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Hindered Settling of Animal Manure

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  2011 Louisville, Kentucky, August 7-10, 2011  1111188.(doi:10.13031/2013.41357)
Authors:   John P Chastain
Keywords:   Treatment, manure, swine, dairy, settling, settling velocity

Most liquid dairy and swine manure has a total solids (TS) greater than 0.5% and hindered settling is the dominate characteristic of the settling process. The defining characteristic of hindered settling is that the particles in the waste do not fall independently, but form a blanket of particles that settle together in a plug fashion. Whenever hindered settling occurs a distinct interface can be observed that separates the supernatant from the settling blanket of particles. Observations from several settling experiments have shown that a consistent pattern describes hindered settling of animal manure. This pattern consists of an initial constant settling velocity period, followed by a period of changing settling velocity that transitions into a final period of very slow compression settling. These three zones are classified as linear hindered settling (LHS), transitional hindered settling (THS), and compression settling (CS). Interface settling velocities for the three zones were empirically determined for liquid dairy manure, milking center wastewater, liquid swine manure, and lagoon sludge-supernatant mixtures. The data indicated that the interface settling velocities associated with the three hindered settling zones varied greatly with the TS concentration and type of manure. In general, swine manure settled faster than dairy manure and increasing the dilution increased the interface settling velocity for all types of manure. The magnitude of the interface velocity during the initial linear zone (LHS) was the most important for selection of the mode of operation and design of a settling basin. The volume of the settled solids was also observed and was shown to depend on TS concentration and manure type. The analysis techniques and data provided in this paper can be used to improve the design of settling basins for dairy and swine facilities.

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