Effect of ferric chloride addition on the removal of Cu and Zn complexes with EDTA during municipal wastewater treatment

The presence of the synthetic chelating agent ethylenediaminetetraacetic acid (EDTA) in municipal wastewater has a significant impact on the efficacy of wastewater treatment plants because pollutant metal–EDTA complexes are difficult to remove. To understand the potential of FeCl3 addition during primary treatment to improve metal removal, laboratory experiments and a full-scale study were conducted. Results of laboratory experiments designed to simulate primary and secondary wastewater treatment indicate that a portion of pollutant metal–EDTA species can be converted into FeEDTA− by addition of FeCl3 during simulated primary treatment. Addition of FeCl3 also resulted in improved metal removal by adsorption. The combination of these two processes improved removal of Cu and Zn by 20% during simulated primary treatment. During full-scale treatment, a slight increase in the concentration of FeEDTA− in secondary effluent was observed when FeCl3 was added during primary treatment. However, FeCl3 addition had no observable effect on metal concentrations in the wastewater effluent. The most noticeable change in EDTA speciation at full scale occurred during primary treatment, where most of the pollutant metal–EDTA complexes were converted into FeEDTA−, irrespective of whether or not FeCl3 was added. The formation of FeEDTA− during primary treatment plays a potentially important role in metal removal and should be considered in future evaluations of wastewater treatment plant performance.