Several schemes for separating solid pollutants from water flowing in a closed conduit were examined. Separation based on the momentum differential between the solids and liquids and the centrifugal movement of a conveying liquid were both examined under turbulent flow conditions. The study indicated these processes to have limited applications, primarily due to the difficulties in withdrawing a significant fraction of separated flow and the deleterious effects of turbulent mixing. However, utilization of the differential momentum of solid particles and water when flowing laminarly through a pipe was found to be a viable separation scheme. Significant experimental separations (greater than 50 percent for pulverized coal and fly ash) were achieved. The experimental situation was modeled theoretically using particles approaching a sink located in a uniform stream. The theory closely predicted observed results for low velocity flows, with increasing flow velocity being the most significant contribution to deviations between experimental results and theory. The ratio of separated flow to flow passing the outlet was also found to affect the separation, but to a lesser degree than flow velocity. An application using small separator pipes in conjunction with a tank was shown to have potential advantages over using a settling tank alone.
A second phenomenon which showed promise was based on the settling of solid particles which occurs in a slurry flowing laminarly through an inclined pipe. A flowing particle bed develops on the bottom of the pipe which can be efficiently and continuously removed. A theoretical model based on discrete particle settling was developed in this study which accurately predicted the substantial separations for fly ash particles flowing laminarly through a one inch diameter pipe. The most significant variable in determining the separation for a given solid in a given experimental situation was the velocity of flow, with higher flow velocities and associated increased turbulence resulting in poorer separations. Utilizing the information from this data, a design was postulated for a pipe settling system which has significant advantages over existing gravity settling systems.
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The work on which this report is based was supported in part by funds provided by the Office of Water Research and Technology, United States Department of the Interior, as authorized under the Water Resources Research Act of 1964.
Sterling, Harry J. Jr. and Wood, Don J., "Hydrodynamic Separation of Solids from Solid-Liquid Mixtures" (1976). KWRRI Research Reports. 106.