Enhancing anaerobic digestion with nano-magnetite: insights into short-term effects and long-term microbial adaptation

Anaerobic digestion is a widely applied biotechnology for renewable energy recovery and waste stabilisation. Nano-magnetite has been reported to enhance anaerobic digestion by facilitating direct interspecies electron transfer, providing redox buffering capacity, and supplying essential trace elements. However, their size-, dosage-, and delivery-dependent effects, especially during long-term operation, remain insufficiently understood. This study systematically evaluated the impacts of nano-magnetite size (20 vs. 50 nm), dosage (10–100 mg/L), and delivery mode (gradual vs. rapid) on anaerobic digestion performance using both short-term batch and long-term semi-continuous assays. Moderate nano-magnetite dosages (10–25 mg/L) enhanced methane production by up to 21.8 %, reduced total solids by 10.2 %, and improved process stability via accelerated volatile fatty acid degradation and enriched hydrogenotrophic methanogens. The dosage of 25 mg/L, 20 nm rapid-dosing group delivered the greatest economic benefit, boosting revenue by 42.5 % compared to the control. This gain was driven by enhanced methane yield and reduced slurry disposal costs, which also contributed to lower carbon dioxide emissions. High nano-magnetite loading (100 mg/L) caused aggregation (up to 45 % for 20 nm), oxidative stress, and extracellular polymeric substance production, shifting communities toward stress-tolerant taxa without notable methane gains, with partial performance recovery was observed after one hydraulic retention time. Environmental and economic analyses confirmed an average revenue enhancement of 28.6 % across nano-magnetite amended groups. However, operational pH near the magnetite isoelectric point increased aggregation risk, indicating the need for particle recovery strategies. Future work should optimise dosing strategies and evaluate nano-magnetite recovery methods under full-scale conditions.