Turbidity is an optical property of water describing how much light is scattered by suspended particles in the water column. High turbidity indicates the presence of suspended matter — including silt, clay, algae, organic particles, microorganisms, or colloidal materials — that reduces water clarity and transparency. Turbidity is measured in Nephelometric Turbidity Units (NTU) or the equivalent Formazin Nephelometric Units (FNU) using light-scattering instruments (turbidimeters/nephelometers).
Turbidity is a key water quality parameter for several reasons:
- Public health indicator: High turbidity in drinking water can shield microorganisms from disinfection (UV, chlorination), reducing treatment effectiveness. Turbidity spikes after rainfall events can indicate pathogen contamination risk.
- Aesthetic quality: Visible turbidity (above 1–4 NTU) is objectionable to consumers and indicates poor water quality perception.
- Disinfection efficiency: Turbidity interferes with UV light penetration and chlorine demand, undermining disinfection effectiveness.
- Environmental indicator: In surface waters, turbidity reflects sediment loading, algal blooms, or pollution events affecting aquatic ecosystems.
- Turbidity testing is part of Ovalab’s water quality testing service.
Measurement Methods
Turbidity measurement is standardised under EN ISO 7027-1:2016:
- EN ISO 7027-1:2016 (Water quality — Determination of turbidity — Part 1: Quantitative methods): Specifies two measurement approaches:
— Nephelometric method (90° scattered light): Light scattered at 90° from the incident beam is measured; results expressed in NTU/FNU. Measurement range typically <0.05–400 NTU. Used for clear and slightly turbid waters (drinking water, treated effluent).
— Attenuance method (transmitted light): Measures attenuation of the transmitted light beam; results expressed in Formazin Attenuation Units (FAU). Measurement range 40–4,000 FAU. Used for more turbid waters (raw water, wastewater). - Calibration: Turbidimeters are calibrated using primary standard formazin suspensions or equivalent secondary standards (StablCal, AMCO Clear). Formazin has been the international primary standard since the 1960s.
- Online/continuous monitoring: Process turbidimeters installed in water treatment plants and distribution systems provide continuous turbidity monitoring, enabling rapid response to quality changes.
- ISO 7027-2:2019: Part 2 covers qualitative methods for visual assessment of turbidity in very clear water samples.
Regulatory Limits
Turbidity regulatory requirements in Europe:
- Directive (EU) 2020/2184 (Drinking Water Directive): Turbidity is an indicator parameter — the parametric value is 1 NTU at the point of treatment (after filtration of surface water), and 4 NTU at the consumer’s tap. Member States are encouraged to achieve the lowest turbidity practicably achievable. High turbidity triggers investigation and potential action to protect public health.
- WHO Guidelines for Drinking-water Quality (4th edition): Recommend turbidity <1 NTU for optimal disinfection effectiveness, with <0.1 NTU ideal for UV disinfection systems.
- Surface water treatment: Filtration systems must consistently reduce turbidity to very low levels (<0.5 NTU in many guidelines) before disinfection to ensure pathogen removal targets (Giardia, Cryptosporidium) are met.
- Environmental water quality: EU Water Framework Directive (2000/60/EC) uses turbidity as a supporting element for ecological status assessment — elevated turbidity can indicate sedimentation stress on aquatic habitats.
Frequently Asked Questions
What causes turbidity in water?
Turbidity in drinking water is caused by suspended and colloidal particles including: clay and silt particles from soil erosion (particularly after rain events), algae and other microorganisms (especially in surface water sources), organic matter from decaying vegetation, precipitation of dissolved minerals (e.g., iron or manganese after redox changes), corrosion products from ageing distribution pipes, and disturbance of settled sediment in reservoirs or distribution systems. Sudden turbidity increases (‘turbidity events’) require immediate investigation as they can indicate contamination breakthrough or treatment failure.
What is the EU limit for turbidity in drinking water?
NTU (Nephelometric Turbidity Units) and FNU (Formazin Nephelometric Units) are numerically equivalent units, both defined by the same measurement principle (90° scattered light from a tungsten lamp or near-infrared LED, per ISO 7027-1:2016). The difference is historical: NTU was the older US-EPA designation (using tungsten light), while FNU is the ISO/European standard designation. In practice, a result of 1 NTU = 1 FNU. Modern instruments report both interchangeably, though ISO and EU standards now prefer FNU terminology for compliance reporting.
How is turbidity measured?
Cryptosporidium parvum and Giardia lamblia are protozoan parasites resistant to chlorine disinfection at normal treatment doses. Their removal relies primarily on physical filtration (coagulation, flocculation, sedimentation, rapid sand or membrane filtration). Turbidity is used as a surrogate indicator of filter performance: low, stable turbidity indicates that the filter is effectively removing particles — including oocysts and cysts. Turbidity breakthrough signals filter problems and potential pathogen passage. WHO guidelines and US EPA regulations use turbidity criteria as a filter performance benchmark for protozoa removal credit.
Why is turbidity important for drinking water safety?
Yes — high turbidity can interfere with several water analysis methods. Turbid samples may cause spectrophotometric interferences in colorimetric tests (e.g., nitrate, nitrite, phosphate determination by photometry), clog membrane filters used in microbiological analysis, and alter apparent concentrations of dissolved parameters. High-turbidity samples typically require filtration or centrifugation prior to analysis. For compliance testing under accreditation standards, sample pre-treatment procedures must be documented and validated to ensure accurate results are not affected by matrix interferences from suspended matter.
What is the difference between NTU and FNU?
Beyond water treatment, turbidity is an important quality parameter in beverage production. In beer brewing, turbidity indicates yeast content and protein haze formation, affecting clarity and shelf life — turbidity meters are used throughout fermentation, filtration, and packaging. In fruit juice and wine production, turbidity correlates with clarity specifications and consumer acceptance. Edible oil turbidity indicates moisture content, wax content, or cold-test performance. Process water quality in food production facilities must meet specific turbidity limits, as water quality directly impacts product microbiological safety and chemical stability.