High-solid anaerobic co-digestion of pig manure with lignite promotes methane production

Abstract

High-solid anaerobic digestion is a common practice to treat organic wastes, but its efficiency of methane production is poor. In this study, the effect of high-solid anaerobic co-digestion (HS-AcoD) of pig manure with lignite (0%–64% based on total solid) on methane production at 37 °C was investigated. The results showed that pH, alkalinity, total ammonia nitrogen, and free ammonia decreased with the increasing ratios of lignite addition during HS-AcoD. After HS-AcoD for 30 days, the highest cumulative methane yield of 175.90 mL g−1 volatile solid was obtained for the treatment with 2% lignite addition, which was 9.26% higher compared with the control treatment without lignite addition (CK). However, the highest cumulative methane yield (the 8% lignite addition treatment) was 36.51% higher than CK after HS-AcoD for 12 days. The modified Gompertz model was fitted well to the methane production (R2 ≥ 0.9863). The synergistic effects of HS-AcoD with lignite ranged from 8.84 to 14.58% when lignite addition accounted for 2–50%. Structural equation models indicated that lignite mainly represented the indirect effect on cumulative methane yield by impacting C/N, pH and total ammonia nitrogen during HS-AcoD. High-throughput sequencing demonstrated that HS-AcoD of pig manure with lignite highly enhanced the relative abundance of Methanosaeta for methanogenesis. Overall, the results suggested that lignite addition could enhance the methanogenes’ activities and improve the methane production during HS-AcoD of pig manure with lignite.