Leveraging oyster shell and zero-valent iron to eliminate excessive acidification and enhance methane production from readily acidified substrates

Anaerobic digestion is a sustainable technology for methane (CH₄) production from organic waste and wastewater. However, its performance is frequently hindered by excessive acidification in readily acidified substrates, such as starch wastewater. Oyster shell (OS), a natural alkaline material, effectively regulates pH and enhances CH₄ production. Nevertheless, its use increases CO₂ levels in biogas, highlighting the need for in-situ conversion of CO₂ into CH₄. This study presents a novel approach by combining OS with zero-valent iron (ZVI), which acts as an indirect hydrogen (H₂) source, to facilitate this conversion and boost methanogenesis from readily acidified substrates. Results demonstrated a 22.8% reduction in CO₂ levels and a significant increase in CH₄ yield to 364 mL/g-COD, surpassing both previously reported values and the theoretical maximum of 350 mL/g-COD. Additionally, the daily CH₄ production rate was increased by 60.3%, with a shorter lag phase and overall duration. This improvement was driven by the synergy between OS and ZVI, which enhanced hydrogen ion consumption, releasing additional CO₂ and H₂ for methanogenesis. The synergistic interaction also promoted extracellular polymeric substances levels, acidogenic and methanogenic steps, key enzyme activities, and enrichments of <i>Methanothrix</i>, hydrogenotrophic methanogens, and <i>Longilinea</i>. Furthermore, an economic assessment revealed significant cost benefits of this approach, offering promising potential for industrial applications. This study provides new insights into leveraging OS and ZVI to eliminate excessive acidification and improve CH₄ production from readily acidified substrates.