Settling Characteristics of Sands Contained in Anaerobically Digested Dairy Manures

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Today?s dairy producers desire to maximize cow comfort and manage the nutrients in manure in an environmentally sound manner. Many dairy farmers would like to use sand for bedding in their freestall barns because of the increased comfort it provides for cows. A negative aspect is that sand bedding inherently mixes with manure to form sand-laden dairy (SLD) manure, which has historically been difficult to deal with effectively. Dairy farmers are increasingly using biological digester systems as a means to control odor and to reduce the potential for environmental pollution (Wright et al., 1999; Parsons, 1984). Anaerobic digesters biologically convert or digest wastes and agricultural byproducts in the absence of oxygen into biogas, a combination of methane and carbon dioxide gases over a period of fifteen to twenty days (Parsons, 1984). These systems can experience failure due to the bedding sand collecting as settled material within the digester, with no easy means for removal. Design solutions are needed to address this problem.

The settling rate of sand in manure that was undergoing anaerobic digestion was investigated. A Hindered Settling Equation, based on boundary layer theory, was selected to model the rate that sand settles in dairy manure while being anaerobically digested for 21 days. The change in viscosity and the total volumetric solids content over time were required as inputs to the Hindered Settling Equation. Laboratory scale digesters were used to anaerobically digest SLD manure and to concurrently monitor sand settling characteristics and changes to manure total solids, total volatile solids and viscosity.

A power law was used to mathematically model manure viscosity. Regression equations were developed from the data collected from the laboratory scale experiment to model total solids, total volatile solids, k, n and shear stress over time during the digestion process. These equations had R2 values that ranged from 0.40 ? 0.96. The Hindered Settling Equation using the modeled manure viscosity term was used to then predict two data sets, one from the laboratory scale experiments and the other from a commercial field scale digester that had been in operation for several years. There was no significant difference between model predictions of sand settling times and the two data sets at an alpha level of 0.05.

A program was developed in Microsoft EXCEL, based on the results from this project, to be utilized as a tool for dairy producers and consultants for the design, construction, and management of anaerobic digesters that receive SLD manures.

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