Technical implications and global warming potential of recovering nitrogen released during continuous thermal drying of sewage sludge

Abstract

Thermal drying and consequent incineration of sewage sludge result in an absolute loss of an important macronutrient – nitrogen. To fulfill the growing food demand, humanity relies more on industrial fixation of nitrogen, primarily via the Haber-Bosch process. The present paper examines the nitrogen release during continuous thermal drying of municipal sewage sludge and its consequent recovery for fertilization. Furthermore, the possibility of nitrogen recovery from condensate is assessed. Finally, the study assesses the global warming potential of the proposed nitrogen recovery system and compares it with the baseline system manufacturing fertilizers from industrially fixed nitrogen. The results of the drying experiments showed that 0.73–1.03 g N-NH3 kg−1 total solids of sewage sludge was released to off-gases during its continuous thermal drying under 160 °C, which corresponds to 41–58% of ionized nitrogen content in raw sewage sludge subjected to thermal drying. The global warming potential of the nitrogen recovery was 28% lower compared to that of the commercial fertilizer production of equivalent properties: 4.1 kg CO2-Equiv. kg−1 N versus 5.7 kg CO2-Equiv. kg−1 N. Still, the sensitivity analysis showed that the results might traverse and lead to a higher global warming potential of 6.2 kg CO2-Equiv. during the nitrogen recovery process under certain process parameters.