Assessing Methods of Removing Metals from Wastewater: The Effect of Ferric Chloride Addition

The presence of cationic pollutant metals in municipal wastewater effluent is a concern because stringent discharge requirements cannot always be met with conventional treatment methods. Attempts to improve metal removal are often unsuccessful because a significant fraction of the cationic metals are complexed by the synthetic chelating agent ethylenediamine tetraacetic acid (EDTA).To identify practical approaches for improving metal removal, an analytical method for measuring metal-EDTA complexes was used to survey metal speciation at a series of wastewater treatment plants. Following these analyses, bench-scale experiments were conducted. The survey data indicated that pollutant metal-EDTA complexes account for a significant fraction of the dissolved metals in wastewater. The bench-scale studies indicated that ferric chloride addition improves the removal of copper and zinc by approximately 20%.To test the results of the bench-scale experiments, a full-scale experiment was conducted by interrupting chemical addition at a municipal wastewater treatment plant that normally adds ferric chloride during primary treatment.