DEFINITIONS, UNITS, AND MEASUREMENT METHODS

ERDC TN-DOER-E8

June 2000

Although popularly called suspended solids, this method is more accurately called nonfilterable

solids (or residue), because the size of separation (about 0.45 m) is not the same as the boundary

between suspended and dissolved solids, which varies among molecules but is generally around

0.1 m.

Turbidity is an optical property of water that causes light to be scattered and absorbed rather than

transmitted in straight lines through the sample. It is caused by the molecules of water itself,

dissolved substances, and organic and inorganic suspended matter. Devices commonly used to

measure turbidity include the Jackson candle turbidimeter, absorptimeters, transmissometers, and

nephelometers (McCarthy, Pyle, and Griffin 1974). All but the nephelometer measure the effects

of both absorption and scattering of light. The nephelometer measures scattered light only, and is

the most commonly used device in colloidal chemistry, potable water treatment, and water quality

management.

Nephelometers use a device such as a photomultiplier tube or silicon photodiode to measure light

that has been scattered at a specific angle, usually 90 deg, from the main light path. The light source

is usually a tungsten filament lamp or a light-emitting diode, and the light path is designed to

minimize stray light falling on the detector. Thus, a zero signal means no light scattered at 90 deg

from the main light path and implies no turbidity.

The ability of a particle to scatter light depends on the size, shape, and relative refractive index of

the particle and on the wavelength of the light (Lillycrop, Howell, and White 1996). The reading

on the instrument depends on many design parameters, including the light source, detector, electrical

circuit, sample container, and optical arrangement. Therefore, two samples with equal suspended

solids concentrations but different size distributions of particles will produce very different turbidity

readings on the same nephelometer; and two different nephelometers may produce different

turbidity readings on the same sample, even if they were calibrated on the same standard (Vanous

1978; APHA et al. 1992; Hach 1972). Although the original Jackson candle turbidimeter was

standardized with a specific fine silica suspension in which one Jackson Turbidity Unit (JTU)

equaled 1.0 mg/L of TSS, modern turbidimeters are no longer standardized against the Jackson

candle, and the term JTU is no longer used.

Modern turbidimeters are standardized against a formazin suspension with a value of

40 Nephelometric Turbidity Units (NTU). The standards should be prepared according to Standard

Methods (APHA et al. 1992). The 400-NTU stock suspension should be prepared monthly, and the

40-NTU standard turbidity suspension should be prepared daily. Experience shows that this

turbidity can be repeatedly prepared within an accuracy of 1 percent (Hach 1972). The formazin

turbidity standard is assigned a value of 40 NTU and can be diluted to any desired value. It reads

approximately 40 JTU on the Jackson candle turbidimeter, but is not exact. The Jackson candle

turbidimeter is no longer an accepted standard method (APHA et al. 1992).

The Secchi disk is a white disk, 20 to 30 cm in diameter, commonly used by oceanographers and

limnologists to indicate water clarity (the opposite of turbidity) instead of measuring turbidity. The

disk is lowered into the water to the depth where the sharp outline disappears into a formless glow.

The depth is then recorded. This method is quick, easy, and cheap, but is somewhat subjective and