The Danish EPA gives green light for application of biochar from sewage sludge for use in farming: Biochar from sewage sludge can now be used as a fertilizer. If the pyrolysis takes place at temperatures > 500˚C for more than 3 minutes, and the process temperature and duration is documented, the Danish Environmental Protection Agency interprets that the process may constitute a controlled waste hygienisation. After Sweden and the Czech Republic, this is the third EU country to take this important step towards closing cycles and securing phosphorus resources.
This is the result of the joint efforts of AquaGreen Denmark, the European Biochar Industry Consortium EBI and many biochar supporters. The EBI calls on the EU Commission to include biochar from sewage sludge in the EU Fertilizer Regulation as an important step towards a safe and sustainable circular economy and agriculture. In the absence of a clear position on pyrolysis as a means of upgrading sewage sludge, the EBI has addressed a position paper to the European Commission. It describes the Pyrolysis process and summarizes the current state of research as follows:
What is pyrolysis?
The heating of biomass in a low-oxygen environment is called pyrolysis. Pyrolysis converts organic carbon into a gas (pyrolysis gas) and fixed/elemental carbon. While organic carbon is degradable and while during its natural degradation, greenhouse gases like CO2 or CH4 are released into the atmosphere, fixed carbon is recalcitrant (resistant to weathering/degradation). Unless it is burned, it will not react with any element and stay in its stable form as C. Thus, it can be considered as a permanent carbon sink if used in a material way (no combustion). The specificities of the pyrolysis process include:
– A temperature and process time high enough to “decompose” and/or “volatilize” major feedstock contaminants, like virus or micropollutants (see below).
– The retention of key nutrients (like phosphorus) in the solid phase.
– The capacity to convert part of the carbon contained in the feedstock into “recalcitrant carbon” in the char, ensuring a stable carbon sink if the char is not
oxidized (burnt). This process is called Pyrolytic Carbon Capture and Storage (PyCCS).
Pyrolysis destroys feedstock pathogens
Sewage sludge originates mainly from human excrements. Naturally, the sludge contains pathogens and pyrogens, which are of public health concern. Standard hygienization of sewage sludge e.g., heating of the sludge to 70°C, does not eliminate spores, pyrogens or pathogens.
The process conditions of pyrolysis (> 350°C for several minutes) are much harsher than approved sterilization conditions (Requiring 132°C for 4 minutes with steam (see CDC Steam Sterilization Disinfection & Sterilization Guidelines) and 250°C to remove pyrogens (bacterial endotoxins) under dry conditions (Dry Heat Sterilization). DNA is denatured at 90 °C, hence pyrolysis removes all pathogens and pyrogens contained in sewage sludge (incl. bacteria, fungi, vira, spores, parasites, antibiotic resistance genes etc), from the final product, i.e. the biochar, thereby eliminating these public health concerns.
Pyrolysis eliminates micropollutants from sewage sludge.
Increasing concern is raised regarding sewage sludge spreading on farmland, due to the presence of micropollutants in sludges. Recent scientific research has demonstrated that pyrolysis will have a destruction or removal effect on several types of micropollutants:
Organic pollutants (pharmaceuticals, hormone disrupting molecules):
Recent scientific evidence shows that, at sufficiently severe pyrolysis temperatures (> 500°C) and residence times (> 3 min), all reference organic contaminants and organic micropollutants were completely or nearly completely degraded or driven off the solid material. A study published by the German Ministry of Environment in 2019 (Bundesumweltamt 2019) investigated pharmaceutical residues of various biosolids after pyrolytic treatments above 500 °C. Following the pyrolysis treatment with operating temperatures above 500°C all values of the investigated pharmaceuticals were below the detection limit. The authors concluded: With thermo-chemical treatments (i.e. pyrolysis) a complete destruction of the pharmaceutical residues is achieved. No further technical treatment measures are necessary.
PFASs have been used in consumer products since the 1940s. They are extremely persistent and accumulate in the environment as well as in our bodies. For this reason, they are often referred to as “forever chemicals.” According to research, some of them cause serious health effects such as cancer and liver damage. Per- and Polyfluoroalkyl Substances (PFAS) are eliminated by the process of pyrolysis. Kundu et al.  found that >90% of PFOS and PFOA in sewage sludge were destroyed in a pyrolysis-combustion integrated process. Evidence from the US EPA Office of Research and Development (2021) work with Bioforcetech’s commercially installed PYREG pyrolysis plant shows that pyrolysis at 600°C for 10 minutes and combustion of pyrolysis gases at 850°C eliminate PFAS from sewage sludge .
Bioforcetech (2021) has reported 38 PFAS compounds that were all kept at or removed to below detection limit in the biochar in their pyrolysis and pyrolysis gas burning process .
Direct land spreading of sewage sludge is a preferred method in some European countries. A potential issue with this method is the elevated content of polycyclic aromatic hydrocarbons (PAH) in sludges. The process of pyrolysis can eliminate the content of those to below detection limits in the biochar with proper design of the pyrolysis process (Moško et al., 2021) demonstrated that slow pyrolysis > 400 °C removed more than 99.8 % of PCB, PAH, and endocrine disrupting and hormonal compounds studied . The conclusion from the study is “high temperature (>600 °C) slow pyrolysis can satisfactory remove organic pollutants from the resulting sludge-char, which could be safely applied as soil improver.
Pyrolysis eliminates microplastics from sewage sludge
Research indicates that sewage sludge is a sink for microplastics and further handling of sewage sludge is critical for potential dispersal. Thus, effective reduction of microplastics in the sludge is an important issue (Rolsky et al., 2020). The elimination of microplastic contaminants can be assured by the high temperature during the treatment and the residence time. Ni et al. 2020  found that “Polyethylene and polypropylene, the two most abundant microplastics in sewage sludge, were entirely degraded when the pyrolysis temperature reached 450 °C.”.
The phosphorus present in the feedstock is retained in the pyrolysis char
Phosphorus must be recovered from sewage sludge in more and more EU member states so that fields can be fertilized with this recycled phosphorus in the future. There are various methods for phosphorus recovery, but pyrolysis at temperatures from 500-800 °C is among the most carbon efficient and leads to a product that is directly useable as a fertilizer for soil applications without the need for any further chemical extraction. The P-availability (P2O5) of the sludge biochar is between 40-80% in ammonium citrate (Friedrich et. al. 2015)  which is a highly suitable method for measuring the value as a P-fertilizer (Kratz, S.; Schnug, E., 2009) . According to the same reference this indicates a highly valuable fertilizer.
 Paz-Ferreiro J, Nieto A, Méndez A, Askeland M, Gascó G (2018) Biochar from Biosolids Pyrolysis: A Review. International Journal of Environmental Research and Public Health, 15, 956
 Removal of PFASs from biosolids using a semi-pilot scale pyrolysis reactor and the application of biosolids derived biochar for the removal of PFASs from contaminated water, Kundu S. et al, Environ. Sci.: Water Res. Technol., 2021, 7, 638–649
 EPA PFAS innovative treatment team (PITT) findings on PFAS destruction technologies, EPA Tools & Resources Webinar February 17, 2021, Gullett B.
 Effect of pyrolysis temperature on removal of organic pollutants present in anaerobically stabilized sewage sludge, Moško J. et al, Chemosphere 265
 Ni et al., 2020: Environ. Sci. Technol. Lett. 2020, 7, 12, 961–967. https://doi.org/10.1021/acs.estlett.0c00740
 Deutsche Gesellschaft für Abfallwirtschaft e.V., 5. Wissenschaftskongress Abfall- und Ressourcen- wirtschaft am 19. und 20. März 2015 an der Universität Innsbruck Kevin Friedrich, Katharina Schuh, Thomas Appel Trockene Klärschlammkarbonisierung – ist ein dezentrales Phosphorrecycling möglich?
 Kratz, S.; Schnug, E., 2009 On the solubility and plant availability of phosphorus from mineral fertilizers – a review, JOURNAL FÜR KULTURPFLANZEN, 61 (1). S. 2–8, 2009, ISSN 0027-7479 VERLAG EUGEN ULMER KG, STUTTGART,