Water Odor Evaluation Methods: Sensory Assessment, Threshold Odor Number Testing, and Instrumental A

Water odor evaluation is a fundamental component of drinking water quality management, combining standardized sensory panel techniques with trace-level instrumental analysis to detect earthy, musty, chlorinous, and sulfurous odors that consumers can perceive at nanogram-per-liter concentrations. This article provides a comprehensive overview of water odor assessment methods, covering recognized standards such as ASTM D1292, Standard Methods 2150, and GB/T 5750.4, applicable sample types, key testing parameters, and practical applications for water utilities and accredited third-party laboratories.

Technical Overview

Odor is one of the most sensitive and immediately perceptible indicators of water quality. The human olfactory system can detect certain naturally occurring compounds, such as geosmin, at concentrations as low as 5 to 10 nanograms per liter, far below the detection limits of many routine chemical analyses. As a result, consumers frequently judge water quality primarily by its smell and taste, making odor evaluation a critical element of water treatment optimization, distribution system management, and regulatory compliance.

Odor in water originates from a variety of sources, including biological activity, treatment processes, and distribution system conditions. Effective evaluation requires a combination of sensory testing performed by trained assessors and instrumental analysis targeting specific odor-causing compounds. Key reference frameworks include ASTM D1292 (Standard Test Method for Odor in Water), Standard Methods 2150 and 2170, GB/T 5750.4 for the examination of drinking water in China, and the general sensory analysis guidance provided in the ISO 6658 and ISO 8586 standards.

• Biological sources: cyanobacteria and actinomycetes producing geosmin and 2-methylisoborneol (2-MIB)
• Treatment-related sources: chlorine, chloramine, and disinfection byproducts such as halophenols and aldehydes
• Distribution system sources: biofilm growth, pipe corrosion, and sulfur-reducing bacteria generating sulfide odors
• Anthropogenic sources: industrial solvents, petroleum hydrocarbons, and agricultural runoff

Testing Samples

Drinking Water Samples

Finished drinking water from treatment plants, distribution networks, household taps, bottled water, and purified water systems represents the most common sample category. These samples are evaluated against aesthetic criteria specifying that water should be free from objectionable odor, as required by standards such as GB 5749 and the guidance of the World Health Organization.

Source Water Samples

Surface water from rivers, lakes, and reservoirs, as well as groundwater, is tested to characterize baseline odor conditions and to provide early warning of algal bloom events. Seasonal monitoring of source water is particularly important during warm months when cyanobacterial activity peaks.

Treatment Process Samples

Samples collected at successive treatment stages, including raw water, sedimentation effluent, filtered water, and finished water, allow utilities to quantify odor removal efficiency across each unit process and to optimize dosing of powdered activated carbon, ozone, or other oxidants.

Wastewater and Reclaimed Water Samples

Treated effluent and reclaimed water destined for reuse applications are evaluated for sulfurous, septic, and medicinal odors that may affect public acceptance or indicate process instability.

Sample Collection and Preservation

Proper sample handling is essential because odor characteristics change rapidly. Samples should be collected in clean glass bottles with PTFE-lined caps, filled headspace-free, kept at 4 degrees Celsius during transport, and analyzed as soon as possible, preferably within 24 hours. Plastic containers should be avoided because they may leach odor-causing compounds or adsorb volatile substances.

Testing Items

Water odor evaluation typically encompasses the following parameters, which may be reported individually or as an integrated sensory profile:

• Odor character and descriptor analysis (earthy, musty, grassy, fishy, chlorinous, medicinal, sulfurous, septic, hydrocarbon-like)
• Odor intensity rating on a graded descriptive scale from imperceptible to very strong
• Threshold Odor Number (TON), expressed as the dilution ratio at which odor is just detectable
• Taste and palatability assessment for confirmed potable samples only
• Quantification of specific odorants, including geosmin, 2-MIB, haloanisoles, halophenols, aldehydes, and reduced sulfur compounds
• Temperature-dependent odor evaluation at ambient and elevated temperatures to reveal masked or temperature-activated odors

Testing Methods

Threshold Odor Number (TON) Method

The TON method, described in Standard Methods 2150 and consistent with ASTM D1292, is the most widely used quantitative odor test. A measured volume of sample is serially diluted with odor-free water until the odor is just detectable by the assessor. The TON is calculated as the total volume of the mixture divided by the volume of the original sample. For example, a sample diluted to 25 milliliters in a total of 100 milliliters yields a TON of 4. Testing is commonly performed at approximately 40 to 60 degrees Celsius to enhance volatility and improve detection sensitivity.

Flavor Profile Analysis (FPA)

FPA, standardized in Standard Methods 2170, employs a trained panel of at least four assessors who independently describe the odor character and rate its intensity on a numerical scale, typically from 0 indicating no perceptible odor to 12 indicating a very strong odor. Panelists compare notes and reach a consensus profile. FPA provides richer diagnostic information than TON because it identifies odor types rather than intensity alone, making it valuable for tracing odor sources.

Flavor Rating Assessment (FRA)

FRA, outlined in Standard Methods 2160, evaluates the acceptability or palatability of drinking water using a structured rating scale. It is suitable for assessing overall consumer acceptability and is applied only to water that has been confirmed safe for ingestion.

Instrumental Analysis of Specific Odorants

Because sensory methods cannot identify the chemical cause of an odor, instrumental techniques are used to confirm specific compounds. Solid-phase microextraction coupled with gas chromatography-mass spectrometry (SPME-GC-MS) achieves detection limits near 1 nanogram per liter for geosmin and 2-MIB. Gas chromatography-olfactometry (GC-O) combines chemical separation with human sniffing to link individual chromatographic peaks with perceived odor, while emerging electronic nose systems provide rapid screening for process monitoring.

Quality Assurance

Reliable sensory results depend on controlled conditions: assessors trained in accordance with ISO 8586, an odor-free testing environment free from perfumes and cleaning agents, reference odor-free water prepared by activated carbon filtration and deionization, and replicate evaluations to establish statistical confidence. Blanks and positive controls using standard odorant solutions should be included in each testing session.

Applications

Water odor evaluation supports a broad range of operational, regulatory, and research objectives across the water industry:

• Treatment optimization: adjusting powdered activated carbon, ozone, or biological activated carbon dosing during taste and odor episodes
• Early warning systems: detecting cyanobacterial blooms in reservoirs before odor reaches the distribution network
• Customer complaint investigation: identifying the compound and source responsible for reported odors at the tap
• Regulatory compliance: demonstrating conformance with sensory requirements in drinking water standards such as GB 5749
• Reclaimed water programs: verifying aesthetic quality for non-potable and indirect potable reuse schemes
• Materials testing: evaluating odor leaching from pipes, fittings, coatings, and membranes in contact with drinking water
• Research and development: assessing the performance of new adsorbents, oxidants, and biological treatment processes

FAQ

Question: What is the Threshold Odor Number (TON)?

Answer: TON is a quantitative measure of odor intensity defined as the dilution factor at which a water sample must be diluted with odor-free water for the odor to be just barely detectable. A higher TON indicates a stronger odor. Many utilities operate with an internal target of a TON of 3 or lower for finished drinking water.

Question: What causes earthy and musty odors in drinking water?

Answer: Earthy and musty odors are most commonly caused by geosmin and 2-methylisoborneol, metabolites produced by cyanobacteria and actinomycetes in source waters. These compounds have extremely low human odor thresholds and are resistant to conventional treatment, often requiring adsorption with activated carbon or oxidation with ozone.

Question: At what temperatures is odor evaluated?

Answer: Sensory evaluation is typically performed at ambient temperature, around 20 degrees Celsius, and at an elevated temperature of approximately 40 to 60 degrees Celsius. Warmer testing increases the volatility of odor compounds and reveals odors that may not be noticeable at room temperature.

Question: How are trace odorants such as geosmin measured in the laboratory?

Answer: Accredited third-party laboratories and research facilities use solid-phase microextraction or closed-loop stripping followed by gas chromatography-mass spectrometry, achieving quantification limits near 1 nanogram per liter, well below human perception thresholds.

Question: Is taste testing of water samples safe?

Answer: Taste testing is performed only on water that has been confirmed chemically and microbiologically safe through prior analysis. For samples of unknown or suspect quality, odor evaluation alone is used, and assessors never ingest the sample.

Question: How frequently should odor monitoring be conducted?

Answer: Water utilities commonly perform routine odor checks daily or weekly at treatment plants and increase monitoring frequency during algal bloom seasons, after treatment changes, or in response to consumer complaints. Source waters prone to cyanobacterial activity benefit from continuous or high-frequency sensory and instrumental surveillance.