Microbial acclimation of thermophilic anaerobic digestate enhances biogas production and biodegradation of polylactic acid in combination with the organic fraction of municipal solid waste (OFMSW)

Bioplastics are a promising alternative to conventional plastics. Their anaerobic co-digestion with the organic fractions of municipal solid waste (OFMSW) is an ideal end-of-life scenario reducing pre-treatment and increasing efficiency and biogas production. Bioplastic degradation is limited under anaerobic digestion (AD) as it requires longer hydraulic retention time (HRT) compared to industrial OFMSW plants' HRTs. Here, three AD runs were conducted sequentially under thermophilic conditions to investigate the effects of inoculum acclimation on enhancing the degradation of polylactic acid (PLA) and OFMSW in mono and co-digestion (PLA + OFMSW). In PLA mono-digestion, microbial acclimation increased biogas production up to +152 % (831 ± 11 NL kgVS^-1) and biogas production rate from 27 to 47 NL kgVS^-1 d^-1 with a 5-day reduction of the lag phase. This improvement was associated with the enrichment of the PLA-degrading bacteria Tepidanaerobacter. In PLA + OFMSW co-digestion, biogas production increased of +69 % (827 ± 69 NL kgVS^-1), the biogas production rate increased to 58 NL kgVS^-1 d^-1 with a lag phase reduction of 7 days. An increase of both protein degraders (Halocella and Acetomicrobium) and Tepidanaerobacter was achieved. In OFMSW mono-digestion, acclimation increased cumulative biogas production to + 22 % (719 ± 25 NL kgVS^-1) with no biogas production rate and lag phase modifications, indicating an already adapted community. A variance in Methanothermobacter and Metanoculleus abundances across treatments was linked to different biomethane productions. Microbial acclimation is a valid and economical approach to enhance biogas production and PLA degradability, alone or with OFMSW, further reducing HRTs enabling sustainable bioplastic and OFMSW waste management.