Effect of ferric chloride addition on the removal of copper and zinc during municipal wastewater treatment

Because pollutant metals can have unfavorable impacts on aquatic life at relatively low concentrations, many areas of the country have developed strict water quality criteria. In order to meet strict metal discharge limits, many wastewater treatment plants must add tertiary treatment systems or optimize existing systems for metal removal. The presence of the synthetic chelating agent ethylenediaminetetraacetic acid (EDTA) in municipal wastewater has a significant impact on the efficacy of metal removal in wastewater treatment plants because pollutant metal-EDTA complexes do not adsorb to sludge particles and therefore are not removed during biological treatment. In many well-run treatment plants, pollutant metal-EDTA complexes must be destroyed for any additional metal removal to be achieved.; One way to destroy pollutant metal-EDTA complexes is through metal-exchange reactions with Fe(III) that result in the formation of Fe(III)-EDTA complexes and free pollutant-metals that can be removed from wastewater by adsorption to sludge particles. Equilibrium calculations indicate that Fe(III)EDTA formation is favored at low pH and high [Fe(III)]. Because wastewater typically is oversaturated with respect to amorphous ferric hydroxide, addition of FeCl3 cannot increase the equilibrium concentration of dissolved Fe(III) in wastewater. However, results of bench-scale tests conducted at the Millbrae (CA) Wastewater Treatment Plant (WWTP) and Sacramento Regional (CA) WWTP that were designed to simulate primary and secondary wastewater treatment indicate that a portion of pollutant metal-EDTA species can be converted into Fe(III)EDTA by addition of FeCl3 during simulated primary treatment. Variation in wastewater pH also affected EDTA speciation; however, in bench-scale tests conversion of pollutant metal-EDTA complexes into Fe(III)EDTA required adjustment to a lower pH than typically encountered during wastewater treatment.; Two phenomena that could contribute to conversion of metal-EDTA complexes into Fe(III)EDTA when FeCl3 is added to wastewater were studied. The first was formation of Fe(III)EDTA in response to decreases in {lcub}Zn 2+{rcub} and {lcub}Cu2+{rcub}. The activity of Zn2+ and Cu2+ decrease when FeCl3 is added to wastewater because the metals adsorb onto the surfaces of iron-containing minerals that precipitate when FeCl3 is added to wastewater. (Abstract shortened by UMI.)...