Pyrolytic conversion of biological sludge: products characterization and integration of biochar into constructed wetlands

The aim of this study is to enhance the value of biological sludge from wastewater treatment plants by pyrolytic conversion into biochar and its integration into pilote-scale vertical flow constructed wetlands (VF-CWs). This approach aligns with the principles of the circular economy, aiming to transform a high-impact waste into a valuable resource for sustainable wastewater treatment. The biochar was produced using a pilot-scale pyrolysis unit developed in previous research. The thermochemical conversion process occurs in a nitrogen-saturated steel chamber, where the feedstock is exposed to a mild flow of inert gas during the thermal phase. Condensable pyrolysis gases were collected using a refrigerant system connected to a thermo-cryostat and analysed to assess the calorific value of the resulting bio-oil. The non-condensable gas fraction was also sampled and characterized for the same purpose. The resulting biochars underwent acid and thermal activation and were characterized in accordance with the European standard EN 12915-1:2009, which refers to carbon materials intended to be used for the treatment of water for human consumption (e.g. ashs from 49% to 76%, surface area from 130 m2/g to 452 m2/g). The best-performing biochars, as determined by the aforementioned criteria, were then integrated in pilote- scale VF-CWs which were fed with the effuent from the clariflocculation stage of a wastewater treatment plant operating in a mixed domestic-industrial textile context, in comparison with systems filled with gravel, thus being tested as quaternary treatments.VF-CW influents and effluents were monitored for chemical oxygen demand (COD), nitrogen and phosphorus cycles, total suspend solid and absorbance values at 254 nm and 420 nm, the latter as rapid and reliable screening parameters of the removal of organic micropollutants containing aromatic moieties and/or chromophores. This decentralized treatment approach embodies the core principles of biomass recycling and circular economy. Moreover, the inclusion of biochar can significantly enhance the performance of these nature-based systems, which typically exhibit limited removal efficiency for various classes of organic micropollutants such as pharmaceuticals and perfluoalkyl substances. We thank GIDA S.p.A for the financial and technical support provider during this work.