Effect of process parameters on phosphorus conversion pathways during hydrothermal treatment of sewage sludge: A review

•Elucidation of P conversion pathways during hydrothermal treatment of sewage sludge.•Effect of temperature, reaction time, acidic-basic treatment, and use of additives.•P recovery yield in the solids improves by increasing temperature and reaction time.•Fast organic P mineralisation to phosphate is usually observed at T > 180 °C.•Alkaline treatment and CaO addition favour conversion of non-apatitic to apatitic P. Sewage sludge represents a renewable source of organic carbon and nutrients such as nitrogen (N), potassium (K), and phosphorus (P) that can be valorised through the recovery of energy carriers (e.g. biofuels) and fertilizers (N, K, and P precipitates). This review analyses>60 recent studies that have investigated P recovery potential from sewage sludge by hydrothermal processes. The effect of process parameters such as temperature, residence time, pressure, solid-to-liquid ratio, and addition of additives on P conversion pathways has been investigated by a critical discussion of the results published in the literature. Results show that temperature is the most influential parameter for P speciation and repartition: the increase in temperature appears to promote the increase in solid P recovery yield, the mineralization of organic P, and the conversion of non-apatitic P into apatitic P. The increase in reaction time has similar effects as temperature, but to a lesser extent. Solid P recovery yield and apatitic P fraction can be enhanced by increasing the medium alkalinity and by adding Ca-containing reactants. Non-apatitic P fraction can be increased by lower medium alkalinity, and by the addition of Fe- and Al-containing reactants. The results of this review provide to researchers and practitioners in the field of sewage sludge management key elements for the best operation of hydrothermal reactors to improve the recovery of P and biofuels. Finally, some new research perspectives and technical challenges are proposed to improve the knowledge and the scaling up of the technology.