Improving anaerobic digestion process against acetate accumulation: insights into organic loading rates and nano magnetite additions

• Acetic acid accumulation signals AD’s microbial imbalance and potential failure. • MNPs boost AD system resilience under acetate stress. • MNP pretreatments stabilize AD microbial structure and metabolic pathways under acetate stress. • MNP post-treatments address immediately to sudden stress-induced disruptions. Anaerobic digestion (AD) offers significant environmental benefits by converting organic waste into biogas and nutrient-rich digestate, supporting circular economy principles. However, acetic acid accumulation during feedstock overload disrupts microbial balance, reducing methane yields and causing instability. This study explored nano magnetite particles (MNPs) as additives to mitigate acetate stress and enhance AD performance. MNPs were applied using pretreatment and post-treatment strategies. Pretreatment with 20 nm and 50 nm MNPs improved methane production by 24.4 % and 19.1 %, respectively, compared to acetate-stressed systems. Post-treatment with 20 nm MNPs resulted in a 15.7 % methane production boost and a 16.4 % volatile solids reduction, facilitating rapid system recovery. MNPs acted as electron conduits, supporting direct interspecies electron transfer and providing bioavailable iron, crucial for methanogenic enzymatic functions. These mechanisms enhanced microbial resilience, mitigated oxidative stress, and restored functional pathways disrupted by acetate accumulation. The size-dependent effects of MNPs indicated that smaller particles (20 nm) were more effective in promoting acetate methanation, while larger particles (50 nm) enhanced microbial viability, highlighting the potential for customised, combined applications under diverse scenarios in AD systems. Economically and environmentally, MNPs enhanced methane production and reduced slurry output by up to 9.4 %, decreasing disposal costs and carbon emissions, thereby improving system sustainability. Future research should optimise MNP dosing strategies and explore recovery and recycling methods to ensure economic feasibility. At the tested concentrations (100 mg/L), MNPs offer a scalable, environmentally compatible solution to enhance AD performance, making them a promising additive for industrial-scale organic waste management.