Phosphorus recovery from hydrothermal conversion products of sewage sludge through wet-chemical methods

Phosphorus (P), as a crucial resource, has attracted more attention for its recycling. Products from hydrothermal reaction of sewage sludge (SS), represent promising sources for efficient P recovery. This work systematically summarizes reaction mechanisms of SS hydrothermal reactions and corresponding distribution, migration, and morphological transformation pathways of P during reactions. The formation of hydrochar is mainly attributed to the polymerization, aromatization, and condensation of remarkably reactive intermediates generated through the degradation of biopolymers in SS, as well as the solid-solid conversion of part of lignin and the precipitation of insoluble humic substances. P in SS is mostly concentrated in solid-phase products (hydrochar) after hydrothermal reaction. Orthophosphate is the dominant P species in hydrochar. The distribution and migration transformation pathways of P are notably influenced by reaction conditions (i.e., reaction temperature, reaction time, initial pH and additives). The reaction time had a small effect on P conversion. Wet chemical extraction is widely used to extract P from hydrochar. The P extraction efficiency of acid extraction is higher than that of alkaline extraction. However, the precipitation of heavy metals in hydrochar limits industrial application of acid leaching. Ultimately, some prospects for the optimization of the P recovery process are put forward.