Supplementation of Schwertmannite improves methane production and heavy metal stabilization during anaerobic swine manure treatment

Abstract Anaerobic conversion of animal manure to methane is an important bioenergy strategy. However, low methane productivity and heavy metal contamination hamper the sustainability of anaerobic swine manure treatment. In this study, we propose using Schwertmannite (Sch, Fe8O8(OH)4.5(SO4)1.75), a natural Fe-rich waste widely existing in acid mine drainage, to enhance biomethanation of swine manure. Results showed that the addition of 5 g/L Sch to the anaerobic digestion of swine manure loaded with Cu, Zn, and As improved the production of methane in batch and continuous-flow stirred tank reactors by 44.1% and 47.6%, respectively. The positive impact of Sch may be related to the stimulated enrichment of Methanosaeta (from 54.3% to 71.5%) and Methanospirillum (from 2.0% to 5.5%) for methanogenesis and the increased contents of extracellular polymeric substances (>2.6 times), especially aromatic and tryptophan-like proteins, for enhanced substrate degradation and anaerobic microbial aggregates formation. After anaerobic treatment by the addition of Sch, Cu, Zn, and As were dominantly bound to residual fractions and oxidizable fractions in solid digestate, and small amounts accumulated in liquid digestate, indicating that the stabilization of toxic metals in the treated swine manure improved greatly. These results demonstrate that Sch can be utilized as a viable and cost-effective enhancer to improve the anaerobic production of methane from heavy metal-bearing animal manure and reduce the risk of heavy metal pollution.