Pyrolysis Solves the Issue of Organic Contaminants in Sewage Sludge while Retaining Carbon—Making the Case for Sewage Sludge Treatment via Pyrolysis

Abstract

Contaminants of emerging concern are a growing burden for sewage sludge recycling. Pyrolysis of sewage sludge could be a solution. Yet, the product of sewage sludge pyrolysis (biochar) is currently not included on the list of eligible fertilizers in the new EU Fertilising Products Regulation. This was justified by insufficient evidence for organic contaminant removal through pyrolysis. Here I summarize the current evidence on this topic covering 20 studies and more than 100 different organic pollutants. The studies demonstrate that pyrolysis reduces the concentration of well-established contaminants, such as PAHs, PCBs, and dioxins, but also emerging ones, i.e., pharmaceuticals, hormones, antibiotics, antibiotic resistance genes, antimicrobials, microplastics, and per- and polyfluoroalkyl substances (PFAS) with very high efficacy (>95 to >99% in most cases). After pyrolysis, the levels of organic contaminant in biochar were typically below the limit of detection. Furthermore, modern pyrolysis units prevent environmental release of contaminants that are only vaporized and not decomposed during pyrolysis by internal combustion of pyrolysis liquids and gases. The evidence for effective organic contaminant removal through pyrolysis is comprehensive, covering all relevant groups of compounds. This and its potential as negative emission technology makes pyrolysis of sewage sludge a great opportunity for sustainable and safe nutrient recycling.