Thermochemical processing of sewage sludges: Comparative analysis of hydrochars and biochars for sustainable phosphorus recovery

Securing sustainable phosphorus (P) resources from municipal sewage sludge is critical for circular nutrient management. This study investigates the effects of hydrothermal carbonization (HTC, 180 °C) and pyrolysis (450 °C and 600 °C) on two different sewage sludges: Stabilized Sewage Sludge (SSS) derived from an extended aeration activated sludge (EAAS) system, and Sludge Cake (SC) obtained from an anaerobic–anoxic–oxic (A/A/O) treatment process. Feedstock origin strongly governed phosphorus speciation, transformation pathways, and char properties during thermochemical processing. The two raw sludges were influenced by their original treatment, with SSS showing higher total phosphorus (TP = 39.04 mg g −1 ) but a lower Ca/P molar ratio (0.20) compared with SC (TP = 26.10 mg g −1 , Ca/P = 0.50), indicating distinct P partitioning between mineral and organic fractions. Hydrochars (SH from SSS, CH from SC) showed moderate total P concentrations (TP) and retained some organic P (OP), whereas pyrolysis produced biochars with higher TP: 59.7 mg g −1 in SB600 (SSS) and 46.0 mg g −1 in CB600 (SC). OP decreased significantly from 6.38 mg g −1 (SSS) and 8.15 mg g −1 (SC) to below 2.5 mg g −1 , while inorganic P (IP) became dominant. In SSS biochars, apatite phosphorus (AP) was the main fraction, while in SC biochars, non-apatite inorganic P (NAIP) remained significant. These contrasting behaviors are attributed to feedstock-dependent mineral compositions, where Fe- and Al-associated fractions in SSS promoted conversion of NAIP into AP, whereas the higher initial Ca content in SC favored NAIP retention. Solid-state 31 P NMR confirmed that orthophosphate (Ortho-P) is the main P species, highlighting its bioavailability and thermal stability. Pyrolysis increased carbonization, decreased H/C and O/C ratios (down to 0.44 and 0.33 at 600 °C), and generated highly porous, thermally stable chars. Overall, these results demonstrate that both sludge type and thermochemical pathway critically influence char properties and phosphorus forms.