11. May 2023
Henriette zu Doha
agriculture | Biochar | biosolids | Co2-Footprint | PFAS | phosphorus fertilizer | Sewage Sludge | Waste

Waste-to-Value: From Sewage Sludge to Natural Ferti­lizer and Carbon Capture

Recy­cling sewage sludge, through the process of pyro­lysis, results in the produc­tion of high-quality biochar, contai­ning valuable phos­phorus, an essen­tial nutrient for plant growth. Moreover, this process 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 (thereby redu­cing CO2 emis­sions), when placed into the soil (CO2 removal certi­fi­cates).

Since 2015, PYREG has been instal­ling its proven, sustainable and scalable biochar produc­tion plants at Waste Water Treat­ment Plants (WWTP) throug­hout Germany, Denmark, Czech Repu­blic, Sweden, as well in the United States.  PYREG’s modular Systems have a small / compact foot­print, which allows for inte­gra­tion with exis­ting WWTP equip­ment such as sludge diges­ters, drying equip­ment, etc.

A signi­fi­cant revenue opportunity

Further­more, the reco­very of phos­phorus in the waste­water treat­ment process ensures inde­pen­dence from costly mineral phos­phorus imports that pollute the envi­ron­ment and the climate. Hence, what may seem like an addi­tional finan­cial burden for local autho­ri­ties, is in reality, a signi­fi­cant revenue oppor­tu­nity, as the carbo­niza­tion recy­cling process not only produces P-ferti­lizer biochar, but also provides rege­ne­ra­tive energy and enables valuable CO2 removal certi­fi­cates.  Hence, WWTPs can benefit from three revenue streams, whilst elimi­na­ting signi­fi­cant costs, such as sewage sludge transportation.

PYREG plants are now in opera­tion at more than 50 loca­tions around the world. And they also serve national envi­ron­mental autho­ri­ties, such as the US Envi­ron­mental Protec­tion Agency (EPA), as study and refe­rence plants; an example is the PYREG plant operated at the Silicon Valley Clean Water (SVCW) WWTP, near San Fran­cisco, California.

Biomass cycles

The heating of biomass in a low oxygen envi­ron­ment is called pyro­lytic carbo­niza­tion. In this process, organic carbon compounds are converted into a process gas and solid elemental carbon. While organic carbon compounds are degra­dable and natural decom­po­si­tion releases green­house gases such as CO2 or methane (CH4) into the atmo­sphere, elemental carbon is stable for thou­sands of years. As long as this carbon is not burned, it does not react with any element and remains in its stable form as C. Thus, it can be considered a perma­nent carbon sink, when it is used as a soil amend­ment in arable farming.

The charac­te­ristics of the carbo­niza­tion process

– A tempe­ra­ture and a process dura­tion, high and long enough, respec­tively, to remove important impu­ri­ties of the starting mate­rial such as viruses or micro­pol­lut­ants to “decom­pose” or “vola­ti­lize”
– The reten­tion of important nutri­ents such as phos­phorus in the solid phase.
– The ability to retain most of the carbon contained in the feedstock into stable carbon in the resul­ting biochar, thus provi­ding a stable carbon sink. This process is referred Biochar Carbon Removal (BCR)

Auto­thermal carbonization

The PYREG process enables the conver­sion of organic resi­dues to biochar with simul­ta­neous reco­very of thermal energy. The core of our tech­no­logy is the PYREG 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. The sewage sludge is almost comple­tely pyro­ly­ti­cally carbo­nized in a controlled process. In the process, the phos­phorus remains comple­tely available for plants.

The vola­tile compon­ents are freed from entrained particles by hot gas filtra­tion and burned flame­l­essly as hot process gas in the combus­tion chamber. The resul­ting combus­tion heat is partly used to heat the reactor, so that the process is ther­mally self-suffi­cient after the start-up phase.  Hence, PYREG Systems do not produce resi­dues such as pyro­lytic oil, which are costly and proble­matic to dispose of.

The FLOX combus­tion, with flue gas recir­cu­la­tion, in conjunc­tion with hot gas filtra­tion, allows very low flue gas emis­sions – espe­ci­ally low amounts of nitrogen oxides and dust – while simul­ta­neously crea­ting biochar and usable waste heat. Thus, PYREG Systems repre­sent a NetZero tech­no­logy, as they require signi­fi­cantly less energy to operate than the rene­wable energy they produce themselves.

Valuable output

The resul­ting excess heat is used for prepa­ra­tory drying of the raw mate­rial or fed into heating networks. Alter­na­tively, it can be used to gene­rate elec­tri­city should that be a goal. The resul­ting biochar can be used as a high-quality ferti­lizer. This is possible because the carbo­niza­tion process at more than 500 °C sani­tizes and decon­ta­mi­nates the dried sludge. And: The phos­phate reco­very rate with this process is more than 98 %.

High ferti­li­zing effect

The commit­ment to resource conser­va­tion requires us to recover phos­phorus from sewage sludge to make it available to farmers. Of the methods for phos­phorus reco­very, carbo­niza­tion at tempe­ra­tures of 500 to 700 °C is among the most carbon effi­cient and results in a product that can used directly as a ferti­lizer for soil appli­ca­tions without further chemical extra­c­tion. In 2021, the Hessian State Labo­ra­tory (LHL) in Giessen, commis­sioned by the Hessian Ministry of the Envi­ron­ment conducted a trial to compare the plant avai­la­bi­lity of ten recy­cled phos­phates with that of triple super­phos­phate (TSP) and with that of sewage sludge. The recy­clates differed in terms of their produc­tion, compo­si­tion and product form.
TSP is a calcium dihy­drogen phos­phate-contai­ning ferti­lizer, which has a converted content of more than 46 diphos­phorus pent­oxide (P2O5). The phos­phorus avai­la­bi­lity of the PYREG carbo­nate reached almost 90 % of the effect of the TSP (regrowth perfor­mance). This TSP ferti­lizer with 46% P2O5 costs curr­ently between 700 and 800 €/t. Thus, muni­ci­pa­li­ties will be able to gene­rate reve­nues instead of incur­ring signi­fi­cant costs.

Climate protec­tion benefits

Compared to conven­tional ferti­lizer, sewage sludge carbo­nates have a nega­tive global warming poten­tial. A study by the German Federal Envi­ron­ment Agency from 2019 comes to the conclu­sion that conven­tional ferti­lizer produc­tion in Germany emits about +1.2 kg CO2-equi­va­lents per kg of P2O5 [1]. Phos­phate reco­very processes such as preci­pi­ta­tion in digested sludge or centrate or sewage sludge ash cause CO2 emis­sions. Compared to the green­house gas poten­tial of these processes PYREG carbo­nates from sewage sludge have a nega­tive global warming poten­tial of -4.01 kg CO2 equi­va­lents per kg P2O5. Conse­quently, the reco­very of phos­phate in the Pyreg process and the final appli­ca­tion of the biochar contri­butes to the fight against global warming and to advance our goal of net zero.

In addi­tion, the phos­phate – reco­very rate of the sewage sludge carbo­nates is more than 98%, which is within the range of other thermal treat­ments and is far better than that of preci­pi­ta­tion processes with a reco­very rate of less than 40 %.

No micro­pla­s­tics

Still the direct appli­ca­tion of sewage sludge onto arable land is still a preferred method in some Euro­pean count­ries. Rese­ar­chers showed that sewage sludge contains signi­fi­cant amounts of micro­pla­s­tics. The elimi­na­tion of micro­pla­stic conta­mi­na­tion can only be achieved by high tempe­ra­tures during treat­ment and a suffi­ci­ently long reten­tion time. Ni et al. 2020 [2] stated that “poly­ethy­lene and poly­pro­py­lene, the two most common micro­pla­s­tics in sewage sludge, are comple­tely degraded at a carbo­niza­tion tempe­ra­ture of 450 °C.

No patho­gens

Sewage sludge mainly consists of human excreta and natu­rally contains patho­gens which by their very nature, are a signi­fi­cant risk to public health. The process condi­tions of pyreg carbo­niza­tion of more than 500 °C for more than ten minutes are more extreme than those of the CDC (Centers for Disease Control and Preven­tion) of the U.S. Depart­ment of Health and Human Services.
Accor­ding to the Steam Steri­liza­tion Disin­fec­tion and steri­liza­tion guide­lines of the CDC, the minimum steri­liza­tion condi­tions are as follows 132 °C for four minutes or 250 °C to remove patho­gens such as bacte­rial endo­to­xins under dry condi­tions (dry heat sterilization).

No conta­mi­nants

In a study published by the Federal Envi­ron­mental Agency in 2019 phar­maceu­tical resi­dues of various bioso­lids were analysed after pyro­lytic treat­ments at over 500 °C[3]. After carbo­niza­tion, all the para­me­ters of the of the analysed phar­maceu­ti­cals were below the detec­tion limit. The authors concluded that ther­mo­che­mical treat­ments such as carbo­niza­tion achieve complete destruc­tion of the drug residues.


Another example: perfluo­ri­nated and poly­fluo­ri­nated alkyl subs­tances (PFAS) are very persis­tent, long-lived and accu­mu­late in the envi­ron­ment and in our bodies. For this reason they are often referred to as “Forever Chemi­cals”. In this regard, a study by the US EPA from the year 2021 shows that the inte­grated carbo­niza­tion and combus­tion process of the PYREG plant operated near San Fran­cisco successfully elimi­nates PFASs [4].


The CO2-emit­ting inci­ne­ra­tion of sewage sludge or the untreated appli­ca­tion to soils  is no longer justi­fiable from the point of view of climate, envi­ron­mental and health aspects. Instead, carbo­niza­tion is a profi­table process for recy­cling the valuable raw material(s) from sewage sludge and supp­lying it to agri­cul­ture as refined biochar.
For muni­ci­pa­li­ties, this has several posi­tive effects: they close mate­rial cycles, meet their decar­bo­niza­tion targets, gene­rate signi­fi­cant amounts of rene­wable energy and create a high-quality, safe and envi­ron­men­tally friendly end product, which they can sell as an alter­na­tive to phos­phorus fertilizer.



  1. Umwelt­bun­desamt, „Ökobi­lan­zi­eller Vergleich der P-Rück­ge­win­nung aus dem Abwas­ser­strom mit der Dünge­mit­tel­pro­duk­tion aus Rohphos­phaten unter Einbe­zie­hung von Umwelt­fol­ge­schäden und deren Vermei­dung“, UBA Texte 13/2019, ISSN 1862–480
  2. Ni et al., 2020, „Micro­pla­s­tics Miti­ga­tion in Sewage Sludge through Pyro­lysis: The Role of Pyro­lysis Tempe­ra­ture“, Environ. Sci. Technol. Lett. 2020, 7, 12, 961–967, https://doi.org/10.1021/acs.estlett.0c00740
  3. Umwelt­bun­desamt, „Arznei­mit­tel­rück­stände in Rezy­klaten der Phos­phor­rück­ge­win­nung aus Klär­schlämmen“, UBA Texte 31/2019
  4. Envi­ron­mental Protec­tion Agency, „PFAS inno­va­tive treat­ment team (PITT) findings on PFAS destruc­tion tech­no­lo­gies“, February 17, 2021, https://www.epa.gov/chemical-research/pfas-innovative-treatment-team-pitt


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