Sustainable Upcycling of Swine Wastewater Sludge: Using Thermal and Citrate Pretreatment to Enhance Volatile Fatty Acid Production

The sustainable management of intensive swine farming is currently bottlenecked by the difficult valorization of metal-rich wastewater sludge. The structural rigidity of this sludge, stabilized by divalent cation bridging, severely limits its anaerobic digestion and overall resource recovery. To optimize the manure management chain, this study comprehensively evaluated various physical and chemical pretreatments to identify the most effective disintegration strategy for enhanced volatile fatty acid (VFA) production. Among the tested conditions, the coupling of thermal hydrolysis with citrate chelation (T/SC) was the most effective, achieving the highest disintegration degree (12.37%) and biopolymer solubilization. Mechanism analysis revealed that, unlike traditional alkaline treatments, which are limited by the severe reprecipitation of magnesium and phosphate, citrate effectively sequestered bridging cations (Ca2+ and Mg2+) via ligand exchange. This synergistic disintegration accelerated the fermentation kinetics, enhancing the total VFA yield 2-fold (1293 mg/L) compared to the control group while maintaining a high-value, butyrate-dominant product profile. These findings demonstrate that targeting ionic bridges via ligand-promoted dissolution provides a highly practical and sustainable strategy to maximize resource recovery and nutrient cycling from metal-laden livestock wastes.