Effect of aqueous phase recycling on iron evolution and environmental assessment during hydrothermal carbonization of dyeing sludge

Iron is usually added as a flocculant in the wastewater treatment process, indicating that the terminal product -sludge will contains. Aqueous phase (AP) is the main byproduct produced during hydrothermal carbonization (HTC) of sludge. Thus, HTC treatment of sludge produces wastewater containing Fe. Therefore, it is necessary to study the migration behavior of Fe during AP recycling process. This study aims to investigate the effect of AP recycling (APR) and catalytic AP recycling (CAPR) on hydrochar properties, iron evolution and environmental assessment during HTC of sludge. The yield of hydrochar increased with the increase of recycle number and CAPR process was more significant than APR (13.96 % for CAPR and 9.9 % for APR). After four AP recycling process, the approximate equilibrium moisture content (AEMC) of WHC (hydrochar obtained in APR) and HHC (hydrochar obtained in CAPR) decreased by 4.62 % and 6.27 %, respectively, indicating AP recycling process was beneficial in improving the dewatering performance of hydrochar. The dehydration and demethylation reaction were strengthened during APR process, while the CAPR process showed enhanced decarboxylation reaction. The AP recycling process facilitated the recovery of Fe from the liquid phase and reached stability after the third cycle. The XRD pattern showed that Fe in the WHC was mainly in the form of lepidocrocite (FeO(OH)) and vivianite (Fe3(PO4)2), while in HHC it was present in magnetite (Fe3O4) and butlerite (Fe(SO4)(OH)& BULL;2H2O) forms. The XPS spectra indicated that most of Fe3+ entered the interior of the hydrochar during the AP recycling process. Life cycle assessment (LCA) results showed that AP recycling process was more eco-friendly, but it will be upto at least three cycles. In theory, the environmental impact categories would continue to decline as AP recycling proceeded in the laboratory scale.