Heavy Metals in Sewage Sludge-Amended Soils

Abstract

Wastewater treatment plants are generating tons of activated sludge daily. In some soils, this residual sludge is a good source of nutrients for plants and an amendment material, but in others, they are undesirable material due to their crop toxicity. In this research, the heavy metal concentration was evaluated in residual sludge and sewage sludge-amended soil in Aguascalientes, Mexico. Sewage sludge (600 t ha-1) was incorporated into a plot near to the "El Nigara" wastewater treatment plant. Sludge was sampled before its application. Soil samples were collected (0-20 cm, depth) before and 16 months after sludge incorporation. Soils and sludge were dried, ground and sieved (mesh 10). Sludge was digested with a mixture of sulfuric and perchloric acids, and the total content of heavy metals (Fe, Cu, Mn, Zn, Pb, Ni, and Cd) was determined by atomic absorption spectrometry. DTPA-extractable heavy metals in sludge were determined. In the soil, only DTPA-extractable heavy metals or heavy metals immediately available for plants were quantified. Sludge showed the greatest concentration of Fe (total and DTPA-extractable). The Relative Recovery Percentage (RRP) is the rate of chemically and physiologically active heavy metal, which is the most available for plant uptake. Zn presented the highest RRP (90%) followed by Fe. Sludge incorporation increased soil concentration of heavy metals within soil permissible limits. Therefore, sewage sludge from "El Niagara" could be incorporated into this kind of soil without any risk. However, more research must be carried out on the residual effects of sludge amendment and subsequent applications.