Pyrolysis of municipal sewage sludge: challenges, opportunities and new valorization routes for biochar, bio-oil, and pyrolysis gas

The efficient management of municipal sewage sludge (MSS) daily produced worldwide by biological wastewater treatment processes is nowadays of utmost importance. Classic treatment/disposal methods are affected by efficiency and/or safety issues. Innovative thermochemical treatments are gaining momentum as promising alternatives. Pyrolysis of MSS can result in the recovery of precious resources, such as nutrients and organic matter, and their conversion into three valuable fractions, i.e. biochar, bio-oil, and pyrolysis gas. These products are employable in innovative biorefinery pathways towards a wide range of value-added materials. In this review, an integrated biorefinery platform for MSS valorization is presented. After a brief introduction on MSS properties and issues related to its management, a deep focus on the influence that the feedstock and pyrolysis conditions have on the product yields and composition was conducted. Innovative valorization routes for biochar, bio-oil and pyrolysis gas were extensively discussed by highlighting challenges, opportunities, advantages and drawbacks. The characteristics required by these products to be efficiently valorized, as well as the main solution for their enhancement, were described. Additionally, economic considerations on MSS pyrolysis derived from full-scale applications conducted at the European and global level were elaborated. Finally, future perspectives about biochar, bio-oil and pyrolysis gas employment in cutting-edge upcycling routes have been reported.