Hydrothermal carbonization as an alternative sanitation technology: process optimization and development of low-cost reactor

Jae Wook Chung; Gabriel Gerner; Ekaterina Ovsyannikova; Alexander Treichler; Urs Baier; Judy Libra; Rolf Krebs

doi:10.12688/openreseurope.14306.2

Hydrothermal carbonization as an alternative sanitation technology: process optimization and development of low-cost reactor

Open Res Eur. 2022 Jun 10:1:139. doi: 10.12688/openreseurope.14306.2. eCollection 2021.

Authors

Jae Wook Chung¹, Gabriel Gerner¹, Ekaterina Ovsyannikova², Alexander Treichler³, Urs Baier³, Judy Libra⁴, Rolf Krebs¹

Affiliations

¹ Institute of Natural Resource Sciences, Zurich University of Applied Sciences, Wädenswil, 8820, Switzerland.
² Institute of Agricultural Engineering, University of Hohenheim, Stuttgart, 70599, Germany.
³ Institute of Chemistry and Biotechnology, Zurich University of Applied Sciences, Wädenswil, 8820, Switzerland.
⁴ Postharvest Technology, Leibniz Institute for Agricultural Engineering and Bioeconomy, Potsdam-Bornim, 14469, Germany.

Abstract

Background: The provision of safe sanitation services is essential for human well-being and environmental integrity, but it is often lacking in less developed communities with insufficient financial and technical resources. Hydrothermal carbonization (HTC) has been suggested as an alternative sanitation technology, producing value-added products from faecal waste. We evaluated the HTC technology for raw human waste treatment in terms of resource recovery. In addition, we constructed and tested a low-cost HTC reactor for its technical feasibility. Methods: Raw human faeces were hydrothermally treated in a mild severity range (≤ 200 °C and ≤ 1 hr). The total energy recovery was analysed from the energy input, higher heating value (HHV) of hydrochar and biomethane potential of process water. The nutrient contents were recovered through struvite precipitation employing process water and acid leachate from hydrochar ash. A bench-scale low-cost reactor (BLR) was developed using widely available materials and tested for human faeces treatment. Results: The hydrochar had HHVs (23.2 - 25.2 MJ/kg) comparable to bituminous coal. The calorific value of hydrochar accounted for more than 90% of the total energy recovery. Around 78% of phosphorus in feedstock was retained in hydrochar ash, while 15% was in process water. 72% of the initial phosphorus can be recovered as struvite when deficient Mg and NH ₄ are supplemented. The experiments with BLR showed stable operation for faecal waste treatment with an energy efficiency comparable to a commercial reactor system. Conclusions: This research presents a proof of concept for the hydrothermal treatment of faecal waste as an alternative sanitation technology, by providing a quantitative evaluation of the resource recovery of energy and nutrients. The experiments with the BLR demonstrate the technical feasibility of the low-cost reactor and support its further development on a larger scale to reach practical implementation.

Keywords: energy; faecal sludge; fertilizer; hydrothermal carbonization; nutrient; resource recovery; sanitation; struvite.

Grants and funding

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 893588.