In the present study, hydrothermal conversion (HTC) of dewatered sewage sludge (DSS) under sub- and near-critical water has been performed to investigate effects of reaction temperature and pressure, moisture content of DSS, and calcium oxide (CaO) additive on evolution profile and characteristics of gas, solid, and liquid products. Although energy recovery rate decreased with increasing temperature and pressure, significant decarboxylation and dehydration reactions led to hydrochars with best fuel quality at 320 degrees C. High moisture content favored decarboxylation reaction but reduced H-2 and CH4 yields. Compared to that in the absence of additive, H-2 yield increased almost 6-fold at 380 degrees C and Ca/C molar ratio of 0.2, resulting in 58% H-2 and 26% CH4 in final fuel gas. The results suggested that mineralization of heteroatomic compounds and dissolution of metals or mineral elements occurred during HTC. Under higher temperature and pressure, heavy metals or mineral elements were prone to be immobilized whereas dehalogenation became more distinct. CaO additive not only facilitated hydrolysis and deamination of organic compounds but also accelerated further fixation of inorganic elements and anions. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.