Distribution of heavy metal contents and chemical fractions in anaerobically digested manure slurry.

Digested slurry samples from twenty-one large-scale anaerobic digestion plants together with intensive pig and dairy farms in Jiangsu Province of China were collected and analyzed for total and dissolved concentrations of Zn, Cu and As, as well as chemical characteristics. The results showed that total concentrations of Zn, Cu and As in digested pig slurries were concentrated to <10, <5 and 0.02-0.1 mg/l, respectively; while <2 and 10-30, <1, and 0.02-0.1 mg/l, respectively, in digested dairy slurries. Lowering the dietary supply of these elements to pig and dairy would be the most effective way to control heavy metal contents in digested manure slurries. Dissolved fractions of Zn, Cu and As accounted for 1-74%, 1-33% and 2-53% of the total concentrations, respectively, in digested pig slurries; and 18-65%, 12-58% and 3-68% in digested dairy slurries. The chemical fractions of heavy metals in digested slurries were not only dependent on the total concentrations of heavy metals in raw manures but also on conditions of digestion and storage. Oxidation pond systems could significantly cripple the total contents of heavy metals in digested slurries, and the removal effect was better in multi-oxidation-pond systems than that in primary-oxidation-pond systems. However, the chemical fractions of heavy metals in digested slurries changed in a complicated manner when stored in oxidation ponds, due to the suspended solid deposition, elements reduction, as well as variations of pH values and oxidation-reduction potential.