Carbonização hidrotérmica para valorização do lodo de esgoto: avaliação do processo e do potencial energético

Abstract

The treatment of sewage sludge (SS) is a global demand, requiring adequate methods that ensure environmental protection and public health. SS is a wet biomass generated in wastewater treatment plants (WWTPs) through aerobic, anaerobic, or primary treatment processes, depending on the configuration of the facility. For final disposal, dewatering methods such as centrifugation and drying are typically required before landfilling, which increases operational costs and processing time. In this context, hydrothermal carbonization (HTC) emerges as a promising alternative, since it enables the thermochemical conversion of wet biomasses as it uses the intrinsic moisture of the feedstock as reaction media and catalyst to produce a carbonaceous solid material known as hydrochar (HC) and a liquid phase named process water (PW). These products have potential applications in energy generation, soil conditioning, agricultural fertilization, and other environmental uses. In this scenario, this study aimed to evaluate the influence of temperature, acid addition, and solids content on the response process parameters (pressure, final pH and phase distribution) and on the energetic properties of the resulting hydrochars. Sewage sludge was collected from a WWTP located in Toledo, Paraná (Brazil), and used as feedstock for HTC. Experiments were conducted in a 0.5 L high-pressure reactor under different operational conditions, varying temperature (160–240 °C), addition of 1% H2SO4, and solids content (5–15%). The sludge and hydrochars were subsequently characterized to determine their physicochemical and energetic properties. The results showed that increasing temperature intensified dehydration and decarboxylation reactions, reducing the hydrochar mass yield (from 79.8% to 56.2%), volatile content, and the H/C and O/C ratios, while increasing aromaticity and ash content. However, higher temperatures also decreased the heating value (from 17.31 to 11.77 MJ·kg-1 ). Acid addition enhanced these conversion pathways, promoting further dehydration and decarboxylation, increasing carbon retention, raising the heating value (from 16.53 to 17.27 MJ·kg-1), and increasing both final pressure (at opening) and gas formation, indicating a higher reaction advance. Regarding solids content at the HC, higher values led to greater hydrochar yields, for example a 69.8% at 12.5% of solids load, increased ash concentration, and produced more carbonized hydrochars, as indicated by shifts observed in the Van Krevelen diagram. Hence, solids load higher than 10% are recommended to achieve desirable energetic properties to the HC for solid fuel use, besides the enhanced process energy efficiency. Overall, temperature, acidic medium, and solids content proved to be key parameters for tailoring the physicochemical and energetic properties of hydrochar, enabling its optimization according to the desired application and scenarios.