Magnetite particles accelerate methanogenic degradation of highly concentrated acetic acid in anaerobic digestion process

Abstract

The anaerobic digestion (AD) process has become significant for its capability to convert organic wastewater into biogas, a valuable energy source. Excessive acetic acid accumulation in the anaerobic digester can inhibit methanogens, ultimately leading to the deterioration of process performance. Herein, the effect of magnetite particles (MP) as an enhancer on the methanogenic degradation of highly-concentrated acetate (6 g COD/L) was examined through long-term sequential AD batch tests. Bioreactors with (AM) and without (AO) MP were compared. AO experienced inhibition and its methane production rate (qm) converged to 0.45 L CH4/g VSS/d after 10 sequential batches (AO10, the 10th batch in a series of the sequential batch tests conducted using bioreactors without MP addition). In contrast, AM achieved 3–425% higher qm through the sequential batches, indicating that MP could counteract the inhibition caused by the highly-concentrated acetate. MP addition to inhibited bioreactors (AO10) successfully restored them, achieving qm of 1.53 L CH4/g VSS/d, 3.4 times increase from AO10 after 8 days lag time, validating its potential as a recovery strategy for inhibited digesters with acetate accumulation. AM exhibited higher microbial populations (1.8–3.8 times) and intracellular activity (9.3 times) compared to AO. MP enriched Methanosaeta, Peptoclostridium, Paraclostridium, OPB41, and genes related to direct interspecies electron transfer and acetate oxidation, potentially driving the improvement of qm through MP-mediated methanogenesis. These findings demonstrated the potential of MP supplementation as an effective strategy to accelerate acetate-utilizing methanogenesis and restore an inhibited anaerobic digester with high acetate accumulation.