Effects of chemical addition on the performance of settling tanks

In the treatment of wastewater, sedimenta tion is more widely used than any other process. Primary settling tanks are designed to remove inorganic and organic settleable solids from wastewater and often act as flow equalization tanks; although they are nor mally not designed for this purpose. Of all wastewater treatment processes used, primary settling tanks probably provide the best return in treatment achieved for the capital invested. In spite of the importance of the sedimenta tion process, relatively little work has been done in recent years to improve the design methods and performance. A review of pub lished work was done in 1964.l Since then, gravity thickening has received considerable attention but there have been relatively few articles published on sedimentation in dilute suspensions.2""15 In recent years many countries have re quired phosphorus (P) removal from waste water. This has resulted in the widespread use of chemical precipitation in treatment. By necessity, in primary treatment plants, and by choice, in a considerable number of secondary treatment plants, chemicals, such as ferric chloride, alum, lime, and polymers, are added ahead of primary settling tanks for the purpose of reducing P concentration to acceptable levels. Increased solids and bio chemical oxygen demand (bod) reductions have been experienced by several plants con current with P removal. The addition of such chemicals ahead of primary settling tanks will create floes. These, together with settle able solids in wastewater, will settle at dif ferent rates to normal gravity settling. The performance of existing settling tanks will therefore be affected, and design criteria for settling tanks operating with chemical addi tion should be reexamined. This paper presents the effects of chemical additions on the performance of primary settling tanks at Sarnia, Ont., and Windsor Ont., wastewater treatment plants, and a pilot plant at the Canada Center for Inland Waters, Burlington, Ont. The results are based on full-scale plant investigations at varying rates of flow and on tube settling tests. The work was carried out over a period of 4 years. Results of velocity and suspended solids con centration measurements in the tanks are reported in a subsequent paper. Based on the results of this study tentative design criteria are proposed for primary settling tanks treating domestic wastewater with chemical addition.