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.

Highly efficient phosphorus recovery from sludge and manure biochars using potassium acetate pre-treatment

“…We recommend ~500°C for pyrolysis of K-doped, P-rich feedstocks that also contain increased levels of PTEs and organic contaminants because (i) the availability of P decreases with pyrolysis temperature, (ii) the availability of PTEs increases with pyrolysis temperature and (iii) organic contaminants present in the feedstock material are almost completely removed at temperatures of ≥500°C as demonstrated previously (Buss, 2021).…”

“…Thermo-chemical conversion processes, in an oxygen-rich atmosphere (incineration) and oxygen-poor atmosphere (pyrolysis), remove organic contaminants from sewage sludge and related materials (Buss, 2021). However, the P availabilities in the products from pyrolysis (biochar) and incineration (ash) are lower than in the feedstock materials (Adhikari et al, 2019;Herzel et al, 2016;Xiao et al, 2018).…”

“…Unlike incineration, pyrolysis treatments operate at lower temperatures and additionally retain around half of the carbon present in the feedstock material. Therefore, pyrolysis significantly reduces the environmental impact of P-recycling (Buss, 2021). We previously demonstrated that doping of sewage sludge with a potassium acetate solution prior to pyrolysis at 700°C converts phosphate minerals of low solubility (Al-, Ca-, Fe-and Mg-phosphates) into highly available K-phosphates (Buss et al, 2020).…”

Biochar can mitigate co-selection and control antibiotic resistant genes (ARGs) in compost and soil

“…This assertion is most likely the case for organic contaminants and not PTEs. Conversion of feedstocks to biochar through pyrolysis decreased the level of various contaminants (dioxins, PAHs, PCBs, antibiotics, ARGs, microplastics, antimicrobials, per- and polyfluoroalkyl substances) with great effectiveness between 95% - 99% ( Buss, 2021 ). The organic contaminants were reduced below detectable limits.…”

Contaminants in biochar and suggested mitigation measures – a review

“…One category of contaminants in biochar are potentially toxic elements (PTEs) that originate from the feedstock material and are enriched during pyrolysis [16]. While PTEs cannot be destroyed, organic contaminants in the feedstock material are typically decomposed during pyrolysis and subsequent combustion of pyrolysis vapors [17]. However, some organic contaminants are formed during pyrolysis and can be introduced into biochar.…”