The aim of this study was to determine how temperature conditions (max. 72oC, 81oC and 43.5oC) during sewage sludge composting
affect the distribution of organically bound heavy metals in humic substances (HS). To accomplish this, the content of Cu, Ni, Zn
and Fe was measured in the two main fractions of HS, humic acids (HAs) and the fulvic fraction (FF). In the feedstock, the FF was
mainly responsible for binding all metals except Ni. During composting in the two series with thermophilic conditions, the content
of Cu and Ni in HAs increased considerably. However, in the third series this increase did not take place. Zn content in HAs nearly
doubled in the first two series, but 92-94% of Zn still remained in the FF. Although Cu, Ni, and Zn content in stable-HAs increased
in all three series, the increase was much greater in the two series with higher temperatures. Fe distribution was not affected by
composting. These results may indicate that thermophilic conditions during composting of sewage sludge should be favored to
reduce the risk of heavy metal pollution when it is used as a soil amendment.