Hydrothermal Carbonization of Sewage Sludge: A Pathway toward Sustainable Energy Transition and Regional Development

The transition toward low-carbon energy systems requires technologies capable of converting residual biomass into value-added energy carriers. In this context, hydrothermal carbonization (HTC) of sewage sludge (SS) has emerged as a promising pathway for producing solid biofuels while addressing persistent challenges in waste management and sanitation. This study evaluates the technological basis, environmental implications, and regional development potential of SS-HTC within renewable energy and decarbonization strategies. HTC enables the direct processing of wet sewage sludge, avoiding energy-intensive drying steps and generating a carbon-rich hydrochar with reduced moisture content and improved fuel properties compared to the raw material. In addition to the solid fraction, the process produces a nutrient-rich liquid stream that can be further valorized, supporting circular resource management and integrated waste-to-energy systems. Although the estimated global production potential of SS-derived hydrochar (approximately 45 million dry tons) is insufficient to replace fossil coal on a large scale, its targeted use in selected industrial sectors can partially substitute mineral coal, contributing to greenhouse gas mitigation and incremental decarbonization. Beyond its technological advantages, SS-HTC offers opportunities for regional development, particularly in areas with expanding wastewater treatment infrastructure. Integrating HTC into sanitation systems enables localized biofuel production, reduces dependence on imported coal, and supports industrial diversification. The State of Paraná, Brazil, with a population of 11.5 million and sanitation coverage of approximately 81%, exemplifies a favorable context in which SS-HTC can enhance regional competitiveness while advancing environmental sustainability and socio-economic development within the broader low-carbon transition.