Direct interspecies electron transfer (DIET), which does not involve mediation by electron carriers, is realized by the addition of conductive materials to an anaerobic digester, which then activates syntrophism between acetogenic and methanogenic microorganisms. This study aimed to investigate the effect of the addition of two conductive materials, zero-valent iron (ZVI) and magnetite, on the methane production and microbial consortium via DIET in the anaerobic digestion of food wastewater. The operation of a batch reactor for food wastewater without the addition of the conductive materials yielded a biochemical methane potential (B_u), maximum methane production rate (R_m), and lag phase time (λ) of 0.380 Nm³ kg⁻¹-VS_added, 15.73 mL day⁻¹, and 0.541 days, respectively. Upon the addition of 1.5% ZVI, B_u and R_m increased significantly to 0.434 Nm³ kg⁻¹-VS_added and 19.63 mL day⁻¹, respectively, and λ was shortened to 0.065 days. Simultaneously, Methanomicrobiales increased from 26.60% to 46.90% and Methanosarcinales decreased from 14.20% to 1.50% as the ZVI input increased from 0% to 1.50%. Magnetite, at an input concentration of 1.00%, significantly increased the B_u and R_m to 0.431 Nm³ kg⁻¹-VS_added and 18.44 mL day⁻¹, respectively. However, although magnetite improves the efficiency of methanogenesis via DIET, the effect thereof on the methanogen community remains unclear.

Activated carbon (AC) may improve methane production, but its performance can depend on substrate loading and incubation time. This study evaluated 60-day batch anaerobic digestion of food wastewater at 38 °C using substrate-to-inoculum (S/I) ratios of 0.5, 1.0 and 1.5 on a volatile-solids mass basis, three nominal AC particle sizes (4, 45 and 150 μm), and AC doses of 5 and 10 g/L. In the processed dataset, at S/I = 1.5 the highest day-5 methane yield was 49.9% above the control, whereas day-60 yields were lower than the corresponding controls in all AC treatments. Endpoint soluble Ni concentrations were lower in all AC treatments, and Co concentrations were generally lower. The metal pattern suggests a possible association between nutrient availability and final methane yield. The results highlight the need to assess early methane gains together with final yield and metal availability. AC plus trace-element supplementation should be evaluated in future studies before operational recommendations are made.