Phosphorus is essential for global food production, but intensive agriculture disrupts its natural cycle, increasing reliance on non-renewable sources. A sustainable alternative is recovering phosphorus from waste streams using thermochemical processes. This work investigates the hydrothermal treatment of sewage sludge digestate to explore phosphorus conversion and speciation, aiming to optimize solid by-product as fertilizer. Through a novel integration of response surface methodology and process severity modeling, the study examines the influence of process parameters (temperature, time) on phosphorus behavior. A sewage sludge digestate, sampled from a wastewater treatment plant, was treated in a batch reactor at temperatures from 250 to 350 °C for 5-45 min. Products were centrifuged into a solid pellet and process water, followed by characterization of their physicochemical properties. Results show that temperature and time significantly impact by-product characteristics and phosphorus speciation. Treatments at 250-300 °C promote organic phosphorus mineralization and increase soluble phosphate concentration, while treatments at 350 °C lead to greater phosphorus recovery in the solid pellet, mostly as calcium phosphates. This research provides a framework for sustainable phosphorus recovery, suggesting hydrothermally treated pellets as potential slow-release fertilizers. Future work should comprehensively investigate the effect of calcium addition on phosphorus precipitation as calcium phosphates.

Pig manure is a renewable source of nutrients, such as phosphorus and nitrogen, that can be used to produce fertilizers. Hydrothermal treatment experiments using real pig manure were conducted to investigate the effect of temperature (107-200 °C) and reaction time (25-95 min) on nutrient conversion and distribution into hydrochar and process water. Sequential extractions were also performed to determine the phosphorus speciation in raw pig manure and hydrochars. The results showed that phosphorus and nitrogen recovery in the hydrochar was consistently above 94 % and 56 %, respectively, for all the experiments. Phosphorus content in the hydrochar increased from 15.4 to 24.6 mg P/g by increasing temperature and/or reaction time. Sequential extractions showed the increase in temperature led to a decrease in non-apatite inorganic phosphorus and organic phosphorus fractions in the hydrochars, while the apatite inorganic phosphorus fraction (Ca phosphates) increased. The concentration of total dissolved nitrogen in process water increased to values higher than 6000 mg N/L, while the ammonium concentration decreased to values lower than 2000 mg N/L by increasing temperature and/or reaction time. These results appear to indicate that the increase in hydrothermal temperature from 107 to 200 °C promoted the extraction and dissolution of organic nitrogen compounds from solid matrices to process water. This study provides valuable insights into P and N transformation during hydrothermal treatment of pig manure, which represent crucial information for developing sustainable treatment processes that aim to minimize waste disposal while enhancing the closure of anthropogenic P and N cycles.