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.

Hydrothermal pretreatment (HTP) enhances the anaerobic digestion (AD) of waste-activated sludge (WAS); however, severe conditions can exacerbate foaming and rheological challenges. This study evaluates whether bioaugmentation with a proprietary microbial consortium, Ydro Process®, can enhance or partially substitute high-severity HTP while advancing renewable biogas recovery from thickened waste-activated sludge (TWAS). Biochemical methane potential (BMP) assays were performed on TWAS subjected to HTP at 70 °C (HTP70) or 170 °C (HTP170), YDRO alone, and combined treatments (YDRO+HTP70; YDRO+HTP170). Performance endpoints included cumulative methane yield, biodegradability, solids reduction and solubilization, particle-size distribution, extracellular polymeric substances (EPS), viscosity, and foaming propensity. YDRO+HTP170 achieved the highest methane yield (257 mL CH4 g−1 COD_added) and biodegradability (67%), corresponding to a 46.8% improvement over untreated TWAS. YDRO+HTP70 yielded 249 mL CH4 g−1 COD_added and 62% biodegradability (+42.3%). Both combinations outperformed conventional HTP alone; notably, YDRO+HTP70 performed comparably to HTP170. All pretreatments reduced particle size and total EPS, increased soluble EPS, and lowered viscosity. Foaming was most pronounced when HTP was applied alone and was mitigated when YDRO was co-applied. These findings indicate that targeted bioaugmentation can synergize with thermal conditioning to enhance sludge digestibility and stabilize operation, enabling moderate-temperature HTP to approach high-severity performance. The combined strategy supports higher on-site biogas generation from TWAS as a renewable energy pathway.