Developing a treatment technology for the removal of trace metals from municipal wastewater

The implementation of bioavailability-based water quality standards for metals in the United Kingdom, Europe and the United States is likely to challenge water utilities to reduce the concentrations of metals in effluent discharges. Municipal wastewater treatment works (WWTWs) are not, however, specifically designed to remove metals, with current treatment technologies providing a relatively low and inconsistent level of removal. New technologies will therefore be required to achieve removal rates to meet potentially more stringent discharge criteria. Improvements in the understanding of the form of metals in effluent is crucial for the development of technologies to reduce metal concentrations in effluent discharges. In the present study, effluent samples were separated into three parts; a particulate fraction (> 0.45 μm), a ‘colloidal’ fraction (0.45 μm – 1 kDa) and a ‘truly dissolved’ (< 1 kDa) size fraction. Soluble (i.e. < 0.45 μm) copper (Cu), lead (Pb), nickel (Ni) and zinc (Zn) was found to be distributed predominantly in the colloidal fraction, and therefore technologies capable of removing particles from the colloidal fraction have good potential to reduce the concentration of these metals in effluent. Bench-scale studies tested the coagulation-flocculation (CF) treatment process and found this treatment approach to be effective for the removal of Cu, Pb and Zn from effluent, but was not effective at removing Ni. Two different CF treatment technologies were assessed, namely, the biopolymer, Floculan, and a conventional coagulant, ferric chloride (FeCl₃). Floculan and FeCl₃ demonstrated similar removal rates for metals (except for Cu, for which Floculan offered a better level of removal), however, Floculan maintained the concentration of dissolved organic carbon (DOC), which offers an important control on metal bioavailability. Floculan therefore simultaneously reduces metal concentrations and bioavailability whereas FeCl₃ reduces metal concentrations and DOC in proportion and therefore bioavailability is unaffected. Floculan offers the opportunity to control the concentrations of trace metals in effluent in an environmentally and financially sustainable manner. This thesis reports research funded by the Natural Environment Research Council and Severn Trent Water.