Comparative performance of material conductivity on anaerobic digestion of palm oil mill effluent under varying Feed-to-Inoculum ratios

Stability in the anaerobic digestion (AD) of palm oil mill effluent (POME) is frequently compromised by temporal fluctuations in organic loading that cause process inhibition. This study aims to compare the effects of material conductivity on AD performance under varying feed-to-inoculum (F/I) ratios. Batch reactors were operated under mesophilic conditions (37 ± 1°C) for 54 d using carbon felt, carbon graphite felt, carbon fiber brush, and polyurethane foam at 3 g·L⁻¹ dosage and F/I ratios of 1.45, 2.16, and 3.62 (g chemical oxygen demand (COD)/gCOD). Material addition improved solids degradation, COD removal, and volatile fatty acid reduction (p < 0.05). Carbon felt achieved the greatest improvement, increasing methane production by 39% and shortening the lag phase by up to 4.0 days, relative to the control. Statistical analysis confirmed that the F/I ratio was the dominant factor influencing methane yield and lag phase (p < 0.05), whereas material conductivity did not significantly affect performance (p > 0.05). Mechanistic investigation integrating kinetic behavior and thermodynamic evaluation found that the observed improvements were driven mainly by microbial retention, biofilm development, and stabilized syntrophic cooperation rather than conductivity-mediated electron transfer. Although direct interspecies electron transfer offers favorable donor-side thermodynamics, its acceptor-side proton demand constrains its practical expression, thereby limiting the correspondence between conductivity and performance. Overall, this study highlights that temporal shock loading commonly encountered in the continuous AD of POME systems can be effectively buffered through material addition, with stabilization driven by microbial adaptation capacity rather than conductivity-dependent mechanisms.