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The removal of metals, such as copper and zinc, from urban stormwater has been identified as a priority by the Auckland Regional Council. Conventional stormwater treatment devices, such as ponds, have proven limited in their ability to remove the fine suspended particle fraction with which significant proportions of these metals are associated. This report examines the potential of developing and applying a novel “floating treatment wetland” concept for the provision of enhanced stormwater treatment, particularly with regards to copper, zinc and fine particulate removal.

Constructed treatment wetlands have traditionally involved the use of free-floating aquatic plants, or sediment-rooted emergent wetland plants, either with water flowing through the root zone (subsurface flow) or amongst the stems (surface flow). Floating treatment wetlands (FTWs) are an innovative variant on these systems that employ rooted, emergent plants (similar to those used in surface and subsurface flow applications) growing as a floating mat on the surface of the water rather than rooted in the sediments. Because of this feature, floating treatment wetlands offer great promise for rainfall-driven stormwater treatment applications as they are littleaffected by fluctuations in water levels that may submerse and adversely stress bottom-rooted plants.