Small solid particles (minerals, silt, organics, microorganisms) form stabilized colloids after they have adsorbed anions. The light incident on a particle is not transmitted, it is reflected. Depending on the concentration, the mean particle diameter, and particle distribution, turbidity can be observed and measured by using one of two methods: turbidimetry and nephelometry. The latter is widely used. A light beam is directed into the water sample and the light is scattered by the particles (Tyndall effect) and converted to a galvanometric reading. This is compared with a standard such as a formazine polymer colloid (1.25 mg L-1 hydrazine sulfate and 12.5 mg L-1 hexamethylenetetramine) and set as 1 FTU formazine turbidity unit). The colloid standard is used because of its stability and reproducibility (Bratby 1980).
In addition to colloidal solid particles in water, the predominant group of solids is mostly non-settleable. These can be removed by porous membrane filtration of the total solids (TS) sample (pore size: Denmark, 1.6 pm; Germany, 0.45 pm; Henze et al. 2002). The TS are now separated into a more defined solid-in-water colloid, often called "dissolved solids", and a fraction of suspended solids.
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