Microwave-Assisted Drying of Sewage Sludge: Kinetic Optimization, Energy Efficiency, and C:N:P Preservation for Biomass Valorization

Drying municipal sewage sludge represents a major energy bottleneck in biomass valorization, and conventional thermal methods often strip away its valuable nutrients. This research explores microwave-assisted drying as a strategy to enhance energy efficiency while preserving the carbon-to-nitrogen-to-phosphorus (C:N:P) stoichiometry critical for agricultural applications. Experimental trials evaluated sludge cakes across a broad spectrum of microwave powers (۱۶۰–۸۰۰ W) and various geometric configurations. The results revealed a consistent three-stage drying process, characterized by a distinct kinetic shift once moisture dropped below a ۵۰% threshold. Below this point, internal steam generation created a strong pressure gradient—often referred to as the “beggar's chicken effect”—that drove thicker samples to dry more rapidly than thinner ones. A power setting of ۴۸۰ W paired with a ۲.۵ cm cake diameter emerged as the optimal configuration, achieving a true thermal efficiency of ۴۹% and a specific energy consumption of just ۱.۹۲ MJ/kg. This performance outpaced conventional hot-air and infrared drying methods, achieving a ۳۴% lower specific energy consumption than hot-air drying. Crucially, the rapid, volumetric heating provided by microwaves mitigated the nitrogen loss typically triggered by prolonged thermal exposure. Consequently, the microwave-dried sludge maintained a C:N ratio of ۱۱, closely mirroring the raw feedstock and ensuring high agronomic value. Ultimately, these findings provide a practical, energy-efficient pathway for transforming biosolids into premium bio-fertilizers and bioenergy feedstocks within circular economy models.

کلیدواژه ها:

Microwave drying ، Sewage sludge ، C:N:P stoichiometry ، Bio-fertilizer ، Energy Efficiency