Influence pathway of iron ions with different valences for methane fermentation of vegetable waste

Anaerobic digestion (AD) is a promising approach for vegetable waste (VW) recycling and energy recovery but methanogenesis is always inhibited by acid accumulation. The addition of a mixture of iron and other elements has proved effective on reducing acid inhibition during AD of VW, but the effect of use of iron alone has rarely been assessed. In the present study, we compared the effects of Fe-0 and Fe2+ on methane fermentation from VW at an organic loading rate of 1.5 gVS/L/d. The results indicated that Fe-0 maintained a pH > 7.7, oxidation reduction potential <-520, and methane production rate (MPR) at 250-300 mL/gVS/d. Partial least squares path modeling and correlation analysis revealed Fe-0 maintained pH for high MPR mainly through enhancing the conversion of propionic and butyric acids to acetic acid, causing the total acid decreasing to 1500 mg/L. In contrast, Fe2+ caused an accumulation of all volatile fatty acids up to 4000 mg/L and aggravated acid inhibition, resulting in a significant reduction in MPR to 148 mL/gVS/d. In all treatments, the influence of ammonia nitrogen was not significant. This study provided a direction for the determination of engineering monitoring indicators and regulation measures in AD of VW.