Synergistic Role of Low-Strength Ultrasound and Co-Digestion in Anaerobic Digestion of Swine Wastewater

Swine manure poses significant challenges for anaerobic digestion due to its low carbon-to-nitrogen (C/N) ratio and elevated ammonia concentrations, both of which restrict methane generation. This study investigated the impact of integrating low-intensity ultrasound with co-digestion of piggery wastewater and food waste leachate. Laboratory-scale upflow anaerobic sludge blanket (UASB) reactors were employed under four operational conditions to evaluate anaerobic digestion performance, track shifts in microbial community structure, and assess the abundance of antibiotic resistance genes (ARGs). Co-digestion significantly enhanced methane production, yielding 1.3–3.2 times more than manure alone, while low-intensity ultrasound further increased methane yields by approximately 36–44% at high loading rates. Moreover, coupling low-intensity ultrasound with co-digestion led to the most rapid recovery following an overloading shock. Unexpectedly, ultrasound treatment alone increased the expression of certain ARGs (tetG, sul1, ermB) and the Integrase gene (intI1), while co-digestion led to a reduction in these genetic markers. These findings clearly indicate that the concurrent application of co-digestion and low-intensity ultrasound achieved the highest methane yield, the fastest recovery after organic overloading, and greater suppression of specific ARGs.