Combining process severity and response surface methodology: a comprehensive approach to phosphorus speciation in sewage sludge hydrothermal treatment

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.