Heavy metals fractionation during the thermophilic phase of sewage sludge composting in aerated static piles.

An aerated static pile system with temperature feedback control was used for the composting of sewage sludge amended with olive tree leaves in a 1:1 (SS1) and 1:2 (SS2) v/v ratio, on two different occasions. The two piles were approximate 20 m3 each of similar dimensions; 2.0 wide at the base, 1.3 m height and 11 m long. Samples were taken from the core of the piles on four occasions: day 0 (establishment), day 15, day 30 and day 60 of the thermophilic phase. Heavy metal (Cu, Ni, Pb and Zn) fractions in each of the samples were monitored using sequential extraction with H2O (for the water soluble fraction), KCL (for the exchangeable fraction), Na2EDTA (for the inorganic bound fraction), NaOH (for the organically bound fraction) and HNO3 due the significant residual fraction of these metals in both piles and in all four samplings, which reduced any losses through leaching when water was added. For Ni the water soluble and the exchangeable fractions were dominant, reaching values of 45% in SS1 and 35% in SS2, resulting in some losses into the leachates. The fraction of metals connected to the organic matter was similar for Ni in both piles, larger in SS2 for Pb than in SS1, whereas Cu and Zn presented far larger values in SS1 than in SS2. There was no common pattern of fractionation variation among the metals in each pile or when comparing both piles.