Exogenous Elemental Sulfur Promoting Methane Production and Simultaneous Ammonia Nitrogen Removal in Anaerobic Digestion of Food Waste: Experimental Verification and Mechanism Analysis

The treatment of food waste (FW) via anaerobic digestion (AD) is frequently plagued by a low methane yield and ammonia (NH₄⁺) inhibition. This study demonstrates that the addition of elemental sulfur (S⁰) effectively mitigates both of these issues. Through batch and continuous experiments, it was found that the specific methane yield was enhanced by up to 48.1% and the NH₄⁺ concentration decreased by 26.9% at the optimal S⁰ dosages of 20 mg/L. Metagenomic analysis revealed a dual mechanism underlying this enhancement: at low dosages, S⁰ provides a sulfur-containing functional group for the biosynthesis of methyl-coenzyme M, thereby accelerating the rate-limiting "methyl-transfer" step in methanogenesis; at high dosages, it promotes the biosynthesis of coenzyme A, which markedly enhances acidogenesis. Furthermore, S⁰ alleviates NH₄⁺ inhibition by fostering a synergistic interaction between sulfate-reducing bacteria and anammox bacteria, which convert NH₄⁺ to N₂. Continuous operation over 140 days confirmed the long-term stability and effectiveness of this S⁰ addition strategy. This study provides mechanistic insights into S⁰-driven methanogenesis in complex organic waste (FW) and offers a cost-effective, sustainable approach to enhancing AD efficiency and stability.