The Nephelometric Turbidity Unit (NTU) is the international standard used to assess water clarity. This metric quantifies the cloudiness or haziness of a liquid, known as turbidity. NTU provides a standardized optical measurement reflecting the presence of microscopic suspended particles. This allows scientists, water treatment operators, and regulatory agencies to compare water quality across different locations and times.
What Turbidity Represents
Turbidity is the physical property that causes water to appear cloudy or murky. It is caused by suspended solids that interfere with the passage of light. These particles are too small to be seen individually, but their collective presence scatters and absorbs light, creating the hazy appearance.
Sources of these solids are varied, including fine particles like clay and silt from soil erosion, and organic matter like algae, plankton, and microbial organisms. In natural bodies of water, turbidity often increases following storm events that wash runoff and sediment into rivers and lakes. Measuring turbidity is a fundamental part of evaluating water quality, reflecting its chemical, physical, and biological conditions.
Nephelometric Turbidity Units Explained
The “Nephelometric” part of NTU refers to the specific technique used to measure light scatter. This method is based on the principle that suspended particles deflect a beam of light. The NTU scale is derived from measuring the intensity of light scattered at a 90-degree angle from the original light source.
A higher NTU value indicates a greater concentration of suspended particles, signifying greater cloudiness. This measurement reflects the light-scattering intensity caused by the particles, not a direct particle count. The use of the 90-degree angle defines the nephelometric method, differentiating it from methods that measure light blockage.
How NTU Values Are Measured
NTU values are determined using a turbidimeter, also known as a nephelometer. The instrument directs a controlled beam of light through the water sample. A sensitive detector is positioned at a 90-degree angle to the path of the light beam.
When light hits suspended particles, it is scattered, and the detector measures the intensity of this scattered light. The more intense the scattered light detected, the higher the NTU reading, indicating greater turbidity. Instruments must be calibrated using known standards, such as Formazin Polymer, to ensure accuracy and consistency.
Why Water Clarity Matters for Health
Maintaining low NTU levels is a requirement for safe public drinking water. The primary health concern related to high turbidity is its ability to interfere with the disinfection process at water treatment plants. Suspended solids physically act as shields for harmful microorganisms, such as viruses and bacteria, protecting them from chemical disinfectants like chlorine.
If these pathogens are shielded, they can enter the public water supply and potentially cause gastrointestinal illnesses. Due to this risk, regulatory bodies like the U.S. Environmental Protection Agency (EPA) have strict standards for filtered drinking water. The EPA requires that the turbidity of water leaving a treatment plant should not exceed 0.3 NTU in 95% of monthly measurements.