Effects of baking soda on Co-hydrothermal carbonization of sewage sludge and Chlorella vulgaris: Improved the environmental friendliness of hydrochar incineration process

Incineration is one of the prevailing methods of sewage sludge management worldwide. However, the pollutants of NOX, SO2, and heavy metals emitted severely limit this disposal method during the incineration process. By adding baking soda to the co-hydrothermal carbonization process of sewage sludge and Chlorella vulgaris, this study improved the cleanliness of hydrochar. The characteristics of hydrochar produced with 0, 4, 8, and 12 g baking soda/15 g sample were evaluated. When added 8 g baking soda/15 g sample, the content of N and S in hydrochar dropped to the lowest, 0.98% and 0.16%, respectively. Compared with the hydrochar produced with no baking soda, the contents of N and S of hydrochar with 8 g baking soda/15 g sample decreased by 66.78% and 80.72%, respectively. TG-FTIR analysis shown that NOX and SOX emissions from the hydrochar combustion process were reduced. As for heavy metals, they can be divided into two categories. One is easy to combine with amino acids hydrolyzed from protein to dissolve in the liquid phase; the other is easy to combine with amino acids and sedimented as a part of hydrochar. The fixation ratios of Zn, Cr, Cu, and Mn of the hydrochar produced with 12 g baking soda/15 g sample reached 74.34%, 93.53%, 79.47%, and 81.74%, respectively. The comprehensive assessment using multi-dimensional decision-making methods showed that the hydrochar produced with 12 g baking soda/15 g sample had the best environmental friendliness.