Multi-approach characterization of organic sediment produced by anaerobic digestion plant fed with pig slurry and stored for a longterm in a lagoon

tThis study combined different approaches to characterize organic sediments produced by an anaerobicdigestion plant feed with pig slurry, and accumulated for many years in a lagoon. The results of allanalyses identified a certain homogeneity of the sediments. As a consequence of the pig diet, the sedimentcontained an high concentration of Zn (about 4 g kg−1) and Cu (about 1.2 g kg−1), which were mostlyassociated to the particles with a size ranging from 2 to 53 m. The sediment was made of large amount oforganic matter, mostly cellulose and recalcitrant molecules, and 30–40% mineral fraction. XANES and XESspectroscopies indicated the presence of zinc phosphate (38%), zinc sulfide (32%), zinc carbonate (19%),and zinc oxide (11%). The presence in the sediment of forms characterized by a very scarce solubility,as also confirmed by the Zn and Cu chemical speciation, indicated a low bioavailability of these metals.However, although their low mobility, the high concentrations of Zn and Cu allowed to consider thesediment not suitable to use as a fertiliser due to the potential risk of metal interaction with the foodchain.