength, the measurement angle and the shape, optical characteristics and particle size distribution of the particles — therefore the mass concentration of suspended matter cannot be calculated from turbidity, and results from different instruments are comparable only with the same measuring principle and apparatus conforming to the standard. The result is reported relative to a calibration standard. Jackson turbidity units (JTU) generally cannot be related to other turbidity units.

COLOUR AND BUBBLES (introduction). Measurements are affected by dissolved light-absorbing substances (colour); this effect is minimized by measuring at wavelengths greater than 800 nm — in this region only carbon black and a blue colour found in some polluted waters slightly affect the result. Air bubbles also interfere — careful handling of the sample minimizes this. The previous edition of the standard (ISO 7027:1999) specified an 860 nm radiation source and a detector perpendicular to the beam.

SAMPLES (clause 4). All containers in contact with the sample are kept scrupulously clean, washed with hydrochloric acid or a surfactant cleaning solution. Samples are collected in glass or plastic bottles and tested as soon as possible; if storage is unavoidable — in a cool, dark place for at most 24 h, and before measurement they are brought to room temperature. Contact of the sample with air and unnecessary temperature changes are avoided.

STANDARDS (5.2). Formazin suspensions are prepared with particle-free water: distilled water filtered twice through a 0.45 µm membrane filter (soaked beforehand for 1 h in 100 ml of distilled water, the first 250 ml of filtrate discarded) or e.g. reverse osmosis water. The standard recommends commercial 4000 FNU formazin suspensions — this avoids the hazards of the toxicity and carcinogenicity of the hydrazine sulfate used to prepare the standard in the laboratory; stabilized commercial suspensions may be stable for up to a year when stored cool and dark. Reagents are stored in hard glass, HDPE or LDPE bottles.

DRINKING WATER — LEGAL REQUIREMENTS. Regulation of the Minister of Health of 22 May 2026 (Journal of Laws 2026, item 748): in Annex 1 Part C turbidity must be "acceptable to consumers and no abnormal change"; Annex 7 gives, for turbidity as an operational parameter, a reference value of 0.3 NTU in 95 % of samples with no sample exceeding 1 NTU (not applicable to groundwater sources where turbidity is caused by iron and manganese), and a minimum monitoring frequency depending on the volume of water supplied. Annex 4 Part B: the measurement uncertainty of turbidity is estimated at 1.0 NTU according to PN-EN ISO 7027.

WHAT WE DO NOT GIVE HERE. The requirements for the apparatus, calibration, measuring procedure and reporting of both methods (5.2.3–5.4) — we only know the titles of these clauses. Therefore we do not state tolerances for wavelength and angle, calibration ranges or rounding rules here.

Method principle

Water containing undissolved particles not only attenuates the radiation passing through it but also diffuses it unequally in all directions; water coloured by dissolved substances only attenuates it. In nephelometry the intensity of radiation diffused at a specified angle to the beam is measured (the method for waters of low turbidity, result in NTU); in turbidimetry — the attenuation of the beam passing through the sample (the method for highly turbid waters, result in FAU).

Because diffusion depends on the wavelength and on the properties of the particles, the result is referred to a calibration standard — a formazin suspension. The effect of sample colour is minimized by measuring in the infrared, at wavelengths above 800 nm.

Applications

• Water intended for human consumption — turbidity "acceptable to consumers and no abnormal change"; as an operational parameter 0.3 NTU in 95 % of samples and none above 1 NTU (Journal of Laws 2026, item 748, Annexes 1 and 7)
• Control of treatment efficiency in operational monitoring — frequency depending on the volume of water supplied
• Waters of low turbidity — nephelometry (NTU)
• Waste waters and other highly turbid waters — turbidimetry (FAU)

Key parameters

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

1. Preparing containers

   Wash all containers in contact with the sample with hydrochloric acid or a surfactant cleaning solution and keep them scrupulously clean (clause 4).

2. Sampling and storage

   Glass or plastic bottles; determination as soon as possible after sampling. If storage is unavoidable — in a cool, dark place, at most 24 h. Avoid contact of the sample with air and unnecessary temperature changes (clause 4).

3. Bringing to room temperature

   Bring samples stored cool to room temperature before measurement. Handle the sample carefully so as not to introduce air bubbles (clause 4, introduction).

4. Choice of method

   Nephelometry — waters of low turbidity, e.g. drinking water (NTU); turbidimetry — highly turbid waters, e.g. waste water (FAU) (clause 1).

5. Calibration and measurement

   The instrument is calibrated with formazin suspensions prepared with particle-free water; a commercial 4000 FNU stock suspension is recommended (5.2). Calibration, the measuring procedure and reporting are described in 5.3 (nephelometry) and 5.4 (turbidimetry) of the standard.

6. Result

   The nephelometric result in NTU (numerically equivalent to FNU), the turbidimetric one in FAU; it is reported relative to the calibration standard, without conversion to suspended matter concentration (5.1).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Hydrazine sulfate is toxic and carcinogenic — the standard recommends commercial formazin suspensions instead of preparing the standard in the laboratory (5.2.2)
• Hydrochloric acid used for washing containers is corrosive — follow the safety data sheet

Frequently asked questions