Pyrolysis heats dried sludge without oxygen and turns it into a lighter solid. At 500 °C that solid comes out practically free of PFAS, microplastics and pharmaceuticals, although PFAS are harder to remove from sludge with industrial inputs. What it does not solve: some of the PFAS move into the gas, which must be burned or cleaned; heavy metals stay in the solid; and the European Biochar Certificate does not allow it to be used in soil, except in countries that permit it.
Contents
What happens to sludge inside the reactor
Pyrolysis heats dried sludge in the absence of oxygen. This leaves a black, mineral-rich solid that weighs considerably less than the sludge fed in: in the studies reviewed, between about a quarter and almost two thirds, depending on the sludge and the temperature.
What matters most to a wastewater operator is what leaves that solid:
· PFAS. In digested sludge, removal exceeds 99% at 400 °C, and at 500 °C they fall below the limit of quantification, both in the lab and at a commercial plant. The same authors warn that below 400 °C, pyrolysis does not work for this.
· But sludge type matters. A 2026 study compared sludges with and without industrial inputs at 500 °C: in sludges without industrial inputs, removal exceeded 95%; in sludges with industrial inputs, it was only between 24% and 88%, and exceeded 95% only at 800 °C.
· Microplastics and pharmaceuticals. In lab trials, microplastics almost disappear at 500 °C and pharmaceuticals fall below the detection limit from 400 °C.
What pyrolysis does not solve
1. The PFAS that leave the solid do not simply disappear. In sludge without industrial inputs, about half of the incoming PFAS was found in the pyrolysis gas and liquid. At a full-scale plant that burns that gas, emissions were below 3% of the incoming PFAS. The 2024 study's authors conclude: the gas must be burned or cleaned.
2. Heavy metals stay. Most remain in the solid. What changes is how much leaches in the TCLP test, and not always for the better: in the same study, leached cadmium fell as temperature rose, while nickel fell up to 500 °C and rose again at 700 °C.
3. The solid is mostly ash. In municipal sludge pyrolysed at 700 °C, the char was 75% ash and 14% carbon.
What the rules say
Frequently asked questions
Can I use the char from my sludge as a fertiliser?
Under the European Biochar Certificate, generally not: biosolids can only be certified as EBC-Basic, which excludes farming and soil use. The exception is countries whose national annex allows it, such as Denmark. It also cannot be sold as a fertilising product under the EU fertilising products regulation. In Ecuador, in Ministerial Agreement 097-A we have not identified a specific regulation. If someone offers to turn your sludge into certified fertiliser, ask under which standard.
Can pyrolysed sludge generate carbon credits?
It can, depending on the methodology: Puro.earth accepts wastewater sludge but counts its fossil carbon as fully re-emitted. And the starting point is low: in one study, municipal sludge char made at 700 °C was 14% carbon, and only the non-fossil part counts.
What should I ask a pyrolysis supplier before sending them my sludge?
Six questions, valid for any supplier, including us:
1. At what temperature, and for how long, does the sludge stay in the reactor? Below 400 °C, pyrolysis is unsuitable for removing PFAS; for biosolid char, the EBC requires more than 500 °C for at least 3 minutes.
2. What happens to the gas? Part of the PFAS leaving the solid ends up there. Ask how it is burned or cleaned, and whether that has been measured.
3. What moisture content does the sludge need on entry, and who dries it?
4. What is the legal destination of the char in your country, and who is responsible for it?
5. Do they have results measured on sludge similar to yours, with third-party lab analysis?
6. What analysis of your sludge do they need before quoting? At minimum heavy metals and moisture, and whether the plant receives industrial wastewater.
Conclusion
Pyrolysis turns a bulky, contaminated problem into a smaller solid that is far cleaner of organic contaminants, but generally not into a fertiliser. The metals remain, some of the PFAS move into the gas, and the European Biochar Certificate accepts this char in soil only where a national annex allows it. The useful question for a wastewater plant is not whether pyrolysis works, but what will happen to the gas and to the char, and who is accountable for each.
Sources and methodology
Standards and protocols:
· European Biochar Certificate (EBC), v10.5E, 14 Aug 2025: section 3 (p. 13) and section 4.9 (p. 15). Official PDF (accessed 7 Oct 2026)· Commission Delegated Regulation (EU) 2021/2088, adding CMC 14 to Regulation (EU) 2019/1009, point 1(a). EUR-Lex (accessed 7 Oct 2026)· Ministry of Environment of Ecuador, Ministerial Agreement 097-A, Official Register 387, 4 Nov 2015, Annex 1, section 5.2.1.3. PDF (accessed 7 Oct 2026)· Puro.earth, Biochar Methodology, Edition 2025 v2, approved 27 Nov 2025, §3.4.5 and §3.4.23–3.4.26. PDF (accessed 7 Oct 2026)· Isometric, Biomass Feedstock Accounting module v1.3, category SC6. Registry (accessed 7 Oct 2026)
Studies:
· Hušek et al. (2024). Biochar 6:31. doi:10.1007/s42773-024-00322-5· McNamara et al. (2026). Water Environment Research 98(4). doi:10.1002/wer.70352· Sørmo et al. (2023). Journal of Hazardous Materials 454:131447. doi:10.1016/j.jhazmat.2023.131447· Ni et al. (2020). Environmental Science & Technology Letters 7(12):961–967. doi:10.1021/acs.estlett.0c00740· Moško et al. (2021). Chemosphere 265:129082. Article record· Zhao et al. (2025). RSC Advances 15:43932–43940. doi:10.1039/D5RA04990C· Zhang et al. (2015). Scientific Reports 5:9406. doi:10.1038/srep09406· Vali et al. (2021). Energies 14(5):1477. doi:10.3390/en14051477
Methodology: every figure comes from the authors' own text or the cited standard, accessed 7 October 2026, and is recorded with its quote and conditions. No indirect citations are used. Where two studies disagree and it changes the reader's decision (PFAS removal by sludge type), both are shown. This article reports no Nina Energy results on sludge: there is no publishable third-party analysis.
