Treatment of Colored and Corrosive Water at Ottawa, Ontario

Abstract

In a previous paper in this Journal (29, 1088 (1933)), an outline was given of the method of treatment of the Ottawa river water at the city of Ottawa water purification plant and some experiences and difficulties of the first year of operation were discussed. It is now five years since this plant was completed and operating and control methods are more firmly established on a routine basis, as will be described in the present paper. The problem is chiefly one of color removal, the processes involved consisting of coagulation followed by settling, rapid sand filtration, corrective treatment with lime and sterilization with chlorine. The source of supply is the Ottawa river, an ample supply of soft colored water of the following characteristics: pH, 6.9 to 7.4; color, 40 to 80; turbidity, 7 to 11 (except for about two weeks during the spring run-off when this increases to as much as 100 p.p.m.); alkalinity, 15 to 38 p.p.m. ; and total hardness, 35 to 70 p.p.m. The coagulant used is alum, the dosage varying from 2| to 2f grains per Imperial gallon, depending upon the raw water conditions. A minimum mixing time of 40 minutes and a minimum settling time of 3 hours and 20 minutes is allowed, the average time for each being twice the minimum. After settling, the water passes through rapid sand filters designed to pass 108 million (Imperial) gallons per acre per day, the average being about one-half this standard rate. This treatment reduces the color of the water after filtration to about 3; and, although the natural alkalinity of the raw water is comparatively low, residual alumina is at a minimum, averaging not more than 0.10 to 0.15 p.p.m. The pH of the filter effluent varies from 5.5 to 6.1. This acidity is corrected by the addition of lime before the water is pumped to the city.