Effect of different flocculants on the characteristics of hydrochar and hydroliquid derived from the hydrothermal treated active sludge. A comparative study

Abstract

In this study, the effect of flocculants on the physicochemical properties of hydrochar and hydroliquid derived from hydrothermal carbonization (HTC) treated sewage sludge (SS) was investigated. The results suggested that the flocculants including polyacrylamides (PAM), polyaluminium chloride (PAC) and FeCl3 can be transferred from the liquid to hydrochar during the HTC treatment process. The PAM monomers produced from thermal degradation can be absorbed on hydrochar. The Al3+ and Fe3+ in hydrochars were in the form of oxides, hydroxides or salts. The transformation of N, S and P during HTC can be affected by the flocculants, and the thermochemical properties of the derived hydrochars exhibited obvious difference. The influence mechanisms on elements transformation can be attributed to phosphate precipitation by metal ions, pH change caused by flocculants hydrolysis and catalysis of the ions. The thermal release of hydrocarbon and N/S containing gases from the three kinds of hydrochars with PAM, PAC and FeCl3 addition demonstrated significant differences. The hydrocarbon released from hydrochar with PAC addition was much lower than the other groups, while the hydrochar with FeCl3 addition exhibited improved S containing gases release. In addition, the water quality parameter, especially total phosphorus (TP) concentration was affected by flocculants. Phosphorus can be enriched in hydrochar by PAC with 95.1% of TP kept in hydrochar. The volatile organics including fatty acids, alkanes, ketones, aldehydes, amides and pyrazines in the liquid phase were also affected by flocculants. The obtained results in this study will provide valuable information for SS utilization by HTC treatment.