Use of Recalcitrant Agriculture Wastes to Produce Biogas and Feasible Biofertilizer

In the ongoing work, the digestion process of recalcitrant agricultural wastes (olive wastes and citrus pulps) mixed in different proportions with, livestock manures, milk serum and maize silage for biogas production was studied. Additionally, the chemical composition and the phytotoxicity of the digestates (each separated in liquid and solid fraction) were evaluated with the purpose of being used as organic fertilizer in agriculture. The results demonstrated that animal manure and recalcitrant agricultural wastes, if properly mixed, produced high percentage of biogas. The digestate chemical compositions differed and varied in respect to the kind of feedstock, and the ratio of their mixing to feed the digesters. The digestate from the digester named Fattoria, mainly powered with animal manures (poultry, cow and sheep), contained less phenols and more active microbial biomass than the digestate from the digester Uliva, mainly fed with olive waste and citrus pulp and in minor extent with animal manure and maize silage. Our data showed that the digestate composition depended on the mix of biomass input. Additionally, the effects of digestate were plant species-specific and a positive correlation between the amount of phenols and the phytotoxic effects of digestate on plants was also well evident. These results suggest that the sustainable disposal of digestates requires a preliminary screening to select the one which better fits the demands of a particular species for optimizing crop production.