Determination of Odor and Taste in Surface Water: Testing Methods, Standards, and Applications

Odor and taste determination is a fundamental organoleptic testing procedure used to evaluate the sensory quality of surface water destined for drinking water supply, recreation, and aquaculture. This article explains the testing principles, applicable standards such as GB/T 5750.4, EN 1622, and ASTM D1296, sampling and analysis procedures, and the practical applications of odor and taste monitoring in surface water quality programs.

Technical Overview

Odor and taste are among the most immediate and sensitive indicators of surface water quality. Because the human nose and palate can detect certain odorous compounds, such as geosmin and 2-methylisoborneol (2-MIB), at nanogram-per-liter concentrations, sensory testing often reveals water quality deterioration before routine chemical analysis does. For this reason, odor and taste determination is included in national and international water quality frameworks as a basic organoleptic parameter.

In surface water, objectionable odors and tastes typically originate from several sources:

• Algal and cyanobacterial blooms producing earthy, musty, and grassy odorous metabolites such as geosmin and 2-MIB
• Actinomycetes and other microorganisms in sediments and biofilms
• Decomposing vegetation and natural organic matter
• Industrial discharges containing phenols, petroleum hydrocarbons, amines, or sulfur compounds
• Agricultural runoff carrying pesticides and fertilizer residues
• Disinfection by-products formed during downstream water treatment

Regulatory frameworks, including the Chinese surface water environmental quality standard GB 3838 and the drinking water quality criterion GB 5749, effectively require source and treated waters to be free from objectionable odor and taste. Sensory testing therefore serves both as a compliance checkpoint and as an early-warning tool for source water protection.

Testing Samples

Rivers and Streams

Grab samples are collected from representative cross-sections, typically at intake points, upstream reference locations, and downstream of suspected discharge zones. Flowing water bodies require samples that reflect the main current rather than stagnant edges.

Lakes and Reservoirs

Reservoir and lake samples are taken at designated monitoring stations, often at multiple depths, because thermal stratification and algal vertical migration can create strongly localized odor episodes. Samples near water treatment plant intakes are prioritized.

Source Water for Water Treatment Plants

Raw water entering treatment works is tested routinely to guide processes such as powdered activated carbon dosing, pre-oxidation, and ozonation. Rapid sensory feedback allows operators to respond to sudden taste-and-odor events.

Recreational and Aquaculture Water

Surface water used for bathing, fisheries, and aquaculture is assessed to ensure that odorous compounds do not impair usability or transfer taints to aquatic products.

Testing Items

Odor and taste determination encompasses the following measurable and descriptive parameters:

• Odor character at ambient temperature, described qualitatively (e.g., earthy, musty, grassy, fishy, chlorine-like, medicinal, sulfide, hydrocarbon, septic)
• Odor character at elevated temperature (approximately 60 °C), which intensifies the release of volatile and semi-volatile odorants
• Odor intensity grade, commonly reported on a descriptive scale from "none" to "very strong"
• Taste character and intensity, assessed only for water confirmed or presumed safe for ingestion
• Threshold Odor Number (TON), a quantitative dilution-based measure of odor strength
• Flavor Profile Analysis (FPA), a panel-based technique that identifies and rates individual sensory attributes

Testing Methods

Qualitative Sensory Description Method

The standard approach described in GB/T 5750.4 requires trained examiners to evaluate a well-mixed sample in a stoppered flask, first at room temperature and again after warming to approximately 60 °C. The examiner records the odor character in standardized descriptive terms and assigns an intensity grade. Taste, where appropriate, is assessed by taking a small amount of water into the mouth without swallowing. The 60 °C determination is particularly important because many odorants become far more perceptible at elevated temperature.

Threshold Odor Number (TON) Dilution Method

For quantitative results, the sample is serially diluted with odor-free dilution water and presented to a panel of trained assessors in a blind sequence. The dilution at which odor is just detectable by the panel is identified, and the Threshold Odor Number is calculated as the ratio of the total volume (sample plus dilution water) to the volume of original sample. Higher TON values indicate stronger odor. This principle is consistent with EN 1622, ASTM D1296, and the dilution procedures in Standard Methods for the Examination of Water and Wastewater.

Flavor Profile Analysis (FPA)

FPA, as described in Standard Methods (Section 2170), uses a panel of at least four trained assessors who independently identify individual sensory attributes and rate their intensity on a numerical scale. Results are discussed and consensus scores are recorded, providing a detailed sensory fingerprint that helps trace odor sources and evaluate treatment performance.

Quality Assurance and Safety Considerations

• Assessors must be trained, screened for sensory acuity, and free from colds or other conditions affecting smell and taste
• Dilution water must be odor-free, typically prepared by distillation or passage through activated carbon
• Testing is performed in a clean, well-ventilated, odor-free environment free from interfering cosmetics, detergents, or tobacco smoke
• Samples are collected in glass containers with minimal headspace, kept cool, and analyzed preferably within 24 hours
• Taste testing is performed only on water known or reasonably presumed to be safe; suspected chemically contaminated water must never be tasted

Accredited third-party laboratories routinely perform these organoleptic tests alongside chemical and biological analyses, integrating sensory results with chromatographic screening for compounds such as geosmin, 2-MIB, and haloanisoles to establish cause-and-effect relationships.

Applications

Odor and taste determination supports a wide range of surface water management activities:

• Compliance monitoring of drinking water sources against surface water and drinking water quality standards
• Early warning during algal bloom and cyanobacteria-driven taste-and-odor episodes in reservoirs
• Investigation of pollution incidents involving industrial spills, illegal discharges, or fish kills
• Optimization of water treatment processes, including activated carbon dosing, ozone and advanced oxidation, and membrane filtration
• Assessment of wastewater discharge impacts on receiving water bodies
• Evaluation of ecological restoration projects and long-term trend monitoring of lakes and rivers
• Verification of water safety for aquaculture, irrigation, and recreational use

FAQ

What is the Threshold Odor Number (TON)?

TON is a quantitative expression of odor strength obtained by diluting a sample with odor-free water until the odor is just barely detectable. It equals the total diluted volume divided by the volume of original sample, so a TON of 4 means the sample had to be diluted fourfold before the odor disappeared.

Why is odor evaluated at both room temperature and 60 °C?

Many volatile and semi-volatile odorants are only weakly perceptible at ambient temperature. Heating the sample to approximately 60 °C increases volatilization and enhances sensory detection, making the warm-water determination a more sensitive screening step.

Is taste testing always performed?

No. Taste assessment is conducted only when the water is known or strongly presumed safe for ingestion. Water suspected of chemical, microbiological, or toxin contamination must never be tasted; in such cases odor testing and instrumental analysis are used exclusively.

Which standards govern odor and taste determination?

Commonly referenced methods include GB/T 5750.4 for organoleptic examination, EN 1622 for the determination of taste and odour in water, ASTM D1296 for odor evaluation, and the dilution and FPA procedures in Standard Methods for the Examination of Water and Wastewater.

How should samples be collected and preserved for sensory testing?

Samples should be collected in clean glass bottles, filled with minimal headspace, sealed tightly, kept at low temperature, and analyzed as soon as possible, ideally within 24 hours, to prevent loss or alteration of odorous compounds.

How are results reported?

Odor and taste results are reported as a descriptive character plus an intensity grade, and, where dilution testing is performed, as a numerical Threshold Odor Number. FPA results additionally list each perceived attribute with a consensus intensity rating.