Turbidity is a measure of the cloudiness or haziness of water caused by suspended particles. It is a key indicator of water quality and treatment effectiveness.
Turbidity is an optical property of water that describes the degree to which light is scattered and absorbed by suspended and colloidal material including clay, silt, organic matter, algae, and microorganisms. It is measured using a nephelometer, which detects light scattered at a 90-degree angle, and reported in nephelometric turbidity units (NTU). Turbidity is distinct from but related to total suspended solids and water color, as it depends not only on particle concentration but also on particle size, shape, and refractive index. In drinking water treatment, turbidity serves as a critical operational parameter: the Surface Water Treatment Rule requires that filtered water turbidity not exceed 1 NTU and must be 0.3 NTU or lower in 95 percent of daily samples. Low turbidity is essential for effective disinfection because particles can shield pathogens from chemical and UV disinfection. In natural waters, high turbidity reduces light penetration for photosynthesis, smothers benthic habitats, and can transport adsorbed pollutants. Continuous turbidity monitoring using online instruments is standard practice in water treatment plants, with real-time data used to optimize coagulant dosing and filter operations.