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

Hydrothermal carbonization (HTC) and hydrothermal liquefaction (HTL) are promising processes to recover energy from wet biomasses, such as sewage sludge, manure, and algae. These biomasses also contain phosphate, which is irreplaceable for agriculture. Their hydrothermal treatment should therefore include phosphate recovery. Otherwise, a negative environmental impact could occur. Phosphate recovery adds value to the production chain. To provide a step towards effective phosphate recovery, this research first investigated how HTC reaction conditions affect the fate of phosphate. This is an important basis for both processes as the temperature in HTC is lower than in HTL. It was found that increasing the temperature of HTC of digested sewage sludge leads to increasing the content of acid-soluble phosphate in the solid product. Next, more specifically, the focus was on pilot-scale HTL. Here the general investigation on phosphate recovery was followed by a more specific study on phosphate recovery as a slow-release fertilizer, struvite. Various efforts depending on the biomass must be done to recover phosphate after HTL. HTL of sludge and manure provided a solid phase suitable for phosphate recovery by its extraction and subsequent precipitation as struvite. For HTL of microalgae Spirulina, struvite precipitation in a one-step approach from the aqueous product was promising. The use of citric acid was found to be effective to improve the purity of struvite, especially in the case of sewage sludge. This research showed that the air-agitated reactor could be used for struvite production after HTL. It showed the potential to harvest ready-to-use fertilizer particles. The findings of this research can improve the further life cycle and techno-economic assessments of HTC/HTL.