The removal of urban pollutants by constructed wetlands during wet weather

The Environment Agency for England and Wales has developed urban runoff treatment wetlands at two selected sites in Outer London. The systems were monitored for a range of determinands including heavy metals, suspended solids and BOD. Initial analysis of the data indicates that during dry weather, removal efficiencies vary greatly. However, during storm events removal efficiencies are higher with mean values of 71% for Zn, 72% for Cd, 69% for Pb, 66% for Cu, 34% for Ni and 81% for Cr at the Dagenham wetland. Mean removal efficiencies of 20% for Cd, 40% for Pb, 36% for Cu, 34% for Ni and 38% for Cr were monitored during storm conditions at the Brentwood wetland, but an overall increase in Zn was measured. Removal of BOD was greater at both sites during storm events with mean removal efficiencies of 24% and 29% at Dagenham and Brentwood, respectively. Suspended solids are reduced on passing through both wetlands during dry weather, but, during storm events there is an overall increase. The metal removal capability of microorganisms isolated from the rhizosphere of Typha latifolia and Phragmites australis is being investigated. Laboratory experiments are assessing the efficiency of two metal tolerant strains to accumulate Pb and Zn. It is believed that this ability may prove to be an important year-round pollutant removal process in the treatment of urban runoff by constructed wetland treatment systems, and preliminary data are presented.