Determining turbidity is extremely important for guaranteeing water quality for a wide variety of purposes in a water treatment plant. For example, the lack of or incorrect monitoring and control of Turbidity can increase the consumption of chlorine for disinfection, increase the consumption of coagulants, as well as the frequency of filter backwashing, leading to higher operating costs.
Turbidity determination, using the nephelometric method, is used in pollution control operations and to check the physical parameter in drinking water, as well as to monitor the efficiency of filtration and ultra-filtration systems in quality processes.
The technique is based on measuring the amount of suspended solid material from light scattered at an angle of 90° to an incident light beam. This angle is used because it offers better sensitivity and less interference with variations in particle size and mass.
Turbidity is directly related to the amount and intensity of light scattered by the analyzed sample. The turbidimeter is the device used for the reading, and Turbidity is expressed in Nephelometric Turbidity Unit (NTU).
There are two widely used standards for standardizing the methodology of Turbidity analysis: ISO 7027 (European) and EPA 180.1 (North American).
The emitted light source is subject to variations in power. To compensate for this variation, transmittance is also measured and the ratio between the signal obtained from each photocell eliminates the errors arising from these variations.
When turbidity is high (typically greater than 1000 NTU) the high concentration of particles tends to cause greater scattering at an angle other than 90°, so it is necessary to include a photocell at an angle of 138° in order to increase sensitivity in higher Turbidity ranges.
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