Augmentation of psychrophilic anaerobic digestion with psychrotolerant Serratia marcescens, calcium phosphate (CaHPO4·2H2O) and hematite (α-Fe2O3) nano-additives

Psychrophilic anaerobic digestion (PAD) requires optimization to improve methane production at low temperatures (<20 °C). This study aimed to improve methane production via bioaugmentation with psychrotolerant <i>Serratia marcescens</i> (SM) and biostimulation with nano-additives, comprising calcium phosphate (CaP) and hematite (α-Fe₂O₃) nanoparticles (NPs), during batch PAD of cattle manure and food waste at 15 °C. The highest methane yields were obtained from treatment with SM and both NPs (163.9 ± 18.0 mL CH₄ g^-1 VS), thereafter with the combination of CaP and α-Fe₂O₃ NPs (143.9 ± 50.2 mL CH₄ g^-1 VS). The lowest yield was observed in the control (70.2 ± 4.9 mL CH₄ g^-1 VS) followed by treatment with SM alone (124.6 ± 20.3 mL CH₄ g^-1 VS). Treatment with CaP and α-Fe₂O₃ NPs reduced the lag phase more than the other treatments. Moreover, the addition of nano-additives biostimulated PAD without significantly altering the microbial community composition. The dominant genera included <i>Bacteroides, Acinetobacter</i>, and <i>Methanosarcina</i> (a mixotrophic methanogen) after batch PAD across all treatments. This research provides new insights on the augmentative effect of SM, CaP and α-Fe₂O₃ NPs on methane production and microbial community dynamics during PAD.