Effect of Prestage Hydrothermal Treatment on the Formation of Struvite vs Vivianite during Semicontinuous Anaerobic Digestion of Sewage Sludge

Anaerobic digestion (AD) with prestage hydrothermal treatment (HT) is an emerging strategy for the sustainable management of sewage sludge and recovery of valuable resources. Both struvite and vivianite can be precipitated during AD process and serve as two important phases for phosphorus (P) and/or nitrogen (N) recovery. Yet the effect of HT conditions on the phase selection and precipitation of struvite vs vivianite in the subsequent AD process remains unclear. This study investigated the evolution and mineralization of P and N in sewage sludge during AD with prestage HT using complementary chemical extraction and X-ray spectroscopy characterizations. Influencing factors such as sludge type and HT temperature were explored. For raw sludge with high molar ratio of Mg/Fe (5.9), HT did not induce the formation of struvite in the HT-derived hydrochars, because the acidic pH of HT slurries was similar to that in the raw sludge. Struvite was observed during the AD of low temperature (90 and 125 degrees C) HT slurries due to the reaction of Mg-phosphate phases with high concentration of NH4+ at pH > 7. For HT slurries prepared at 155 degrees C, the pH was always below 7 during the subsequent AD process, preventing struvite precipitation in the AD solids. For raw sludge with low molar ratio of Mg/Fe (1.1), HT at 90 and 125 degrees C induced the formation of vivianite in the hydrochars, as compared to the formation of strengite at 155 degrees C HT. In comparison with the corresponding hydrochars, more vivianite was precipitated during the subsequent AD via microbial reduction of Fe(III) species. Results from this study suggest that the formation of struvite or vivianite in the final AD solids is highly dependent on the Mg/Fe molar ratio and/or the pH of AD suspensions. Meanwhile, both HT and AD also strongly affected N speciation evolution in sewage sludge. Along with the release of large amounts of ammonia, HT favored the accumulation of quaternary- and pyrrole-N in the hydrochars, while only pyrrole-N was enriched in the AD solids. This study sheds light on the recycling and reclamation of P and N during HT-AD of sewage sludge.

Automated summary

This study investigated the evolution and mineralization of P and N in sewage sludge during AD with prestage HT, revealing that the formation of struvite or vivianite in the final AD solids is highly dependent on the Mg/Fe molar ratio and/or the pH of AD suspensions. Additionally, both HT and AD strongly affected N speciation evolution in sewage sludge.