A Simple Color Test of Floc in Treating Colored Water

Abstract

In order to obtain data which would be useful in the design of the 34 million gallon rapid sand filtration plant for the City of Ottawa, Canada, a trial filter was designed by the consulting engineers and operated under their supervision for a seven months' period. This trial filter consisted of two independent units, each including a concrete mixing chamber, settling tank, filter pump, control apparatus, chemical administration equipment and other necessary essentials including a laboratory. Each complete unit was designed to treat 48,000 Imperial gallons per day at the rate of 105 million Imperial gallons (125 million U. S. gallons) per acre per day. In practice one filter was always operated as a control and variations made on the other. After the necessary preliminary tuning up to get both filters operating exactly alike a single change would be made in some feature of the treatment or method of operation in one filter and the results compared with those from the control. Ottawa river water has a color of 65 to 80; a turbidity of 8 to 45; a total hardness of 45 to 70; a pH of 7.0 to 7.4 and an alkalinity of 18 to 38 p.p.m. To obtain good settling and long filter runs two things were necessary. The first was the conversion of all the precipitated material into well formed settleable floe; the second was providing suitable conditions under which most of this floe could settle out. During the first three months work on the trial filter the filter runs were short, settling seemed poor and the amount of floe passing over to the filters large. There was no simple rapid method of determining how much floe was actually settling in the sedimentation tanks or passing over to the filters. It soon became evident that if we were to understand the process in detail we had to find a rapid method of estimating the quantity of floe in samples of the water taken at any point. The gravimetric method was too slow and cumbersome for our purpose and would have been useless for checking or controlling the process at any stage.