Impact of Physical, Chemical, Biological, and Thermal Pretreatments on the Hydrolysis and Solubilization of TWAS Under Anaerobic Conditions

Anaerobic digestion (AD) of thickened waste-activated sludge (TWAS) is widely applied for sludge stabilization and renewable energy recovery; however, hydrolysis of complex organics often limits fermentation performance. This study evaluated the effects of multiple pretreatment strategies on solubilization, volatile fatty acids (VFAs) production, and extracellular polymeric substances (EPS) during 80 h mesophilic batch fermentation. Pretreatments included hydrothermal treatment (HTP; 70, 90, and 170 °C), ultrasonication (US; 3000, 5000, and 10,000 KJ/kg TS), chemical pretreatment (acidic pH 4 and alkaline pH 10), and biological augmentation using YDRO Process® (YDRO®; 5%, 10%, 15% v/v). Across feedstock pretreatments, HTP generated the greatest improvements in solubilization, increasing SCOD by 56–113-fold and producing substantial acetate levels, particularly at 70 °C, alongside substantial phosphorus release. Ultrasonication resulted in moderate solubilization (28–56-fold) and elevated soluble phosphorus and ammonia. Acidic pretreatment maximized soluble phosphorus, but showed limited VFAs production, whereas alkaline pretreatment rapidly increased soluble EPS due to pH-induced cell disruption. Bioaugmentation achieved the highest total COD but yielded comparatively low soluble fractions. Following fermentation, HTP 170 °C consistently outperformed other treatments, maintaining elevated soluble COD and producing the highest acetate concentration. EPS analysis revealed extensive protein and polysaccharide degradation in thermal and bioaugmented systems, indicating active utilization during fermentation. Overall, the results demonstrate that targeted pretreatment strategies significantly enhance organic solubilization, EPS disruption, and VFAs yields, with thermal pretreatment showing the greatest potential to accelerate hydrolysis and acidogenesis. These findings provide valuable insights for optimizing the pre-methanogenic stages of AD and improving the efficiency of sludge treatment and resource recovery.