Unveiling the synergy of volatile fatty acids and ammonia nitrogen in optimizing methane production during dry anaerobic digestion of pig manure and corn straw

Dry anaerobic digestion (AD) is vulnerable to volatile fatty acids (VFA) and ammonia nitrogen. Understanding their synergy is crucial for optimizing strategies. In this study, the digestive performance, microbial dynamics, metabolic pathways, and synergistic relationships were investigated in the dry AD process by adding different concentrations of acetic acid (2, 5, and 8 mg/g) and urea (1, 5 mg/g N). Results showed that acid inhibition was caused by excessive pH reduction due to VFA accumulation. Propionic acid was the primary form of VFA accumulation. The inhibition of VFA accumulation could be alleviated using a low concentration of ammonia nitrogen (1 mg/g N urea) or be strengthened using a high concentration of ammonia nitrogen (5 mg/g N urea). Compared with the control group (82.2 mL/g VS), VFA and 1 mg/g N urea had a synergistic facilitation effect on the methane production (increasing by 1.7 %-18.1 %). This synergistic effect shortened the lag phases, enhanced the activity of Caldicoprobacter and Methanosarcina, increased the expression of genes encoding functional enzymes involved in the acetoclastic-methanogenic pathway. By contrast, VFA and 5 mg/g N urea had a synergistic inhibition effect on methane production (decreasing by 65.7 %–69.8 %). This synergistic effect prolonged lag phases, increased the relative abundance of ammonia-tolerant and acid-tolerant microbial (such as Methanoculleus) and reduced the expression of genes encoding functional enzymes involved in the hydrogenotrophic-methanogenic pathway. These results improve the understanding of methane regulation during dry AD.