Effects of wet oxidation and coagulation on the stabilization of heavy metals in municipal sludges

Excess activated sludge(S1) and mechanically dewatered sludge(S2) were collected from the municipal wastewater treatment plant of Changsha.The morphology change and chemical fractions of heavy metals(Zn,Cu,Pb,Cd,Hg,As) in sewage dewatered sludge were analyzed.And the stabilization mechanism of heavy metals was discussed by comparison with the excess activated sludge.The results showed that the total amount of the heavy metals in the dewatered sludge was less than the acidity limit value in the pollutant control standards for the agricultural sludge,and accorded with the grade B of the sludge shale requirements.Zn,Cu,Pb,Cd,Hg,As mostly existed in the residue form.Unstable state of these heavy metals were converted into stable state to varying degrees after treatment.In particular,increase of Cu,Hg was the largest.The bioavailability of these heavy metals in S2 were significantly lower than in S1.Meanwhile,the reduction of bioavailability was evident.The bioavailability of the heavy metals in S2 was in the order of ZnPbCuCdAsHg.The heavy metals(Zn,Cu,Pb,Cd,Hg,As) were stabilized evidently by the technique of wet oxidation and coagulation.The results may provide a scientific basis for safe disposal of the sludge.