Imroving energetic and material efficiency by hydrothermal carbonisation of sewage sludge; a parametric study

Sewage sludge is a phosphorus rich biogenic residue that can be converted via Hydrothermal Carbonisation (HTC) to a combustible hydrochar and process liquor. To enhance fuel properties of the hydrochar and dissolve phosphorus into liquid phase, the addition of CO₂ to the HTC was investigated. Using a Central Composite Design, the impact of temperature and time were taken into account. Linear modelling and ANOVA indicated little impact of time. The hydrochar dewaterability is enhanced by temperature and CO₂ addition, whereas the calorific values of the hydrochar is lower than that of the substrate as a consequence of the increasing ash content. CO₂ addition enhances phosphorus release to about 2 % into the liquor. Concluding, the CO₂ has positive impact on the product properties, although the experimental condition are insufficient for industrial applications.