Synthesis of novel adsorbent materials from sewage sludge and their application for wastewater treatment within a circular economy perspective

The water scarcity, resource depletion and global pollution are moving the world's governmental bodies towards the implementation of stricter regulations on pollution discharge, thus technologies to produce clean water are receiving growing attention. On the other hand, municipal wastewater treatment plants produce a large amount of sewage sludge, which is a problematic waste containing pathogens, heavy metals and toxic organic chemicals that are harmful to humans and the ecological system. In Italy, for example, approximately 3.24 million tonnes of sewage sludge are produced every year, which are mainly treated through subsequent biological technologies, dewatering, and drying steps, thus obtaining a solid that is mainly burnt for energy. However, it can find more valuable applications, namely as a precursor of activated carbons to be employed for the removal of pollutants such as dyes, heavy metals and chemicals from various industrial activities. The SIRENA project funded by the Tuscany Region aims at the valorization of the sewage sludge generated by the municipal wastewater treatment plant of Cuoiodepur S.p.A. to produce activated carbon to be used to treat wastewater, reintroducing it into the water cycle, thus developing virtuous circular economy paths. For this purpose, two different streams provided by Cuoiodepur S.p.A. have been considered as feedstock, one generated by the purification of industrial wastewater from the tanning sector, and one recovered at the exit of the MBR (Membrane BioReactor) section for civil wastewater treatment. As benchmark, these sewage sludges were pyrolyzed under a flow of nitrogen at different temperatures (400, 500 and 600 °C) to study the role of temperature on the degree of carbonization and on the specific surface area of the obtained biochars. These were characterized by FT-IR, elemental analysis and N2 physisorption analysis, demonstrating the occurrence of the carbonization process during the pyrolysis treatment but the development of still limited surface areas, responsible for the insufficient extent of the abatement of COD of the municipal wastewaters of Cuoiodepur S.p.A. These results pushed us to the adoption of activating agents, as CO2 and FeCl3, to increase the specific surface area and the microporosity of the material, thus improving the adsorption capacity. Therefore, this work contributes to the valorization of a waste, as the sewage sludge, to obtain an adsorbent material that can be used within the same industrial process in a circular economy perspective.