Fly ash and HO assisted hydrothermal carbonization for improving the nitrogen and sulfur removal from sewage sludge.

In this study, fly ash and hydrogen peroxide (HO) assisted hydrothermal carbonization (HTC) was used to improve the removal efficiency of nitrogen (N) and sulfur (S) from sewage sludge (SS). The removal rate and distribution of N and S in hydrochar were evaluated, and properties of the aqueous phase were analyzed to illustrate the N and S transformation mechanism during fly ash and HO assisted HTC treatment of SS. The results suggested that during HTC process assisted by fly ash (10% of raw SS), dehydration, decarboxylation and hydrolysis of SS were strengthened due to the catalysis effect. The N and S removal were promoted marginally. For hydrochar achieved from HTC process with HO addition, the N and S removal were improved slightly due to the biopolymer oxidization by ‧OH released from HO decomposition. While for HTC treatment with fly ash and HO supplementation, a positive synergistic effect on N and S removal was observed. The N and S removal obtained from fly ash (10% of raw SS) and HO (48 g/L) assisted HTC increased to 81.71% and 62.83%, respectively, from those of 69.53% and 49.92% in control group. N and S removal mechanism analysis suggested that hydroxyl radicals (‧OH) produced by HO decomposition will destroy SS structure, and the biopolymers such as polysaccharides and proteins will be decomposed to release N and S into the liquid residue. In addition, the fly ash acts as the catalyst will decrease the energy need for denification and desulfartion. Consequently, N and S removal efficiency was enhanced by fly ash and HO assisted HTC treatment.