The Origin of the 15-Degree Standard for Drinking Water Color

If you have ever reviewed a drinking water quality report, you have likely encountered the standard that color should not exceed 15 degrees (or 15 TCU). This number appears consistently in regulations worldwide, from China's GB 5749 standard to guidelines in Canada and beyond . But where does this specific figure come from?

The Hazen Scale Origin

The story begins in the late 19th century with an American chemist named Allen Hazen. In the 1890s, Hazen developed a standardized method for measuring the color of water while working with the American Public Health Association (APHA) . He created a visual scale using stable liquid color standards made from platinum-cobalt chloroplatinate solutions .

The scale ranged from 0—representing distilled water as the "water white" baseline—up to a stock solution of 500 units. Intermediate values were obtained by dilution . This method, first published in 1905 in "Standard Methods for the Examination of Water," became known by several names: the APHA scale, the Pt-Co scale, or simply the Hazen scale in honor of its creator .

Why 15 Degrees Specifically?

The selection of 15 degrees as the acceptable limit was not arbitrary—it was based on human perception studies. Research demonstrated that levels of color above 15 TCU (True Color Units) can be detected in a glass of water by most people .

At values below this threshold:

Below 15 Hazen, the color of water in a clear glass container may not be detectable to the average person

When color exceeds 15 degrees, it becomes noticeable and potentially objectionable to consumers

As one Chinese water quality expert explained: "Color greater than 15 degrees can be detected by most people; greater than 30 degrees can be detected by everyone and causes disgust" . This aesthetic concern is fundamental—if water looks unpleasant, consumers may seek alternative, potentially unsafe, sources .

From Perception to Regulation

The 15-degree standard serves multiple purposes beyond aesthetics. Setting the limit at or below this threshold ensures:

Consumer acceptance—Water remains visually appealing

Rapid problem notification—If distribution system issues cause color formation, consumers will notice and report it quickly

Treatment effectiveness—Excess color removal prior to chlorination reduces the production of disinfection byproducts like trihalomethanes