Long-term performance and microbial dynamics of a two-stage expanded granular sludge bed system treating pig slurry...

Pig slurry represents a major environmental challenge due to its increasing production and its potential to cause severe water and soil pollution if not properly managed. In this context, high-rate anaerobic digestion (AD) has emerged as a promising strategy to treat such high-strength waste streams while enabling energy recovery. However, its performance under real-world conditions and associated microbial dynamics remain insufficiently explored. In this study, a two-stage AD system based on expanded granular sludge bed (EGSB) reactors was operated under long-term real conditions to evaluate process performance and microbial adaptation during pig slurry treatment. The system was progressively intensified by reducing the total hydraulic retention time (HRT) from 12 to 1.6 days across different operational stages, while maintaining stable performance at high organic loading rates (OLR). Chemical oxygen demand (COD) removal efficiencies of up to 75% were achieved, together with high volumetric biogas production rates of up to 1.7 m³ m^-3 d^-1, demonstrating the potential of this configuration for efficient waste valorization within a circular economy framework. Additionally, microbial analyses revealed clear differentiation between reactors, with the first stage favoring hydrolytic and fermentative bacteria, including Clostridium sensu stricto 1, while the second stage supported a more stable methanogenic community dominated by Methanosaeta. Overall, the results demonstrate that the two-stage EGSB configuration enables stable and efficient treatment of pig slurry under intensified conditions, providing mechanistic insights into microbial adaptation and a strong basis for the large-scale implementation of high-rate anaerobic technologies for livestock waste management.