Hydrothermal carbonization of sewage sludge: Effect of aqueous phase recycling

In this study, the aqueous phase produced from the hydrothermal carbonization (HTC) of sewage sludge (SS) with pure water was reused for the HTC of fresh SS, aiming to maximize energy recovery from the aqueous phase. The aqueous phase was recycled four times. The effects of aqueous phase recycling on the properties of the aqueous phase and hydrochar produced at temperatures of 200, 230, and 260 °C were studied. The hydrochar yield always decreased with increasing temperature regardless of whether the aqueous phase was recycled. The C and N contents and higher heating values of the hydrochars produced with aqueous phase recycling were all higher than that of the hydrochar produced with pure water and slightly increased as the number of recycling times increased. The O/C and H/C atomic ratios of the hydrochars revealed that dehydration was the main reaction pathway during HTC of SS. The pH value of the aqueous phase increased as the number of recycling times increased, indicating that the concentration of NH4+-N in the aqueous phase increased. The process of aqueous phase recycling affected the carbonization of the hydrochar. These results suggest that aqueous phase recycling was favorable for energy recovery from the aqueous phase produced from the HTC of SS and for improving the combustion properties of the hydrochar. Thus, we believe that aqueous phase recycling was a promising strategy for energy recovery, aqueous phase disposal, and the production of high-quality hydrochar with respect to the HTC of SS.