Biochar – Recovery Material from Pyrolysis of Sewage Sludge: A Review

The treatment of sewage sludge (SS) is becoming one of the most important issues of the wastewater treatment cycle in European Union. Pyrolysis of SS is typically accompanied by production of pyrolysis oil, gas (syngas) and solid char product called either charcoal or biochar, when applied in agriculture. The main aim of this review is to summarize the knowledge on disposal of SS, thermal treatment of SS, especially pyrolysis, the characterization of produced sludge-derived biochar and to summarize the requirements for certification of SS biochar in agricultural application. The description of analyses includes determination of solid yield of biochar, immobilization of heavy metals in SS biochar, and structure of biochar including the surface morphology and specific surface area. It is concluded that pyrolysis represents an eco-friendly treatment of SS giving eco-friendly products, particularly biochar, and therefore it may represent a solution in terms of circular economy, carbon sequestration, contaminant immobilization, greenhouse gas reduction, soil fertilization and improvement of water retention.