Effect of Interstage Hydrothermal Treatment on Anaerobic Digestion of Sewage Sludge: Speciation Evolution of Phosphorus, Iron, and Sulfur

Anaerobic digestion (AD) with interstage hydrothermal treatment (HT) (i.e., AD–HT–AD) is an emerging technology for energy and nutrient recovery from sewage sludge. Yet, systematic understanding on the speciation evolution of nutrient phosphorus (P) and other redox-sensitive elements iron (Fe) and sulfur (S) during AD–HT–AD is still missing. This study investigated the coupled changes of P, Fe, and S speciation during AD–HT–AD of sewage sludge using complementary chemical extraction, microscopy, and spectroscopy analyses. Results indicated that after the first-stage AD a significant fraction of Fe was present as vivianite in the AD solids due to the microbial reduction of strengite and other Fe(III) species. During HT of the anaerobically digested solids, less vivianite but more strengite formed in the HT hydrochars with increasing HT temperature. Partial auto-oxidation of vivianite by H2O at high HT temperatures (155 and 185 °C) induced the formation of strengite. For the second-stage AD, more vivianite formed in AD solids as compared to that in their corresponding hydrochars from higher HT temperatures (155 and 185 °C). Regarding S speciation, both AD stages and HT favored the formation of iron sulfides. Results from this study suggest that the cycling of both P and S is strongly coupled with Fe chemistry. This study sheds light on the reaction mechanisms during AD–HT–AD of sewage sludge and the nutrient recycling and reclamation options for sewage sludge management.