Liquid digestate features as fertilizer: carbon fractions, phytotoxicity and microbiological analysis

The circular economy promotes the use of renewable fuels as an alternative to natural gas. Anaerobic digestion for waste management produces methane, carbon dioxide and a residue-the digestate- which must be recovered. This residue can be separated into two parts, namely the liquid and solid fractions, the former characterized by its large volume, presence of nutrients in mineral forms, and highly variable composition. Here we studied the fertilizing capacity of the liquid fractions obtained from the waste derived from artichoke canning (LF-Ar), orange juice manufacturing (LF-Or) and pig slurry (LF-Sl). To this end, we examined the physical-chemical parameters, carbon fractions, phytotoxicity and presence of pathogens in these fractions. The liquid fraction derived from fruit and vegetables had a low nutrient content compared to that of slurry ( ̴1.0 kg total-N m-3 vs. 5.6 kg total-N m-3 respectively). The NH4+-N content of the fractions ranged between 70-93% of total N. Given the permissible dose in non-vulnerable areas, LF-Sl, LF-Ar and LF-Or would provide 0.9-1.0 t of organic matter ha-1 and 134, 128 and 98 kg of C ha-1 from the total humic extract, respectively. The proportion of humic acids in the total humic extract was 59%, 51% and 34% respectively. The slurry digestate showed phytotoxicity probably due to high salinity, so it should be diluted based on the needs of the crop. On the basis of our findings, the characterized liquid fractions could be recovered in agricultural soils in line to circular economy principles.