Mitigating overload-induced stress in anaerobic digestion: long-term performance and fate of nano magnetite additives

Anaerobic digestion (AD) is known as an effective sustainable waste management and energy recovery technology. However, feedstock overload disrupts microbial stability by promoting volatile fatty acid (VFA) accumulation, reactive oxygen species (ROS) generation, and foam formation, leading to reduced methane yields and potential process failure. This study investigated the long-term application of nano magnetite particles (MNPs) to mitigate overload-induced stress under semi-continuous AD operation. MNPs (20 nm and 50 nm) were applied across various dosages (10-100 mg/L) and dosing strategies. Moderate additions (10-25 mg/L) significantly improved system performance, enhancing average methane production by up to 8.6 %, reducing hydrogen content by 27 %, and lowering propionic acid concentrations by 81.2 % compared to untreated overload systems. DIET-associated genes (e.g., pilA) increased by 27.4-36.5 %, and the abundance of versatile methanogens (Methanosarcina) rose by 219.6 %. In contrast, high-dosage MNPs (100 mg/L) induced microbial stress through increased ROS, iron solubilisation, and nanoparticle aggregation (up to 60 %) under an average pH of 7.18, increasing toxicity and foaming risks. Economic analysis revealed that 25 mg/L 50 nm MNPs improved revenue ninefold and reduced slurry disposal costs by 21.3 %. However, excessive MNP accumulation highlights the need for cautious dosage control. This study demonstrates the potential of MNP application to recover overloaded AD systems and provides practical insights into long-term nanoparticle behaviour, supporting the design of resilient and sustainable AD operations. Future work should explore strategies for MNP recovery and reuse, while assess their long-term environmental fate in digestate applications.