Removal of metals and phosphorus recovery from urban anaerobically digested sludge by electro-Fenton treatment

Abstract

To our knowledge, this work presents the first application of electro-Fenton (EF) process to sludge washing. Suspensions of anaerobically digested sludge (0.50 wt%) from a municipal wastewater treatment facility were electrolyzed with addition of Na2SO4 and Fe2+ at pH 3.0, using a stirred tank reactor with a boron-doped diamond (BDD) or RuO2-based anode and an air-diffusion cathode that produced H2O2. The effect of the sludge content in suspensions and applied current density (j) was examined. High quantities of Cr, Pb, Cd, Zn, Fe and P were leached at pH 3.0, whereas Cu showed the opposite trend. Aeration only enhanced Pb and Zn leaching, whereas the use of Fenton's reagent with 15 mM H2O2 solubilized 16.0% Cr, 23.0% P, 42.6% Fe and 56.0% Pb, with total leaching of Cd, Cu and Zn. EF with BDD anode at high j caused total precipitation of Cr, Pb and Fe, 40% Cd leaching and total solubilization of Cu and Zn. The RuO2-based anode enhanced the entrapment of Cr, Fe and P in the solid fraction of the sludge, but promoted a high transport of Cd, Cu and Zn to the liquid phase. P recovery was about 74%–79% in all EF treatments. The soluble organic carbon increased in most cases except for EF with BDD, where it decreased markedly, in agreement with the high oxidation power of this anode. The sludge dewaterability was largely improved in all treatments, attaining up to 97%, consistent with the scission of many extracellular polymeric components.