Abstract: Abstract: Carbon and nitrogen emissions during sewage sludge treatment are important sources of greenhouse gases and environmental pollutants. The fixation degrees of carbon and nitrogen have been an issue deserving significant attention and consideration when choosing a treatment process for sewage sludge. Hydrothermal carbonization (HTC) is an emerging technology to treat wet biomasses aimed at producing biochar materials. Studies have demonstrated that HTC of wet biomasses including sewage sludge results in the formation of biochar in a relatively cheap and sustainable way. However, the data necessary to understand how multiple processing conditions influence carbon and nitrogen fixed in sludge biochar from HTC are currently lacking. In the present study, the influences of hydrothermal temperature (150-250℃), solid content (5%-15%), and reaction time (2-6 h) on the fixations of carbon and nitrogen in sludge biochar were investigated using a 3-level, 3-factor Box-Behnken experimental design. The results showed that the relationships between the carbon and nitrogen fixation and tested factors can be quantitatively described by multivariate quadratic equations with R2 of 0.9925 and 0.9903, respectively. Carbon and nitrogen fixation rates of 36.6%-52.9%, and 20.4%-42.5%, respectively were obtained under the tested hydrothermal carbonization conditions. Both the maximum carbon and nitrogen fixation rates were achieved at a hydrothermal temperature of 150℃, solid content of 10%, and reaction time of 2 h. The carbon fixation rate was negatively correlated with hydrothermal temperature and reaction time, but positively correlated with solid content. The significant effects (p<0.05) of hydrothermal temperature, solid content, and reaction time on carbon fixation rate were in a decreasing order. Yet, the nitrogen fixation rate was only significantly (p<0.05) and negatively related to hydrothermal temperature. The interaction between hydrothermal temperature and solid content had a significant effect (p<0.05) on carbon fixation, and the coupling of low hydrothermal temperature (≤169℃) and high solid content (≥7%) could maintain a fairly high carbon fixation rate (≥50%). The interactions between hydrothermal temperature and reaction time, and between hydrothermal temperature and solid content significantly (p<0.05) influenced nitrogen fixation. Compared with raising solid content and shorten reaction time, decreasing hydrothermal temperature had a more evident enhancing effect on increasing the nitrogen fixation rate. A low hydrothermal temperature was the key factor in bringing in a higher nitrogen fixation rate. The results from this study can help quantitatively monitor carbon and nitrogen emissions during sludge hydrothermal carbonization.