Addition of iron ore tailings to increase the efficiency of anaerobic digestion of swine manure: ecotoxicological and elemental analyses in digestates

The anaerobic digestion process (AD) occurs via series of biochemical reactions, producing biogas as a renewable fuel and digestate, a rich by-product in trace elements (TEs), which has been directly disposed on soils as organic fertilizer. The application of mineral additives can result higher biogas production and CH4 yields in AD process; however, the chronical effects of these additives in digestates have not been reported in the literature. In this way, this research aims to analyze the chronic effects of digestates from swine manure (SD) and swine manure with iron ore tailings as additive (SDIOT) on survival and reproduction of Enchytraeus crypticus. The fresh samples were collected from sequential batch processes conducted at mesophilic conditions. The TEs and pH ranges in feedstock and digestates were determined in fresh matter (mg L−1), including analyses of heavy metals and nutrients, in which ranges concentrations were discussed according to biogas and CH4 productions, and compared with previous data of literature. These analyses were followed by chronic tests of digestates on reproduction of E. crypticus in Oxisols, in which tests performance met the validity criteria stablished according to ISO 16387 standard. The means of treatments were compared using the Holm–Sidak method, and the means of control groups were compared using t test (α = 0.05). The chronic effect on survival and reproduction of organisms might have occurred through synergistic effect of metals in digestates; however, more analyses must be needed to understand the toxicity effect of these products, specially under different climate conditions. Graphical abstract Article Highlights The application of swine manure resulted in a chronic effect to Enchytraeus crypticus. The synergistic effect of metals might have effects on organisms survival. Available fractions of metals and chemical properties of soils must also be evaluated. Climate change might influence on the speciation of metals within the soils.