Turbidity Determination by the Scattering (Nephelometric) Method

Turbidity determination by the scattering method (nephelometry) is the internationally recognized technique for quantifying suspended particles in water and other transparent liquids by measuring the intensity of light scattered at a 90-degree angle. Conforming to standards such as ISO 7027-1, ASTM D7315, and GB/T 5750.4, the method offers rapid, highly sensitive, and reproducible results across measurement ranges from ultrapure water to heavily loaded effluents. This article explains the measurement principle, applicable sample types, testing procedures, typical applications, and frequently asked questions.

Technical Overview

Turbidity is an optical characteristic of a liquid that describes how much suspended, colloidal, and sub-microscopic matter scatters and attenuates a beam of light passing through the sample. The scattering method, also known as nephelometry, is the preferred technique for low- and mid-range turbidity determination because it is inherently more sensitive than direct transmission (attenuation) measurement.

In a nephelometric turbidity measurement, a collimated light beam is directed through a sample cell. Particles in suspension scatter the incident light in all directions, and a photodetector positioned at 90 degrees to the incident beam measures the intensity of the scattered light. For dilute suspensions, the scattered intensity is approximately proportional to the particle concentration, allowing the instrument to report results directly in turbidity units after calibration with certified standards.

• Governing standards: ISO 7027-1 (water quality — determination of turbidity), ASTM D7315, EN ISO 7027-1, US EPA Method 180.1, and GB/T 5750.4 for drinking water examination.
• Reporting units: NTU (Nephelometric Turbidity Units, typical for tungsten-light instruments) and FNU (Formazin Nephelometric Units, typical for 860 nm infrared instruments); both are formazin-based and numerically equivalent in most applications.
• Light sources: tungsten-filament lamps operating in the 400–680 nm band, or monochromatic infrared LEDs centered at 860 nm, which minimize interference from dissolved color.
• Instrument types: benchtop laboratory nephelometers, portable field meters, and continuous online turbidity analyzers for process monitoring.
• Detection geometry: 90-degree scattered-light detection for compliance measurements; ratio-based instruments additionally monitor transmitted and forward-scattered light to extend the working range and compensate for sample color.

Testing Samples

Drinking Water and Tap Water

Treated drinking water is the most common sample matrix. Regulatory limits are typically very low—often 1 NTU or less—so the scattering method is specified because of its excellent sensitivity at low turbidity levels. Samples are collected in clean glass or plastic containers with minimal headspace and analyzed as soon as possible after collection.

Natural Waters

Surface water (rivers, lakes, reservoirs), groundwater, and well water are routinely tested to characterize raw water quality, track storm events, and evaluate treatment requirements. These samples may contain varying particle sizes and organic color, which must be considered during measurement.

Wastewater and Treated Effluent

Municipal and industrial wastewater samples, including secondary and tertiary effluents, are tested to verify treatment performance and discharge compliance. High-turbidity samples are typically diluted or measured using a ratio-mode or transmission-based extension of the scattering method.

Process Water and Ultrapure Water

Pharmaceutical purified water, water for injection, boiler feed water, and electronics-grade ultrapure water are tested at sub-NTU levels using high-sensitivity laser or white-light nephelometers with low-stray-light optics.

Beverages and Liquid Food Products

Beer, wine, fruit juice, sugar syrup, and other transparent or lightly colored liquid products can be evaluated for clarity, filterability, and stability, provided the method is validated for the specific matrix.

Testing Items

The core deliverable of the test is a quantitative turbidity value, supported by calibration and quality-control documentation:

• Turbidity value of the sample, reported in NTU or FNU, with typical direct measurement ranges from 0 to 40 NTU (compliance range) and extended ranges up to 1,000 NTU or more using ratio mode or dilution.
• Low-level turbidity determination in the 0–1 NTU range for pure water systems, with detection capabilities down to the milli-NTU level on laser-based instruments.
• Calibration verification using certified formazin or stabilized turbidity standards traceable to recognized reference materials.
• Repeatability and precision assessment through replicate measurements, expressed as relative standard deviation.
• Interference evaluation covering dissolved color, air bubbles, floating matter, and particle settling that may bias the result.

Testing Methods

Principle and Instrumentation

A nephelometer consists of a stable light source, a sample cell compartment with defined optical geometry, a scattered-light detector positioned at 90 degrees to the beam, and signal-processing electronics. ISO 7027-1 specifies an 860 nm infrared source to eliminate color effects, while US EPA Method 180.1 specifies a tungsten lamp with a broad visible spectrum. The scattered-light signal is compared with the calibration curve and displayed directly in turbidity units.

Reagents and Calibration Standards

The primary calibration reference is formazin, a polymer suspension prepared by reacting hexamethylenetetramine with hydrazine sulfate under defined conditions, yielding a 4,000 NTU stock that is diluted to 400 NTU and further to working standards. Commercially stabilized formazin suspensions or traceable polymer-bead standards may be used for routine calibration, provided they are validated against freshly prepared formazin. Ultrapure water with turbidity below 0.05 NTU serves as the zero standard.

Calibration Procedure

• Allow standards and instrument to equilibrate at the same ambient temperature.
• Prepare at least five calibration points spanning the intended measurement range (for example, 0, 0.1, 0.5, 1, 2, 5, 10, 20, and 40 NTU for low-range work).
• Fill clean, matched sample cells, cap them, and wipe the optical surfaces with lint-free wipes; apply a thin film of silicone oil if minor surface scratches are present.
• Insert standards in a consistent orientation (cell indexing) and record the instrument response.
• Verify calibration with an independent check standard immediately after calibration and at defined intervals during use.

Sample Preparation and Measurement

• Bring samples to room temperature to prevent condensation on the cell and to stabilize particle suspension.
• Gently invert the sample bottle to homogenize suspended solids without introducing air; wait until visible bubbles dissipate, or degas by brief vacuum or ultrasonic treatment if foaming is an issue.
• Rinse the sample cell with the sample at least twice, then fill to the mark and cap.
• Wipe the cell exterior, apply cell indexing, and place it in the instrument; take the reading promptly to avoid particle settling.
• For samples above the calibrated range, dilute with turbidity-free water, measure, and multiply the result by the dilution factor.
• Analyze samples as soon as practicable—preferably the same day—since biological activity and particle flocculation can alter turbidity during storage.

Quality Control

Each analytical batch should include a blank, a mid-range check standard, and duplicate samples. Acceptance criteria for check standards (commonly within ±10 percent of the assigned value, or ±0.05 NTU at low levels) must be met before results are reported. Results, instrument identification, calibration status, and any deviations are documented in the final test report issued by accredited third-party laboratories.

Applications

Turbidity determination by the scattering method supports a wide spectrum of regulatory, industrial, and research objectives:

• Drinking water treatment control: optimizing coagulant dosing, monitoring flocculation and sedimentation efficiency, and detecting filter breakthrough in real time.
• Regulatory compliance: demonstrating conformance with national drinking water standards such as GB 5749, the EU Drinking Water Directive (EN ISO 7027-1 basis), and WHO guidance recommending turbidity below 5 NTU and ideally below 1 NTU for effective disinfection.
• Wastewater management: verifying suspended solids removal in secondary and tertiary treatment and supporting discharge permit reporting.
• Pharmaceutical and biotechnology manufacturing: monitoring purified water loops, detecting column breakthrough, and confirming product clarity.
• Food and beverage quality control: assessing beer and wine haze, juice clarity, syrup filtration performance, and finished-product stability.
• Electronics and power industries: controlling particles in ultrapure water systems for semiconductor fabrication and boiler water circuits.
• Environmental monitoring: watershed studies, stormwater assessment, aquaculture management, and limnological research.

FAQ

What is the difference between NTU and FNU?

Both units are defined by formazin calibration. NTU is associated with instruments using a broadband tungsten light source under US EPA Method 180.1, whereas FNU is associated with ISO 7027-1 instruments using an 860 nm infrared source. For most practical purposes the two units are equivalent, but the reporting unit should match the method applied.

Why is the detector placed at 90 degrees to the light beam?

The 90-degree geometry maximizes sensitivity to suspended particles while minimizing interference from the intense unscattered incident beam and from absorption by dissolved substances, delivering the best signal-to-noise performance at low turbidity levels.

How does the scattering method differ from the transmission (attenuation) method?

The scattering method measures light deflected by particles and is superior at low turbidity, whereas the transmission method measures the decrease in directly transmitted light and is better suited to very turbid samples. ISO 7027-1 defines both, and the scattering method is designated for turbidity below approximately 40 FNU.

What interferences can affect the results?

Common interferences include dissolved true color that absorbs at the measurement wavelength, air bubbles and foam, condensation on the cell, dirt or scratches on optical surfaces, stray light in the instrument, and settling or flotation of large particles. Most interferences are controlled through proper sample handling, degassing, cell cleanliness, and the use of infrared optics.

How often should a turbidimeter be calibrated?

Manufacturers and standards typically recommend full multi-point calibration at least quarterly, with independent check-standard verification performed daily or before each analytical batch. Instruments used for critical low-level measurements require more frequent verification.

What is the typical measurement range and detection limit?

Compliance-grade nephelometers cover roughly 0 to 1,000 NTU, with the most accurate range below 40 NTU. High-performance laser nephelometers can detect turbidity below 0.01 NTU, making them suitable for ultrapure water monitoring, while ratio-mode instruments extend reliable measurement to several thousand NTU.

Can colored or viscous samples be tested?

Yes, with precautions. Strongly colored samples are best measured with an 860 nm infrared instrument to suppress color bias, and viscous matrices require validation of sample cell cleanliness, degassing, and homogenization. In complex cases, an accredited third-party laboratory can confirm method suitability before testing.