Thermophilic anaerobic digestion of polylactic acid, polyethylene and polypropylene microplastics: effect of inoculum-substrate ratio and microbiome

Microplastics (MPs) generated from major plastic polymers have impacted the environment and formulation of an end-of-life scenario is a need of the hour. In the current study, the effects of inoculum to substrate ratios (ISR) 2, 4 and 6 on the MPs from polyethylene (PE), polypropylene (PP) and polylactic acid (PLA) under thermophilic and mesophilic anaerobic digestion (AD) conditions was studied. The results indicated thermophilic AD to be a prospective method for PLA degradation with a maximum cumulative biogas production of 894.08 NmL/gVS at ISR4 and 89.62% of volatile fatty acids (VFA) was utilised during 148 days of incubation. However, the thermophilic AD of PP and PE was observed to be highly inefficient with a maximum biogas production of 111.64 and 47.48 NmL/gVS and also resulted in VFA accumulation. Under mesophilic AD conditions, PLA degradation was highly inefficient due to long hydrolysis time, whilst inhibition was noticed with both PP and PE. The microbiological study revealed the abundance of Firmicutes and Synergistota, genus D8A-2, Thermovirga and Candidatus Caldatribacterium during thermophilic AD of PLA. An abundance of Methanothermobacter indicated hydrogenotrophic methane production as the major pathway for methanogenesis during thermophilic AD of MPs. An abundance of PWY-3781 associated with detoxification of reactive oxygen species was observed in the AD of PP and PE. Overall, the study provided insight into the prospects for improving thermophilic AD for PLA.