The presences of heavy metals in composts raise great concern about the adverse environmental impact, because the excessive application of compost may lead to the accumulation of the metals in the soil. Therefore, 3 compost samples composed of 6 subsamples were collected from Sanandaj MSW Composting Plant. The samples were physically divided into fractions of different particle size (F> 2 , F1.2-2, F0.8-1.2, F0.4-0.8, F0.2-0.4, F0.1-0.2 and F< 0.1 mm) using dry-sieving process. The bulk sample was reserved as control. The samples were analyzed for Zn, Cu, Cd, pb, Ni and Cr content using standard methods. The results showed that there was a significant difference in heavy metals content between each treatment and control (P<0.05), indicating that the heavy metal content of individuals particles of MSW compost are strongly depended on particle size. Furthermore, with decrease of particle size there is a trend to increase heavy metals concentration. So that, the lowest concentration of Zn, Cu, Cd, Pb, Ni and Cr were found in course fraction (F> 2-mm) with average values of 235.2, 78.6, 1.0, 128.3, 34.7 and 21.9 ppm, respectively. However, the highest concentration of these elements was found in finest fraction (F < 0.1 mm) with average values of 1020.1, 952.5, 3.5, 328.5, 74.9 and 58.8 ppm, respectively. The results also showed that the particles smaller than 0.8 mm were significantly contaminated with the surveyed heavy metals and failed to meet the Canadian Council of Ministers of the Environment (CCME) Standard for agricultural proposes. Based upon the result of the assessment, the courses compost particles larger than 0.8 mm can be used as soil amendment without environmental pollution and threat to human health. Therefore, the application of polluted composts according to particle size can be one of the important ways to reduce environmental pollution.
