Enhancing anaerobic digestion of swine manure using magnetite: Insights into methane production and organic acids metabolism

Abstract

This study evaluated the impact of increasing total ammonia nitrogen (TAN; 0–12,000  mg/L) representative of swine manure anaerobic digestion (AD), and the mitigating effect of magnetite supplementation. In control reactors, methane yield peaked at 164  mL/g COD at 750  mg TAN/L, then declined with higher TAN, while magnetite improved yields to 175 and 149  mL/g COD at 750 and 1,500  mg/L, respectively. The 30 % inhibitory concentration (IC₃₀) for free ammonia nitrogen (FAN) increased from 126 to 141  mg/L with magnetite, whereas IC₆₀ and IC₉₀ showed negligible changes, indicating limited mitigation at higher FAN levels. Specific methanogenic activity (SMA) tests demonstrated that magnetite-enhanced direct interspecies electron transfer (DIET) supported partial recovery of methane production from acetate and butyrate at 1,500  mg TAN/L, while lactate showed minimal improvement, and propionate remained strongly inhibited. Propionate accumulation confirmed its role as a metabolic bottleneck under ammonia stress. Overall, magnetite improved methanogenic activity under moderate TAN but was ineffective under elevated ammonia stress.