Comparison of Heavy Metal Speciation of Sludge During Mesophilic and Thermophilic Anaerobic Digestion

Abstract

PURPOSE: Mobility of representative heavy metals during the anaerobic digestion of sludge under mesophilic and thermophilic conditions was compared. METHODS: The exchangeable, carbonates (F1), Fe and Mn oxides (F2), organic matter, sulfides (F3) and residual (F4) fractions of heavy metals in sludge were fractionated by the modified BCR procedure. Inductively coupled plasma-optical emission spectrometry (ICP-OES) was used to determine the Cu, Ni and Zn concentrations of the different fractions. RESULTS: The mobile fraction (F1, F2 and F3) of Cu, Ni and Zn increased with the progression of the anaerobic digestion process. The mobile fraction of Cu, Ni and Zn under thermophilic conditions was greater than under mesophilic conditions. The mobile fraction of Cu, Ni and Zn also increased with the increase of the ammonia concentration, especially under mesophilic conditions. The acid-soluble fraction levels of the studied metals were ranked as follows Ni > Zn > Cu. CONCLUSIONS: The mobility of the representative heavy metals (Cu, Ni and Zn) during the anaerobic digested of sludge under thermophilic conditions was greater than under mesophilic conditions. These results show that more attention to the negative impacts of heavy metals present in anaerobically digested sludge under thermophilic conditions before land applying.