Hydrothermal carbonization of anaerobic granular sludge: Effect of process temperature on nutrients availability and energy gain from produced hydrochar

Abstract Anaerobic granular sludge (AGS) has been applied for most highly efficacy anaerobic digestion systems like upflow anaerobic sludge blanket and expanded granular sludge bed reactors. As a by-product from these systems, AGS is prospected as a promising resource for energy and nutrients recovery from wastewater. In this study, hydrothermal carbonization (HTC) of AGS was investigated at different temperatures (160–240 °C) regarding the distributions of C, N and P in the hydrothermal products to maximize the utilization efficiency of AGS. Elemental composition and fuel characteristics of the hydrochar were evaluated. Results indicated that the percentages of C in hydrochar increased from 43.79% to 49.81% with the increase in HTC temperature, while N showed an opposite trend, decreasing from 9.58% to 5.49%. The higher heating value of hydrochar increased up to a maximum of 24 MJ/kg at 240 °C from 20 MJ/kg at 160 °C. However, the hydrochar yield decreased remarkably from 62% to 32%. As a result, the highest net energy output was about 6.86 MJ/kg achieved at 160 °C. Results from the van Krevelen diagram suggested that dehydration and decarboxylation reactions occurred during the HTC of AGS. In addition, the thermogravimetric analysis implied that the combustion of the produced hydrochar could be completed in two phases rather than the one phase as the raw AGS. With regard to other resources utilization, HTC was proved to be effective for AGS to immobilize and recycle phosphorus. The increase in HTC temperature exerted a limited effect on P distribution, resulting in less than 5% being released into the liquid at all tested HTC temperatures. Most of P were immobilized into the produced hydrochar where the bioavailable P fractions > 80%.