Dewatered sewage sludge was upgraded to hydrochar using hydrothermal conversion in sub- and near-critical water. Three characteristic temperature regimes responsible for the upgrading were identified. Drastic hydrolysis of carbohydrates, amide II or secondary amines occurred at 200 degrees C while noticeable decarboxylation initiated above 260 degrees C. Elevated temperature improved porosity but did not induce higher surface area. Aliphatic C was mainly transformed to aromatic hydrocarbon rather than aromatic C-O in subcritical water, whereas COO/N-C=O and aromatic C-O were decomposed to carbohydrate C at 380 degrees C. Below 300 degrees C, carbon functionalities in hydrochars were thermally stable and faster decomposition of N than (C) under bar-(C, H) resulted in dramatic decline of N/C. Above 300 degrees C, (C) under bar -H was gradually polymerized to aromatic (C) under bar-(C, H) which was considerably transformed to (C) under bar-(O, N) and (C) under bar -H at 380 degrees C. CaO favored intense destruction of aromatic C-C/C-H, anomeric O-C-O, (C) under bar -H and (C) under bar-(O, N) functionalities but introduced more aromatic C-O and O=(C) under bar -O. (C) 2016 Elsevier Ltd. All rights reserved.