Compounds in water that are perceived as giving it a taste are generally inorganic substances present in concentrations much higher than those of organic pollutants. The salt concentration in water should be approximately the same as in saliva for the water to taste neutral (1). The concentrations of sodium, magnesium, and calcium chloride at which 50% of a tasting panel found an offensive taste were 465, 47, and 350 mg/litre, respectively (2).

Of the ions that may be present in water, iron can be tasted in distilled water at a concentration of about 0.05 mg/litre, copper at about 2.5 mg/litre, manganese at about 3.5 mg/litre, and zinc at about 5 mg/litre (3). Iron, in particular, is suspected of affecting the taste of water in practice (4).

Organic compounds can cause organoleptic effects in water (5) at taste and odour threshold concentrations that can vary from milligrams to nanograms per litre. The compounds concerned include humic substances, hydrophilic acids, carboxylic acids, peptides and amino acids, carbohydrates, and hydrocarbons (6). 

The organisms most often linked to taste and odour problems are actinomycetes and various types of algae, but other aquatic organisms, such as protozoa and fungi, have been implicated from time to time. 

Earthy-musty tastes and odours are produced by certain cyanobacteria (blue-green algae), actinomycetes, and a few fungi. The substances produced by actinomycetes and cyanobacteria that cause tastes and odours in drinking-water have been extensively discussed in the literature (7–10) and include geosmin, methyl-isoborneol (MIB), and cardin-4-ene-1-ol. Growing algae produce numerous volatile and nonvolatile organic substances, including aliphatic alcohols, aldehydes, ketones, esters, thioesters, and sulfides. 

Occasionally, taste and odour problems in water are caused by other bacteria, fungi, zooplankton, and nemathelminthes. Ferrobacteria in water-distribution systems may produce tastes and odours (11), and some species of Pseudomonas can cause a swampy odour (12), whereas others can convert sulfur-containing amino acids into hydrogen sulfide, methylthiol, and dimethylpolysulfide (13).

Halogenated hydrocarbon solvents are the synthetic contaminants of drinking-water most frequently found because of the huge amounts produced, and their very diffuse use, chemical and biological stability, volatility, and negligible adsorption by soil and sediments.

Water treatment often includes storage, slow sand filtration, or activated carbon filtration. Microorganisms can grow in the equipment used for these purposes and can then cause tastes and odours. The biological degradation of organic compounds in raw water can also lead to the production of substances such as phenols, aldehydes, and alkylbenzenes that cause taste and odour problems (14). In addition, the chemicals used in water treatment as coagulants, oxidants, or disinfectants can interact with organic compounds in water and occasionally produce tastes and odours.

Ozone is one of the most efficient agents in removing tastes and odours, but its use can lead to the formation of intermediate reaction products (15). In particular the formation of aliphatic aldehydes, which has been frequently reported in the literature (16), leads to the development of fruity, fragrant, and orange-like odours (17).

The free halogens used as water disinfectants can produce undesirable tastes and odours in the water. For chlorine residuals, threshold values vary significantly with pH: 75 µg/litre at pH 5 as compared with 450 µg/litre at pH 9 (18). Hypochlorous acid, hypochlorite ion, monochloramine, and dichloramine have odour thresholds ranging from 0.15 to 0.65 mg/litre (19).

Taste and odour problems that develop in water-treatment plants are frequently an indirect consequence of chlorination (20). The odour threshold values of chlorinated by-products are generally significantly lower than those of the original products; for example, odour thresholds for phenol, 4-chlorophenol, and 2,4-dichlorophenol are 1000–5000, 0.5–1200, and 2–210 µg/litre, respectively.