ABSTRACT
In addition to their function as providers of biological treatment for wastewater, discussed on pp. 69-71, constructed wetlands have become an increasingly important method of enhancing the quality of run-off water in cities in Europe, the UK, the USA and Canada. They also have immense importance in controlling stream flows and downstream erosion, and restoring impaired habitats for aquatic and terrestrial species. While there is considerable variability in the efficiency of wetlands at removing contaminants, it has been shown that they can remove 70 per cent of excess nutrients, and destroy bacteria and viruses. Heavy metals may be accumulated either in sediments, or are associated with organic materials, and many pesticides, oil and grease are broken down by microbes and plants. They have also been developed to treat the acidic water quality of coal-mining and ore-processing plants. For instance, it was found that wetlands created in western Pennsylvania to treat water from old mine sites resulted in improved water quality as well as furnishing habitat for birds, mammals, reptiles and amphibians.68
