Attapulgite enhances methane production from anaerobic digestion of pig slurry by changing enzyme activities and microbial community

This study evaluated the effect of attapulgite addition on the anaerobic digestion of pig slurry. Results showed that attapulgite addition increased methane yield by 8.9%–37.3% and affected methane production kinetics. Attapulgite with the addition loading of 10 g/L obtained the highest methane yield of 210.4 mL/g volatile solids. Attapulgite accelerated the rates of hydrolysis, acetogenesis, and methanogenesis, as evidenced by the increases in the activities of β-glucosidase, protease, dehydrogenase, and coenzyme F420; the abundance of hydrolytic and acetogenic bacteria (Clostridiales, Syntrophobacterales, and Fibrobacterales); and the abundance of methanogenic microorganisms (Methanomicrobiales). These phenomena can be ascribed to both the reduced ammonia nitrogen due to attapulgite adsorption and the increased trace elements, such as Ca2+, Mg2+, K+, and Fe3+ ions, due to attapulgite release. However, excessive dose of attapulgite (≥30 g/L) decreased the promotion of methane production by reducing the enzyme activities of hydrolytic and methanogenic steps and lowering the abundance of hydrolytic, acetogenic, and methanogenic microorganisms due to the high levels of free ammonia and metal ions, such as Mg2+, at the initial phase of digestion. These results provided insights into the improvement of methane production during the anaerobic digestion of animal slurry.