Material and energy recovery from sewage sludge
Currently, sewage sludge is used for agricultural purposes by direct land application or as compost and is incinerated or landfilled. In the Czech Republic, land application or composting is the predominant material used. In contrast, in Western European countries (e.g., Germany, Austria, Switzerland, the Netherlands, Belgium), incineration or co-incineration predominates. This approach is mainly based on concerns about environmental contamination by pollutants contained in sludge, such as heavy metals, microplastics, pathogens, organic pollutants (PAHs, per- and polyfluorinated substances (PFAS), flame retardants, pharmaceuticals, hormones or endocrine disruptors, etc.). These substances are removed during incineration and the resulting ash from mono-incineration is rich in phosphorus, a critical resource for the EU. For this reason, some countries, such as Germany, Austria, and Switzerland have already made it mandatory to recover (regenerate) phosphorus from the ash from the incineration of sewage sludge or directly from sewage sludge itself.
Within the Research Group of Waste Management and Sustainable Technologies, the team of Associate Professor Pohořelý is working on the thermochemical treatment of sewage sludge, in particular, the pyrolysis and incineration of sewage sludge with subsequent use of the solid products of the processes. The results of the research supported by projects “TH03020119 – Material transformation of sewage sludge into a fertilizer with increased phosphorus content” and “QK21020022 – Complex evaluation of the application of sewage sludge in agriculture concerning emerging pollutants” were successfully used in collaboration with HST Hydrosystémy, Ltd. to optimize the operation of a commercial unit for pyrolysis of sewage sludge at the Trutnov – Bohulavice wastewater treatment plant. The practical conclusions were that pyrolysis should be carried out at a temperature of at least 500°C (with temperatures above 600°C during nominal plant operation) to:
- achieve sufficient transfer of sludge energy to the primary pyrolysis gas, which also serves to heat the pyrolysis itself and to pre-dry the sludge,
- remove organic pollutants, pathogens, and microplastics present,
- and to create sufficient porosity in the sludge char.
(Energies 2020, J Anal Appl Pyrolysis 2021, J Anal Appl Pyrolysis 2022, Sci Total Environ 2024, Chemosphere 2021).
These results contribute to the modification of European and Czech legislation concerning the production and use of sludge-char in agriculture. For example, thanks to the cooperation sludge char produced at the Trutnov WWTP has been removed from the waste catalog and certified by the Central Institute for Supervising and Testing in Agriculture as a soil amendment (Karbofert T1) in the Czech Republic. Members of the team also act as scientific experts in the discussions led by the group for pyrolyzed sludges of the European Biochar Industry Consortium (EBI), which is tasked with initiating the modification of Regulation (EU) 2019/1009 of the European Parliament and of the Council laying down rules on the making available on the market of EU fertilizing products by creating its own Component Material Category for pyrolyzed sewage sludge.
In addition, the group led by Associate Professor Pohořelý is working on the following topics:
- Fluidized bed incineration of sewage sludge with the possibility of using the ash to recover phosphorus and other useful raw materials, which has been described in detail in a review article in J Environ Manage 2021.
- Legislation in the field of sewage sludge management at the national and international levels, e.g. in specific collaboration with scientific colleagues in Japan, conclusions published in Clean Techn Environ Policy 2023.
- Removal of per- and polyfluorinated substances (PFAS) and organic fluorine from sewage sludge by pyrolysis at laboratory and application scale at the Bohuslavice – Trutnov WWTP, in cooperation with the Institute of Microbiology of the CAS, with results published in Biochar 2024 – in press.
- Research on sewage sludge incineration, supported by the project “TK05020001 Fluidized bed technology for decentralized energy utilization of dried sewage sludge”, which will result in project documentation and a prototype of a 0.5 MW fluidized bed incinerator.
- Energies 13, 4087, 2020. DOI
- Chemosphere 265, 129082, 2021. DOI
- J. Anal. Appl. Pyrolysis 156, 105085, 2021. DOI
- J. Anal. Appl. Pyrolysis 161, 105387, 2022. DOI
- J. Environ. Manage. 315, 115090, 2022. DOI
- Clean Techn Environ Policy, 2023. DOI
- Sci Total Environ. 918, 170572, 2024. DOI
- Biochar, 2024 in press