Document Type

Thesis - University Access Only

Award Date

1999

Degree Name

Master of Science (MS)

Department / School

Civil Engineering

Abstract

A previous study for the City of Sioux Falls has indicated that waste heat available during the summer could be used to heat the secondary biosolids digester. Heating of the secondary digester to thermophilic temperatures was determined to have the potential to reduce pathogens. It was desired to find out if this reduction in pathogen densities was sufficient to meet Title 40, Part 503 of the Code of Federal Regulations for pathogen reduction. This research was conducted to determine if heating a secondary digester could reduce fecal coliforms to the required levels and also to develop a model to predict fecal coliform decay at full scale for a specified temperature Lab studies were conducted to determine the kinetics of fecal coliform destruction and their relationship to temperature. First order coefficients were calculated from lab scale data. Coefficients for fecal coliform destruction increased with temperature from 0.29 per day at 37°C to 81.2 per day at 55°C. A full scale secondary digester was operated at 47°C and monitored to determine the extent of fecal coliform destruction. Fecal coliform densities in the digester were reduced by roughly two logs compared to the influent densities. Fecal coliform density in the digester operated at 47°C never fell below the 1000 MPN/g TS regulatory limit. Densities were also observed to be highly variable. A steady state, first order decay model developed from the lab scale studies was found to be relatively accurate for predicting the long-term fecal coliform density trend for mesophilically digested biosolids heated to thermophilic temperatures. Modeling indicated that a temperature of 55°C might be ideal for reducing the average fecal coliform density below the regulatory limit. This temperature was also shown to be effective in the previous study. Temperature and detention time were shown to be important factors in fecal coliform destruction by the lab scale studies. Comparing the batch reactor studies performed in the lab to the completely stirred tank reactor (CSTR) operations in the field, it was noticed that a batch or plug flow type reactor might be ideal for fecal coliform destruction to ensure adequate detention time and reduce the effects of short-circuiting.

Library of Congress Subject Headings

Sewage sludge -- Disinfection Sewage -- Microbiology

Format

application/pdf

Number of Pages

126

Publisher

South Dakota State University

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