Water & Wastewater Treatment Magazine
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Top tips for… turbidity monitoring The impact that high levels of turbidity and suspended solids can have on the aquatic environment makes it a particularly important parameter to measure. Here, Dr John Gaffney, Continuous Water & Gas Analyser Product Manager at ABB, explains why and outlines some top tips for achieving maximum performance from turbidity measurement equipment. The Knowledge www.wwtonline.co.uk | WWT | JULY 2018 | 35 Caption if needed P rotecting the UK's waterways against pol- lution is a full-time job, making continuous, accurate measurement of potential con- taminants an essential part of any water treatment process. Turbidity is a particularly important parameter to meas- ure because of the impact suspended solids can have on the aquatic environment. When measuring turbidity, one of the first challenges is to define a unit of turbidity that can be used to compare measurements from different devices. Turbidity units have no physical value, making it difficult to compare values, especially where they are measured using different devices and different methods of measurement. Two standards aimed at helping the production of turbidimeters that are comparable to each other are the EPA 180.11 method and the ISO7027 standard. Both standards use a scattering angle of 90°, which is called nephelometric arrangement, but each of them uses a differ- ent wavelength and different prescriptions on the geometry of the illumination and detec- tion. The EPA standard was designed to harmonize the de- sign of turbidimeters that used incandescent light sources, with illumination wavelengths peaking in the green region of the spectrum. These devices work well at low turbidity values, owing to the shorter wavelength. EPA measurements are expressed in Nephelometric Turbidity Units, or NTU. The ISO standard, on the other hand, stipulates the use of light in the near infrared region. Using a longer wavelength reduces the ef- fect of absorption by organic contaminants on the meas- urement significantly. ISO measurements are expressed in Formazin Nephelometric Units, or FNU. Whichever method is used, certain key steps can be taken to ensure that sensors used for measuring turbidity and TSS offer maximum performance and accuracy.