Evaluation of the applicability of herbal medicine waste as a new bulking agent for sewage sludge bio-drying and the produced bio-coal

Dehydrating and valorizing sewage sludge is essential for sustainable waste management, yet conventional thermal drying often requires substantial energy and lacks economic feasibility. This study is the first to apply herbal medicine waste as a bulking agent for bio-drying of sewage sludge, evaluating whether external heat input can be avoided while enhancing drying performance and improving the quality of recoverable bio-coal. Sewage sludge was blended with 0%, 10%, 20%, and 30% herbal medicine waste and treated for twenty days in lab-scale reactors while temperature, moisture content, calorific values, and microbial communities were monitored. As the mixing ratio increased, biologically generated heat intensified; notably, the thirty-percent condition achieved sufficient microbial heat generation despite a low initial C/N of 11.36, raising the peak temperature to approximately 45 °C. This condition yielded a final moisture content of 36.66% and a moisture reduction of 44.28%. Sequencing of 16S rRNA revealed a shift from anaerobic or facultative groups to aerobic and thermotolerant dominants, aligning with the rapid early-stage drying phase, with Acinetobacter and Pseudomonas prominently observed. The bio-dried solids from the thirty-percent mixture showed improved fuel properties, including higher fixed carbon, a higher heating value of 16.47 MJ/kg, a lower heating value of 14.91 MJ/kg, and a fuel ratio of 0.29, while chlorine and sulfur decreased by 96.1% and 97.6%, enhancing combustion suitability and environmental safety. Overall, bio-drying assisted by herbal medicine waste offers a low-energy, economically viable, and scalable pathway to produce cleaner, high-quality bio-coal from sewage sludge.