Agronomic valorization of sewage sludge: The potential of thermal drying to achieve sanitation and biological stability

Abstract

This work aims to contribute to the discussion of using a waste (sewage sludge) to obtain a valuable product (organic-rich fertilizer), based on a circular economy perspective. The composition of sewage sludge (SS) was considered, highlighting important characteristics as raw material for fertilizer production (e.g., macronutrients, microbiological contamination, phytotoxicity). Particularly, the biological stability of SS was evaluated measuring the oxygen uptake rate (OUR) using the Oxitop® method, and the phytotoxicity using germination tests. The potential of thermal drying to stabilize the SS was evaluated at two temperatures (100 and 130°C). The results showed that the organic matter (OM), N, and P levels are within the range of interest for agronomic valorization, while the content of some potentially toxic metals (e.g., Zn and Cu) may exceed the legal limits. The phytotoxicity assessment showed that raw SS samples negatively impacted the germination phase. Concerning the biological stability, results showed that raw anaerobic digested samples were characterized by OUR values above 50 mmol O2/kg OM.h (the limit for stable SS). Although thermal treatment of SS at 100 and 130°C eliminated pathogens and significantly stabilized SS to OUR values below 50 mmol O2/kg OM.h, it was not enough to reach the limit established for a stable fertilizer (25 mmol O2/kg OM.h). Globally, SS has potential for soil application, within the scope of circular economy, but it is essential to control its microbiological contamination and stability.