Anaerobic digestion (AD) with inter-stage hydrothermal (HT) treatment of sewage sludge (i.e. AD-HT-AD) is a promising technique for energy recovery and nutrient recovery/recycling. Both AD and HT treatments impose significant impacts on the reclamation/recycling of critical elements, such as iron (Fe) and phosphorus (P). However, the co-evolution of these elements and their transformation mechanisms during the combined AD-HT-AD treatment of sewage sludge is poorly understood. This study conducted AD (15 days) – HT (90, 125, 155 and 185 °C) – AD (73 days) and investigated Fe and P speciation in the raw sludge, AD solids, and HT-derived hydrochars using complementary chemical extraction, X-ray absorption spectroscopy, and X-ray diffraction analyses. A significant fraction (41%) of Fe is present as vivianite in the digested solids from the first stage AD treatment of sewage sludge. After the inter-stage HT treatment of the digested solids, the fraction of vivianite in the HT-derived hydrochars decreased with increasing HT temperature and with the precipitation of more strengite. This is possibly due to the oxidation of vivianite by oxygen introduced from the headspace in the HT treatment reactors. After HT and in the subsequent AD treatment of HT-derived hydrochars, much more vivianite was formed, which can be attributed to the microbial reduction of strengite and other Fe(III) species. The results from this work suggest that Fe plays important roles in controlling P mineralogy during AD and HT treatments of sludges. This work also provides insights into the reaction mechanisms during AD-HT-AD treatments of sludges and can help evaluate the nutrient recycling and reclamation options for sludges.

Co-evolution of iron and phosphorous speciation during anaerobic digestion with inter-stage hydrothermal treatment of sewage sludge

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.