Nutrient removal and plant biomass in a subsurface flow constructed wetland in Brisbane, Australia

Four native plant species (Baumea articulata, Carex fascicularis, Philydrum lanuginosum and Schoenoplectus mucronatus) are being investigated for their suitability in subsurface flow wetlands. The pilot scale Oxley Wetland, Brisbane, consists of 4 cells with different sized gravel (5 mm and 20 mm). The project aims to investigate nutrient removal rates and removal efficiency; nutrient storage in plant biomass; effect of cropping on plant regrowth, and the effect of gravel size on both water treatment and plant growth. Average daily mass removal rates ranged from 7.3 Kgha^-1d^-1 NH₄-N in Cell D to 4.6 Kgha^-1d^-1 in Cell C i.e. 37%-22% removal efficiency respectively; 5.2 Kgha^-1d^-1 NOx-N in Cell C to 1.3 Kgha^-1d^-1 in Cell A (i.e. 75%-22% removal efficiency) and 0.8 Kgha^-1d^-1 PO₄-P in Cell A to 0.1 Kgha^-1d^-1 in Cell C (i.e. 10%-1% removal efficiency). Cell A was the youngest wetland with new 5 mm gravel. Plant biomass was highest for Baumea and Carex. Gravel size does not appear to have affected biomass and recovery following cropping. Carex consistently had the highest harvested above ground biomass with high re-growth following cropping. Cropping appears to have retarded growth of the other three species with Schoenoplectus consistently having slowest regrowth. Plant biomass and nutrient storage was highest in Cell A and accounted for 11% of nitrogen removal and 3% of phosphorus removal.