Mitigating VFAs accumulation and enhancing anaerobic stability in continuous-flow leachate from waste transfer stations using zero-valent iron

The accumulation of volatile fatty acids (VFAs) during anaerobic digestion of leachate from waste transfer stations can destabilize the system. Zero-valent iron (ZVI), with its strong reducing properties, has the significant potential to enhance anaerobic digestion by optimizing the microenvironment for anaerobic microorganisms, enriching microbial communities and promoting direct interspecies electron transfer (DIET). This study examined three continuous-flow scenarios: a control without ZVI (0 # ), initial ZVI addition (1 # ), and ZVI addition during significant VFAs accumulation (2 # ). Both ZVI-supplemented systems outperformed the control, with 2 # demonstrating superior performance. Compared to the control, 2 # reduced VFAs accumulation by 82.45 %, increased pH by 2.51 units, improved COD removal by 67.03 %, enhanced methane production by 0.41 L/gCOD, and increased methane concentration by 32.41 %. ZVI addition reduced the oxidation-reduction potential (ORP) by 131–183 mV in 2 # , facilitating anaerobic conditions conducive to methanogenic activity. Microbial sequencing revealed that VFAs-oxidizing bacteria and methanogens in 2 # were 6.28–14.64 % and 40.80–69.98 % more abundant, respectively, than in 0 # , significantly enhancing VFAs degradation. The enrichment of DIET-related microorganisms, such as Methanobacterium and Syntrophomonas , was also higher in 2 # . ZVI addition during VFAs accumulation stabilizes the anaerobic process, offering a cost-effective and eco-friendly solution for treating leachate and other high-strength organic wastewaters.