Chemical and Biomolecular Engineering Faculty Research


Aerobic Sludge Digestion under Low Dissolved Oxygen Concentrations

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Publication Date

Fall 2004


Low dissolved oxygen (DO) concentrations occur commonly in aerobic digesters treating thickened sludge, with benefits of smaller digester size, much reduced aeration cost, and higher digestion temperature (especially important for plants in colder areas). The effects of low DO concentrationson digestion kinetics were studied using the sludge from municipal wastewater treatment plants in Akron, Ohio, and Los Lunas, New Mexico. The experiments were conducted in both batch digestion and amixedmode of continuous, fed-batch, and batch operations. The low DO condition was clearlyadvantageous in eliminating the need for pH control because of the simultaneous occurrence of nitrification and denitrification. However, when compared with fully aerobic (high DO) systems under constant pH control (rare in full-scale plants), low DO concentrations and a higher solids loadinghad a negative effect on the specific volatile solids (VS) digestion kinetics. Nonetheless, the overall(volumetric)digestion performance depends not only on the specific digestion kinetics, but also the solids concentration, pH, and digester temperature. All of the latter factors favor thelow DO digestion of thickened sludge. The significant effect of temperature on low DO digestion was confirmed in the mixed-mode study with the Akron sludge. When compared with the well-known empirical correlation between VS reduction and the product (temperature × solids retention time),the experimental data followed thesametrend, but were lower than the correlation predictions. The latter was attributed to the lower digestible VS in the Akron sludge, the slower digestion at low DO concentrations, or both. Through model simulation, the first-order decay constant (kd )was estimated as 0.004 h-1 in the mixed-mode operations, much lower than those (0.011 to 0.029 h-1) obtained in batch digestion. The findings suggested that the interactions among sludges with differenttreatmentages may have a substantially negative effect on digestionkinetics. The use of multistage digesters, especially with small front-end reactors, may be advantageous in both "process" kinetics and "biological reaction" kinetics for sludge digestion.





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