Measurement of Low Turbidities

F many years, turbidity remaining on the instruments used and on ing in finished water has been used the procedures employed for their calias an indication of the efficiency of the bration. The purpose of this paper is treatment process for removing susto review some of the optical properties pended solids. Hudson 1 has cited a of suspended matter that affect turnumber of examples in which turbidity bidity measurements and to suggest reduction has paralleled reductions of procedures for obtaining more repremicroorganisms, manganese, bacteria, sentative measurements of relatively and viruses. Although the maximum low-turbidity waters. acceptable turbidity in finished water as The measurement of turbidity in a listed in the USPHS drinking water liquid is based upon the property posstandards of 1962,2 is 5 units, 1 unit is sessed by suspended matter of scattersuggested as a more desirable limit for ing light incident upon it. For waters routinely produced water. AWWA having turbidities exceeding approxiGroup 2225 M-Water Quality Crimately 5 Jackson units, transmittance teria 3 has recommended a limit of 0.1 measurements to determine the reducturbidity unit as standard for a hightion in intensity of the incident beam quality finished water. caused by scattering are preferred. For the lower turbidities, which are of Jackson Candle Turbidimeter concern in finished waters, the scattered Measurements made with the J acklight must be measured directly because son candle turbidimeter," an empirical the intensity of the incident light is instrument, are the primary standard not substantially reduced in passing for turbidity in water, although direct through the nearly clear water. measurements are restricted to turMost of the commercial instruments bidities higher than 25 units. For available for measuring low turbidities lower turbidities, the samples are gengive comparatively good indications of erally compared with dilutions of a fuller's earth suspension or with natuthe amount of light scattered in one ral sediments previously standardized particular direction, predominantly at with the Jackson candle turbidimeter. 90° from the incident beam. There This comparison may be made visually is no basis for the current practice of or photometrically. The turbidity calibrating the 90° turbidimeters with values obtained vary widely, dependclays in terms of Jackson units, how-