Recy­cling sewage sludge, through pyro­lysis, results in high-quality biochar, contai­ning valuable phos­phorus, an essen­tial nutrient for plant growth. Moreover, carbo­niza­tion offers muni­ci­pa­li­ties a safe and profi­table solu­tion in terms of the circular economy as signi­fi­cant rene­wable energy is gene­rated and reused, while the phos­pho­rous-rich biochar provides both agri­cul­tural bene­fits and seques­ters carbon, when given into the soil.
However, controlled carbo­niza­tion has another signi­fi­cant advan­tage: it sani­tizes toxic and conta­mi­nated sewage sludge, thus protec­ting people and nature.

Computer-controlled process

The core of the PYREG tech­no­logy is the patented reactor in combi­na­tion with the down­stream FLOX combus­tion chamber (“FLOX” stands for flame­less oxida­tion). In the reactor, the raw mate­rial is heated largely in the absence of air at high tempe­ra­tures of around 500 to 700 °C for several minutes.  The computer-controlled process para­me­ters – such as speed of convey­ance of the feed mate­rial, tempe­ra­ture and air supply, is the key to recy­cling success. In the process, the phos­phorus remains comple­tely available for plants. And moreover: This treat­ment of sewage sludge offers great poten­tial for the removal of many pollut­ants of high ecolo­gical and human health impact.

Carbo­niza­tion destroys feedstock pathogens

Sewage sludge origi­nates mainly from human excre­ments. Natu­rally, the sludge contains spores, patho­gens, and pyro­gens, which are of public health concern.[1] Stan­dard hygie­niza­tion of sewage sludge (e.g. heating of the sludge to 70 °C), does not elimi­nate all these contaminants.

The process condi­tions of pyro­lysis (> 500 °C for more than three minutes) are much harsher even than approved steri­liza­tion condi­tions. Accor­dingly, pyro­lysis elimi­nates all pathogens[2] and pyro­gens contained in sewage sludge – inclu­ding bacteria, fungi, vira, spores, para­sites, anti­biotic resis­tance genes etc. The final product, i.e. the biochar, is free of threats for public health.

Pyro­lysis elimi­nates micro­pol­lut­ants from sewage sludge

Incre­asing concern is raised regar­ding the spre­a­ding of sewage sludge on farm­land, due to the presence of micro­pol­lut­ants in sludges. Recent scien­tific rese­arch has demons­trated that pyro­lysis will destroy or remove several types of micropollutants:

Organic pollut­ants (e.g. phar­maceu­ti­cals, hormone disrupting molecules):

Scien­tific evidence shows that at suffi­ci­ently high pyro­lysis tempe­ra­tures (> 500 °C) and long dura­tions (> 3 min), all refe­rence organic conta­mi­nants and micro­pol­lut­ants were comple­tely or nearly comple­tely degraded or driven off the solid mate­rial. A study published in 2019 by the German Ministry of Envi­ron­ment (Bundesumweltamt)[3] analyzed the resi­dues of various phar­maceu­tical bioso­lids after pyro­lytic treat­ment above 500 °C. After the process, all of the inves­ti­gated phar­maceu­ti­cals were below the detec­tion limit. The authors conclude: “With thermo-chemical treat­ments (i.e. pyro­lysis) a complete destruc­tion of the phar­maceu­tical resi­dues is achieved. No further tech­nical treat­ment measures are necessary.”

PFAS:

Per- and Poly­fluo­ro­alkyl Subs­tances (PFAS) are elimi­nated by the process of pyro­lysis. Kundu et al. (2021)[4] 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)[5] carried out on the US-based company Bioforcetech’s commer­ci­ally installed PYREG 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)[6] 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.

PAH:

Spre­a­ding sewage sludge on agri­cul­tural land is very common in Europe, although sludges poten­ti­ally contain elevated levels of toxic poly­cy­clic aromatic hydro­car­bons (PAH). Properly desi­gned pyro­lysis processes can elimi­nate these chemical compounds, resul­ting in biochar with a PAH content below limit values or even detec­tion limits: Moško et al. (2021)[7] demons­trated that slow pyro­lysis at > 400 °C removes more than 99.8 % of the studied PCB, PAH, endo­crine-disrupting chemi­cals, and hormonal compounds. The authors state: “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 micro­pla­stic from sewage sludge

Rese­arch indi­cates that sewage sludge is a sink for micro­pla­stic. Thus, effec­tive reduc­tion of the plastic frag­ments is critical for poten­tial dispersal.[8] Ni et al. (2020)[9] found that “Poly­ethy­lene and poly­pro­py­lene, the two most abun­dant micro­pla­s­tics in sewage sludge, were enti­rely degraded when the pyro­lysis tempe­ra­ture reached 450 °C.” Total concen­tra­tions of micro­pla­stic were reduced from 550.8 – 960.9 to 1.4 – 2.3 particles/g at pyro­lysis tempe­ra­tures of 500 °C. No micro­pla­stic with a particle size of 10-50 μm remained.

To illus­trate the beha­vior of plastic during high tempe­ra­ture treat­ment (for example during pyro­lysis), the thermal decom­po­si­tion curves of PE and PP are shown in Figure 1. PE and PP thermal degra­da­tion shows a dramatic mass loss between 400 °C and 500 °C, while above 500 °C “the mate­rial degraded comple­tely without leaving any noti­ceable residue.”[10] PET, a highly rele­vant plastic type regar­ding sewage sludge, starts to decom­pose at a tempe­ra­ture above 450 °C and tran­si­tions to the gas phase. PET decom­po­si­tion is termi­nated in less than one minute (α = 1) at tempe­ra­tures above 500 °C[11]. The cracked gases are of high calo­rific value and can be used for energy produc­tion. Thus, the pyro­lysis of sewage sludge is a good method to drasti­cally reduce micro­pla­stic in the envi­ron­ment.

 Figure 1: TG scans of PE and PP measured at a constant heating rate in two diffe­rent test envi­ron­ments: inert atmo­sphere and in air.[12]

Sources:

[1]  Huygens, D., Garcia-Gutierrez, P., Orveillon, G., Schil­laci, C., Delre, A., Orgi­azzi, A., Wojda, P., Tonini, D., Egle, L., Jones, A., Pistocchi, A. and Lugato, E., Scree­ning risk assess­ment of organic pollut­ants and envi­ron­mental impacts from sewage sludge manage­ment, EUR 31238 EN, Publi­ca­tions Office of the Euro­pean Union, Luxem­bourg, 2022, ISBN 978-92-76-57322-7 (online), doi:10.2760/541579 (online), JRC129690.

[2]  Paz-Ferreiro, Jorge, Aurora Nieto, Ana Méndez, Matthew Peter James Askeland, and Gabriel Gascó. 2018. “Biochar from Bioso­lids Pyro­lysis: A Review” Inter­na­tional Journal of Envi­ron­mental Rese­arch and Public Health 15, no. 5: 956. https://doi.org/10.3390/ijerph15050956

[3]  Bundes­um­weltamt (2019) Arznei­mit­tel­rück­stände in Rezy­klaten der Phos­phor­rück­ge­win­nung aus Klär­schlämmen, Umwelt­for­schungs­plan des Bundes­mi­nis­te­riums für Umwelt, Natur­schutz und nukleare Sicher­heit, Forschungs­kenn­zahl 3715 33 401 0, UBA-FB 002724 (https://www.umweltbundesamt.de/sites/default/files/medien/1410/publikationen/2019-03-29_texte_31-2019_arzneimittelrueckstaende-klaerschlamm_v2.pdf)

[4]  Kundu, S., Patel, S., Halder, P., Patel, T., Mary­bali, M. H., Pramanik, B. K., Praz-Ferreiro, J., Figuei­redo, C. C., Berg­mann, D., Sura­pa­neni, A., Megharaj, M., Shah, K., 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, Environ. Sci.: Water Res. Technol., 2021, 7, 638–649

[5]  Gullet, B., EPA PFAS inno­va­tive treat­ment team (PITT) findings on PFAS destruc­tion tech­no­lo­gies, EPA Tools & Resources Webinar February 17, 2021

[6]  https://ccag.ca.gov/wp-content/uploads/2020/02/BFT_FEB_2020-1.pdf

[7]  Moško J, Pohořelý M, Cajt­haml T, Jere­miáš M, Robles-Aguilar AA, Skoblia S, Beňo Z, Inne­ma­nová P, Linhar­tová L, Mich­alí­ková K, Meers E. Effect of pyro­lysis tempe­ra­ture on removal of organic pollut­ants present in anae­ro­bically stabi­lized sewage sludge. Chemo­sphere. 2021 Feb;265:129082. doi: 10.1016/j.chemosphere.2020.129082. Epub 2020 Nov 23. PMID: 33309446

[8]  Charles Rolsky, Varun Kelkar, Erin Driver, Rolf U. Halden, Muni­cipal sewage sludge as a source of micro­pla­s­tics in the envi­ron­ment, Current Opinion in Envi­ron­mental Science & Health, Volume 14, 2020, Pages 16-22, ISSN 2468-5844, https://doi.org/10.1016/j.coesh.2019.12.001.

[9]  Ni, B., Zhu, Z., Li, W., Yan, X., Wei, W., Xu, Q., Xia, Z., Dai, X., & Sun, J. (2020). Micro­pla­s­tics Miti­ga­tion in Sewage Sludge through Pyro­lysis: The Role of Pyro­lysis Tempe­ra­ture. Envi­ron­mental Science and Tech­no­logy Letters, 7, 961-967.

[10]  Sudip Ray, Ralph P. Cooney, chapter 9 – Thermal degra­da­tion of polymer and polymer compo­sites, Myer Kutz, Hand­book of envi­ron­mental degra­da­tion of mate­rials (third edition), william Andrew publi­shing, 2018, pp. 185-206, https://doi.org/10.1016/B978-0-323-52472-8.00009-5

[11]  Osman, A.I., Farrell, C., Al-Muhtaseb, A.H. et al. Pyro­lysis kinetic model­ling of abun­dant plastic waste (PET) and in-situ emis­sion moni­to­ring. Environ Sci Eur 32, 112 (2020). https://doi.org/10.1186/s12302-020-00390-x https://www.researchgate.net/figure/Reaction-progress-a-versus-the-temperature-for-the-PET-pyrolysis-where-the-coloured-and_fig1_343994995.

[12]  Sudip Ray, Ralph P. Cooney, chapter 9 – Thermal degra­da­tion of polymer and polymer compo­sites, Myer Kutz, Hand­book of envi­ron­mental degra­da­tion of mate­rials (third edition), william Andrew publi­shing, 2018, pp. 185-206, https://doi.org/10.1016/B978-0-323-52472-8.00009-5