Effects of solar drying on heavy metals availability and phytotoxicity in municipal sewage sludge under semi-arid climate

This study aimed to assess the effectiveness of solar drying process, for limed and non-limed sludge, in terms of reducing heavy metals mobility and toxicity in a sewage sludge (SS) generated from an activated sludge wastewater treatment plant (WWTP). The total quantification of heavy metals indicated a high concentration of Chromium (Cr), Copper (Cu), Lead (Pb) and Nickel (Ni) (2739.09, 174.48, 68.59 and 31.97 mg/kg respectively). At the first stage of solar drying (for limed and non-limed sludge), speciation analysis showed that Cr, Pb and Cu are mostly associated to the oxidizable fraction; contrary wise, Ni is mostly associated to the reducible fraction. By the end of the solar drying process, the oxidizable fraction decreased significantly from 80 to 15%, 80 to 25% and from 70 to 27% respectively for Cr, Cu and Pb. On the other hand, the residual fraction (the most stable in the environment) increased significantly from 17 to 78%, 18% to 73% and 28 to 71% respectively for Cr, Cu and Pb. Phytotoxicity risk assessment during the solar drying process revealed a significant increase of the germination index (GI) for both Cress and Turnip (from 4.3 to 98.5% and 15 to 94% respectively). These results highlighted the considerable effectiveness of solar drying process in terms of heavy metals' mobility and phytotoxicity reduction. Thus, the solar dried sludge could be potentially applied on the agricultural soils. (C) 2020 Elsevier B.V. All rights reserved.