Turbidity (Primary Standard: less than 1.0 NTU generally, or 0.5 NTU where – “direct filtration” is used, 95% of the time during disinfection; maximum of 5.0 NTU at any time)
Turbidity is cloudiness or haziness in water, caused by a dispersion or scattering of light by dissolved particles that are the same size as the wavelengths of fight used to illuminate them. (The visible part of the light spectrum includes light with wavelengths from about 0.40 microns for violet light to about 0.77 microns for red light.) It is measured by analyzing the scattered light intensity at a right angle or 90° to the path of the test light beam.- This technique is called nephelometry, and that is the source of the “N” in the NTU unit of turbidity.
The particles that cause turbidity may be clear or opaque, light or dark colored, crystalline or amorphous, made of various chemical compositions. Some particles may reflect light; others may absorb or refract the light, and it is therefore impossible to convert turbidity values to concentrations of sediment in mg/L. However, most turbidity is just “dirt” and “dust; composed of alumina or silica or alumino-silicate – the most prevalent minerals in the Earth’s crust and the stuff of nearly all rocks except limestone. Thus, an analysis of turbid water nearly always shows aluminum, and it’s difficult to distinguish between this naturally occurring aluminum and aluminum floc (from the coagulation step in the treatment train used by farge water works) that has gotten past the filters. That is unfortunate, because blobs of floc may well contain dangerous levels of toxic or infectious sediment. (That is the purpose of flocculation/coagulation – to concentrate turbidity and dirt by agglomeration so they can be removed more efficiently by the large granular bed filters.)
Turbidity is the only contaminant that must be determined daily (every four hours at a minimum; preferably, continuously in “real time”) by waterworks operators, because low turbidity is necessary for effective disinfection. Pathogens are easily shielded from the effects of disinfectants by particles and sediment in the water. But the turbidity measured at the treatment plant bears no resemblance to the turbidity that exists at the far reaches of the distribution system. There, many decades of sedimentation, biological growths, and corrosion products accumulate, become encrusted, and slough off again to produce a steady supply of new particles which clog filters, score valves, and cause foul tastes and odors. This is the turbidity that matters to our products.