Effects of charred sewage sludge on carbon sequestration and nitrogen availability at a mediterranean soil

Abstract

The thermochemical carbonization of sewage sludge (SS) through hydrothermal carbonization (HTC) or dry pyrolysis has been proposed to achieve SS hygienization and increase its biochemical stability. Paneque et al. (2017) showed a higher presence of aromatic and heterocyclic N compounds in pyrolyzed-SS than in HTC-treated SS. This suggest a lower degradability, thus a lower C-sequestration potential, of the former compared to the latter. In addition, the transformation of peptide-like N of SS into heterocyclic aromatic N during thermal treatments is expected to reduce N availability, limiting the use of this residue as a direct source of N for plants. Therefore, the main goal of this work was to study the biodegradability and N availability of pyrolyzed and HTC-treated SS along the time after their addition to a soil. Thus, we performed a 10-month greenhouse incubation experiment, using a Mediterranean Cambisol as matrix and tested the following treatments: control (soil alone), raw SS, SS-hydrochar (HTC 200 °C, 30 min) SS-pyrochar (dry pyrolysis, 600°C, 1 h). The SS was produced in a pilot-scale waste-water treatment plant and enriched with 13C and 15N by the addition of 15NH4Cl, K15NO3 and 13C-glucose during the treatment. The distribution of 13C and 15N among the soil and plants was monitored after 1 and 3 months, as well as at the end of the experiment. Our results confirmed that both C and N in the pyrochar were biochemically more stable than those in the hydrochar. However, HTC was able to reduce the amount of very labile compounds that otherwise would be degraded within the first month. In addition, the hydrochar showed a slightly lower amount of plant available N than raw SS, whereas the amount f N derived from the pyrochar used for plant growth was negligible. This study confirmed that dry pyrolysis achieved a more effective stabilization of SS than HTC, and hence that pyrochars are more appropriate to increase the C sequestration potential of soils. However, both HTC and, in a greater extent, HTC limit the use of the N contained in SS for plant growth. Reference: Paneque M., De la Rosa J.M., Kern J., Reza M.T., Knicker H. (2017) Hydrothermal carbonization and pyrolysis of sewage sludges: What happen to carbon and nitrogen? Journal of Analytical and Applied Pyrolysis, 128, 314-323.