Removal of toxic metals from sewage sludge by EDTA and hydrodynamic cavitation and use of the sludge as fertilizer

Abstract

Sewage sludge (SS) is rich in plant nutrients, including P, N, and organic C, but often contains toxic metals (TMs), which hinders its potential use in agriculture. The efficiency of removal of TMs by washing with ethylenediamine tetraacetate (EDTA), in combination with hydrodynamic cavitation (HC) and the usability of washed sewage sludge as fertilizer were investigated. The environmental risk was assessed. During 8 wash batches an average 35, 68, 47 and 45 % of Pb, Zn, Cd and Cu, respectively, as well as 22 and 5 % Mn and Fe were removed from the SS. The process solutions and EDTA were recycled at a pH gradient of 12.5–2, which was achieved by adding quicklime (CaO) and then acidification by H2SO4, so that no wastewater was produced, only solid waste (ReSoil® method). The quality of the recycled process solutions (they remained unsaturated with salts) and the efficiency of the washing process were maintained across all batches. On average, 46 % of the EDTA was lost during the process and was replenished. The initial leachability of EDTA-mobilized Pb, Zn, Cu, Cr and Fe remaining in the washed SS increased 6-, 17-, 3-, 11- and 11-fold, respectively, but not to hazardous levels except for Zn. After washing, P and K remained in the SS, plant-available P increased 3.3-fold, while total N and C were reduced by 20.28 and 2.44 %, respectively. Washed SS was used as fertilizer in the pot experiment. The yield of Brassica juncea did not improve, the uptake of TMs by the plants and the leaching of TMs from the soil were minimal. Our study highlighted the drawbacks and potential feasibility of the new SS washing method.