Enhanced thermophilic anaerobic co-digestion of food waste with bioplastics by applied electrical voltage input

The increasing adoption of bioplastics (BP), particularly polylactic acid (PLA), in food packaging has led to a growing presence of BP in food waste (FW), raising new challenges for anaerobic digestion (AD) systems. However, BPs exhibit low biodegradability, and since they are co-collected with FW, pretreatment is often impractical. In addition, their inclusion adds an unavoidable increase in organic loading rate (OLR) in AD. To address these challenges, promoting direct interspecies electron transfer (DIET) could offer a promising solution by maintaining process stability under high OLR and enhancing the anaerobic biodegradability (conversion to biogas). Four thermophilic continuous reactors were operated under control and electrical voltage (EVR) at hydraulic retention times (HRTs) of 50 and 40 d. PLA was added as an additional substrate at 1.0-5.0% (w/w of FW). At 50-day HRT, control and EVR exhibited stable performance up to 5.0% PLA addition (corresponding to 31% feedstock COD concentration), while > 30% higher CH₄ production yield (MPY) was attained in the EVR. However, at 40-day HRT, the control failed at 2.5% BP addition due to higher OLR, while EVR maintained stable performance up to 5.0% PLA addition. Enhanced performance in the EVR can be attributed to enrichment of DIET-associated methanogens (Methanothrix) and genes (pilA, ATPase) together with ldhL, which upregulated the lactate-to-pyruvate oxidation and propionate transformation, as supported by the random forest model, collectively enabling tolerance of higher OLR (4.3 → 5.7 g COD/L/d). Furthermore, Fourier transform infrared spectroscopy and MPY calculations for PLA confirmed the enhanced anaerobic biodegradation under EV conditions.