Turbidity is a measure of the clarity or cloudiness of water, indicating the presence of suspended particles. It reflects how much light is scattered or absorbed by these particles when light passes through a water sample. Evaluating turbidity is a fundamental aspect of assessing water quality in various environments. This measurement offers insights into the physical characteristics of water.
Understanding Turbidity and Its Importance
Turbidity in water can originate from various sources, including suspended solids like silt, clay, and organic matter. Microscopic organisms such as algae and plankton also contribute to water cloudiness. Human activities like agricultural runoff, urban stormwater discharge, construction, and mining can introduce significant amounts of sediment and pollutants into water bodies, increasing turbidity.
Measuring turbidity is important across different sectors. In drinking water treatment, high turbidity can shield harmful microorganisms from disinfection, posing public health risks. For aquatic ecosystems, increased turbidity reduces light penetration, hindering photosynthesis for submerged plants and affecting fish oxygen absorption. Industrial processes, such as brewing or pharmaceuticals, also monitor turbidity to ensure product quality and prevent equipment issues.
Traditional Measurement Approaches
Historically, simpler methods were used to estimate water clarity. The Secchi disk, a circular disk typically 20 to 30 centimeters in diameter, is lowered into the water until it disappears from view. The depth at which it vanishes is then recorded as the Secchi depth, providing an inverse measure of water clarity.
While straightforward and inexpensive, the Secchi disk method has limitations. Readings can be subjective, varying between observers due to eyesight and perception. Environmental factors like sunlight intensity, glare, and wave action also influence accuracy. The method indicates overall water transparency but does not differentiate particle types. Another older method, the Jackson Candle Turbidimeter, involved visually comparing water clarity against a candle flame, yielding Jackson Turbidity Units (JTU).
Modern Instrumental Measurement
Modern turbidity measurement relies on instruments called turbidimeters, which operate on the principle of nephelometry. These devices direct a beam of light through a water sample. Suspended particles within the sample scatter this light in various directions.
A detector positioned at a 90-degree angle to the incident light beam measures the intensity of the scattered light. This 90-degree scattering measurement, known as nephelometry, is effective for assessing low to moderate turbidity levels. The amount of scattered light is directly proportional to the concentration and size of the suspended particles. Turbidimeters are calibrated using formazin, a synthetic polymer suspension, to ensure accurate and consistent readings. These instruments may utilize different light sources, including white light or infrared LEDs.
Interpreting Turbidity Readings
Turbidity readings are typically expressed in specific units. The most common units are Nephelometric Turbidity Units (NTU) and Formazin Nephelometric Units (FNU). NTU is widely used in the United States, often associated with the U.S. Environmental Protection Agency (EPA) methods, while FNU is prevalent in Europe, adhering to ISO standards. Both NTU and FNU measurements are based on light scattered at a 90-degree angle.
An older unit, the Jackson Turbidity Unit (JTU), originated from the visual Jackson Candle method but is less precise than modern instrumental measurements. In practical terms, a higher NTU or FNU value indicates greater turbidity. For drinking water, ideal turbidity levels are typically very low, often aimed at less than 1 NTU.