Use of Fe3O4 nanoparticles in reactor co-digestion of residues from 1G2G ethanol biorefinery: microbiological routes and operational aspects

The co-digestion of residues from the sugarcane industry has already proven to be a highly attractive process for biogas production through anaerobic digestion (AD). The use of residues such as vinasse (1G) filter cake (1G) and deacetylation liquor (2G) in operation in a continuous CSTR reactor showed a possibility of integration of 1G and 2G ethanol biorefineries through AD in previous work by our research group. The use of nanoparticles (NP) is a favorable way to optimize AD processes, as these additives serve as a means of introducing nutrients into the process in a more assertive way from the point of view of distribution and interaction with microorganisms. In this context, the present work proposed the optimization of the co-digestion of vinasse, filter cake, and deacetylation liquor in a continuous reactor through the addition of Fe3O4 NP, by the purpose of comparison results with the operation of the same substrates and the same condition but without NP. Initially, tests were carried out in batches with different concentrations of NPs, to evaluate the best concentration to be added in the continuous reactor. A concentration of 5 mg L-1 was chosen, and it was added to each increase in organic rate load (ORL) used in the process. CH4 production reached maximum values of 2.8 ± 0.1 NLCH4 gVS-1 and organic matter removal 71 ± 0.9%, in phase VI, with ORL of 5.5 gVS L-1 day-1. This production was 90% higher than the reactor co-digestion operation without the presence of NP. Furthermore, according to the results of pH, alkalinity, it can be concluded that the methanogenesis stabilized at 60 days of operation, being 30 days before when there was no NP added. The development of AD was stable, with low variations in the oxidation-reduction potential (ORP) and with stable organic acid (OA) concentrations, indicating the possibility of route propionic acid to produce CH4. The main methanogenic Archeae found was Methanoculleus, indicating that the predominant metabolic route was that of syntrophic acetate oxidation (SAO) coupled with hydrogenotrophic methanogenesis. The use of Fe3O4 NP managed to improve the AD operation of residues from the 1G2G ethanol production industry and did not modify the microbial community present, only stimulated their growth. ### Competing Interest Statement The authors have declared no competing interest.