Turbidity is basically a measurement of cloudiness in a water sample. It is caused by suspended matter, such as clay, silt, fine particles of organic and inorganic matter, and microscopic organisms. A turbid sample may not appear clear to the naked eye. Many things depending on the water source may cause turbidity. If you are dealing with surface water turbidity may be increase by soil erosion, waste discharge, runoff, stirred up sediment due to bottom feeders, microorganisms, and algae growth. Groundwater has higher turbidity for entirely different reasons. Groundwater typically will contain minerals from the geologic formations, and high levels of iron, manganese and tannins can cause the water to be discolored thus a higher turbidity.
How Is It Regulated?
The World Health Organization says that the turbidity should be no higher 5 NTU but ideally under 1 NTU. EPA’s surface water treatment rule requires systems using surface water or groundwater under the direct influence of surface water to 1.) disinfect their water and 2.) filter the water or meet the following criteria for avoiding filtration so the following contaminants are controlled at the following levels. Turbidity: at no time turbidity can go above 5 NTU. Systems that filter must ensure the turbidity goes no higher than 1 NTU (0.5 NTU for conventional or direct filtration) in at least 95% of samples taken in any month. As of January 1, 2002, turbidity must not exceed 1 NTU and 0.3 NTU in at least 95% of samples in any month.
How Do We Measure It?
The instrument used to measure turbidity is called a turbidimeter or nephelometer. These meters measure the intensity of light scattered at 90 degrees as a beam of light is passed through the water sample. The instrument can be a sophisticated laboratory meter, a simple handheld unit or equipment design for in-line or constant measurement.
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