19. July 2022
Henriette zu Doha
Biochar | phosphorus fertilizer | Pyrolysis | Sewage Sludge

Biochar from sewage sludge: the phos­phorus ferti­lizer for a safe and sustainable agriculture

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.

Per- and Poly­fluo­ro­alkyl Subs­tances (PFAS) are elimi­nated by the process of pyro­lysis. Kundu et al. [2] 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 [3].
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 [4].

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 [5]. 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 [6] 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) [7] which is a highly suitable method for measu­ring the value as a P-ferti­lizer (Kratz, S.; Schnug, E., 2009) [8]. According to the same refe­rence this indi­cates a highly valu­able fertilizer.


[1] 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
[2] 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
[3] EPA PFAS inno­va­tive treat­ment team (PITT) findings on PFAS dest­ruc­tion tech­no­lo­gies, EPA Tools & Resources Webinar February 17, 2021, Gullett B.
[4] https://ccag.ca.gov/wp-content/uploads/2020/02/BFT_FEB_2020-1.pdf
[5] Effect of pyro­lysis tempe­ra­ture on removal of organic pollut­ants present in anae­ro­bi­cally stabi­lized sewage sludge, Moško J. et al, Chemo­s­phere 265
(2021) 12982
[6] Ni et al., 2020: Environ. Sci. Technol. Lett. 2020, 7, 12, 961–967. https://doi.org/10.1021/acs.estlett.0c00740
[7] 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?
[8] 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,



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