Color (Secondary Standard: 15 “Color Units”)
“Color” in water refers to the well-known yellowish-brownish tint caused by the presence of humus, a complex and variable collection of organic molecules derived from rotting wood and other plant materials containing cellulose, humic acids, tannins and lignins. It is common in surface waters wherever there is a lot of dead vegetation, as in southern swamps, northern bogs, and areas with logging/paper pulp operations. The final product is the result of centuries of microbial transformations, and every molecule is different, but they are all large polymers of benzene rings with hydroxyl, carboxyl, and methoxy groups such as this illustrative but hypothetical example proposed by Christman and Ghassemi
[Chemical Nature of Organic Color in Water,” JAWWA 58:6.722-741.1966]
Color is measured by comparison with a special mixture of cobalt and platinum salts, called the Hazen Standard, which has that tint. 15 Color Units is very pale – just barely discernable without side-by-side comparisons. Some can be removed by activated carbon, but only with difficulty. Treatment with lime or other source of calcium ion will precipitate much of it, for removal by fine-filtration. Flocculation and coagulation with “alum” is even better. RO, UF and NF membranes remove color very well.
Color and Taste & Odor: Many odorous molecules are produced as fragments of color molecules after they are attacked by chemical disinfectants. In addition to the phenols, these include chemicals with names like geraniol (which smells like geraniums), pinene (from pine trees), camphor, cinnamic alcohol, vanillin, etc., which can produce some very exotic mixtures of odors.
Color and Sediment: The vanillin that comes from humus is the same as the vanillin from oak barrels that is so prized in aged wines and spirits. Those products also contain other high molecular weight, humus-like molecules that give an amber hue to brandy and whiskey. The largest of them will precipitate with iron, manganese, calcium, zinc, and other metal ions, which means that highballs made with very hard water may produce ugly sediment in the glass. Cheap liquors whose color comes from charred sugar (caramel) instead of aging in wood do not produce such sediments.