Automatic Control of Continuous Wastewater-Treatment Process for Simultaneous Removal of Hazardous Metals with Hypochlorite and Iron (II)

Abstract

A continuous and automatically-controlled wastewater treatment system is described for simultaneous removal of hazardous metals. The process consists of treatment of wastewater at pH 10 with sodium hypochlorite and subsequent treatment with excess amount of iron(II)sulfate. The hypochlorite concentration was kept at 1.0 mmol.dm^-3 in the treatment tank using a hypochlorite probe. The concentration of the iron(II) was maintained at 5.5 mg.dm^-3 of dissolved oxygen (DO) using a DO meter as the detector. In the subsequent s tep, 1.75 times as much iron(II) as that added in the preceding step was added. Thus ha zardous metals were coprecipitated with the resulting hydrous iron(III) and iron(II) oxides. As (V), Cd(II), Cr(VI), Cu(II), Mn(VII), Pb(II), and Zn(II) were simultaneously removed even in the presence of ammonia, citrate, cyanide, ethylenediamine, oxalate, tartrate, and thiocyanate. These metals were not leached out of resulting sludges on leaching tests. The system was also applied to seawaters with satisfactory results.