Phosphate recovery in the processes of hydrothermal treatment of phosphate-rich biomasses

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Hydrothermal carbonization (HTC) and hydrothermal liquefaction (HTL) are effective methods for recovering energy from wet biomasses like sewage sludge, manure, and algae, which also contain valuable phosphate essential for agriculture. To mitigate potential environmental impacts, phosphate recovery should be integrated into these processes, enhancing their production value. This research first explored how HTC reaction conditions influence phosphate behavior, noting that higher temperatures during HTC of digested sewage sludge increase acid-soluble phosphate in the solid product. The study then shifted to pilot-scale HTL, examining phosphate recovery specifically as struvite, a slow-release fertilizer. Different strategies are required to recover phosphate post-HTL depending on the biomass used. HTL of sludge and manure yielded a solid phase suitable for phosphate extraction and precipitation as struvite. For microalgae Spirulina, a promising one-step struvite precipitation approach was developed. The use of citric acid effectively enhanced struvite purity, particularly from sewage sludge. The research demonstrated that an air-agitated reactor could facilitate struvite production after HTL, showcasing the potential to produce ready-to-use fertilizer particles. These findings can enhance the life cycle and techno-economic assessments of HTC/HTL processes.