Improving biomethane production from co-digestion of fruit and vegetable waste through inoculum mixtures and recirculation of liquid biofertilizer

In Brazil, there are 41 food supply centers, and 72 warehouses distributed in 22 Federation Units, which generate about 42,000 tons of organic waste monthly, normally destined for landfills. This study investigated the energy use of these residues for different inocula and liquid biofertilizer to increase biomethane production in the anaerobic digestion of fruit and vegetable residues (VWF). The experiment used waste collected from the CEASA-PE supply center, which generates about 990 tons of organic waste per month. Different inoculum mixtures, composed of digestate, rumen liquid and anaerobic domestic sludge, were tested in combination with a liquid biofertilizer co-product. The application of the experimental design of mixtures based on the centroid simplex method and the response surface methodology, allowed us to systematically evaluate the interactions between the inocula and identify optimal proportions to produce biomethane. The results revealed that the combination of 25% biofertilizer and 75% rumen liquid provided the highest biomethane production, reaching 453.1 Nml/gSV, an increase of approximately 4 times in relation to the positive control. On the other hand, the mixture of 25% biofertilizer and 75% digestate resulted in a significantly lower yield of 256.7 Nml/gSV, about 2.5 times the control. In addition, based on the estimated amount of VWF of CEASA-PE, a potential for generating electricity from biomethane of 124 MWh/month\(\:{\eta\:}_{elec}\) was calculated, evidencing the energy and economic feasibility of the application of this technology for the use of urban organic waste.