Degree Name

Master of Science (MS)

Semester of Degree Completion

1987

Thesis Director

William A. Weiler

Abstract

Commonly, waste activated sludge is applied to farmland as a fertilizer supplement and soil conditioner. The use of waste activated sludge is not only an efficient way of sludge disposal, but also an economical one. Much concern has been raised about the safety of such a practice. Harmful pathogens, like Salmonella, are commonly present in waste material. Treatment at wastewater plants is successful in eliminating these organisms from final effluent, but the solids generated are full of indigenous microorganisms, including Salmonella.

Work of this paper attempts to determine the safety of the sludge product after simulated winter conditions present throughout the United States (particularly in the Midwest) and the effects those conditions might have on the bacterial population of the resultant sludge. Conditions of refrigeration (4C), frozen storage (-25C), and repeated freezing and thawing were controlled under varied concentrations of waste activated sludge (0, 5, and 10 percent) on Salmonella paratyphi A. Initially low (ca. 3.1x105 cells/ml) and high (ca. 4.7x107 cells/ml) concentrations of cells were established and bacterial counts were taken at one week intervals to determine the survival of these cells under the three conditions.

Initial concentrations of the bacterial population seem to have no apparent influence on their survival rate. Storage under refrigeration, frozen, and repeated freeze-thawing strongly influence the survival of cells, with freezing and thawing producing the greatest mortality over time and refrigeration having the least affect. Greater concentrations of solids seemed to protect and increase the survival of Salmonella over time for low populations of cells, while for high populations of cells the presence of solids tended to increase mortality. The explanation seems to be attributed to the association of cells with solids, more so when the cell concentration is high than when it is low.

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