Impact of key parameters on the hydrothermal carbonization process of sewage sludge: A systematic review

The hydrothermal carbonization (HTC) process is a promising green technology for reducing and reusing sewage sludge and can be a powerful tool against the current problems of energy scarcity, climate imbalance, and environmental pollution. Despite its admirable robustness, HTC can be easily tuned by controlling temperature and reaction time, as well as by biomass:water mass ratio and pH. The systematized knowledge of the relationship between these parameters is fundamental to successful innovative research aiming at scaling up the process. In this review, a bibliometric analysis based on ten years of scientific publications was carried out. The PRISMA guidelines were adopted to select forty-seven works as a source of information and critical discussion on key findings of reactions performed in high-pressure reactors. The global scientific community is mainly concerned with solid products (hydrochar) and the study of mechanisms of removal/adsorption of nutrients, minerals, and pollutants. The gaseous phase is in the background, opening up numerous research possibilities. Future outlooks are focused on coupling HTC with biorefineries to strengthen the valorization of sewage sludge, contribute to the production of biofuels and biofertilizers, and benefit communities currently without access to environmental sanitation.