Syntrophic biofilm architecture rather than surface chemistry governs the long-term performance of palm oil mill effluent digestion amended with carbon-conductive materials.

The application of carbon-conductive materials to enhance palm oil mill effluent digestion from sustained semi-continuous operations has been limited in the literature. This study compared carbon felt (CF) and carbon graphite felt (CGF) at 3 g·L-1 over 150 days in 400 mL duplicate reactors at 37 ± 1°C and 1.5 g volatile solids (VS)added·L-1·day-1. Under steady-state operation (days 30 - 150), CF raised the volumetric methane production rate by 22% relative to the control (0.922 ± 0.096 vs. 0.756 ± 0.080 NL-CH4·Lreactor⁻1·d⁻1) and CGF by 11%, and the specific methane yield gave identical improvements. VS removal followed the same order (CF 88 ± 4%; CGF 78 ± 4%; control 77 ± 3%), and CF was the only configuration whose effluent volatile fatty acids stayed below its own influent throughout the experiment. Pred[icted functional gene profiles were broadly similar, and relative community composition did not track performance; instead, CF retained substantially greater attached biomass, and the absolute abundance of syntrophic bacteria and Methanothrix-dominated methanogens was the highest in CF, paralleling its methane yield. The less conductive material (CF) outperformed the more conductive one (CGF), indicating that the biofilm-carrying capacity on a more colonizable architecture, rather than surface chemistry, community composition, or a distinct gene repertoire, best explains the difference between the two carriers. These findings support CF as a practical fixed-media strategy for energy recovery in POME digesters.