Thermal Runaway of Dried Sewage Sludge Granules in Storage Tanks

Abstract

Elimination of the sewage sludge from urban wastewater occurs exclusively by incineration in Switzerland, because recycling as fertilizer in agriculture has been banned since October 2006. Two options exist for elimination, either thermal elimination of a thickened sludge in a household waste incineration plant, or drying of the thickened sludge to yield a granular material fit for storage and transport. The high organic content of this material makes it a valuable alternative fuel for the cement industry. This elimination route is hampered, however, by the tendency of this material to self-heating and occasional thermal runaway resulting in fire. This research project deals with the investigation of the characteristics of these dried sewage sludge granules and the possible reasons for the observed self-heating phenomenon. It was found that a cascade of reactions and a series of circumstances are responsible for the thermal runaway, where each of them isolated would not lead to an event. Mainly responsible for triggering the initial temperature raise is the oxidation of Fe(II) compounds, added to the wastewater as FeClSO4 for phosphate precipitation, and present in this lower oxidation state due to the reducing conditions in the digesters. Sulfate reduction to sulfide by sulfate-reducing bacteria (SRB) presents an additional source of latent heat in the final product once brought in contact with air as demonstrated by the observed formation of the high temperature allotrope ?-sulfur.