| In this dissertation, the development and validation of a mathematical model of the autoheated aerobic sewage sludge digestion process, AUTOHEAT, is presented along with a design example illustrating model use. This model was developed to: (1) provide a better understanding of the relationships between reactor design and operating parameters on process performance, and (2) serve as a rational basis for the preliminary evaluation of autoheated aerobic sludge digestion for specific situations and also as a methodology for process design.; AUTOHEAT is similar to the previous models of the autoheated aerobic digestion process proposed by Andrews and Kambhu (1973), Koenig (1974), Vismara (1985), and Baines et al. (1985) in several aspects. All are simulation models containing three interrelated submodels; a materials (substrate) balance, an oxygen balance, and a heat balance; which are solved sequentially to predict process temperature and the degree of sludge stabilization. In addition, an iterative solution strategy in which iteration occurs until the absolute value of the difference between heat inputs and losses approaches zero is a common requirement.; However, AUTOHEAT generates characterizations of process performance that differ significantly from these four previous models. This is a reflection of a number of differences, which vary in significance, between AUTOHEAT and previous models. The most important is the difference in the formulation of a materials balance. Although AUTOHEAT shares an underlying assumption of first-order microbial kinetics with the Andrews and Kambhu and Koenig models, the Arrhenius and empirical relationships developed to describe the effect of temperature on reaction-rate coefficients differentiate AUTOHEAT from all previous models. This difference is particularly important since it influences not only predictions of the degree of sludge stabilization, but also the subsequent estimation of biological heat production and other heat balance elements.; A four step process was employed to test the validity of AUTOHEAT. Experimental data from a number of sources (Martin, 1988; Kelly, 1990, Morgan et al., 1984; Booth and Tramontini, 1984; Messenger, 1991; and Appleton, unpublished) were used. From the results of this testing process, it was concluded that this model is a representative mathematical description of the autoheated aerobic sludge digestion process. |
