A field trial comparing the fate and removal performance of phosphorus (P) in two parallel stormwater
retention ponds, one retrofitted with a Floating TreatmentWetland (FTW) and one without any vegetation
(a control), was carried out near Auckland, New Zealand. Results suggest that inclusion of a FTW would
significantly improve P removal efficiency exhibiting 27% lower TP outlet event mean concentrations
(EMCs) than a conventional retention pond. The low SRP inlet EMC did not allow the performance of
either pond to be differentiated.
Inlet particulate bound P (PP)is thoughtto have been associated with particulate copper onfine particles
like colloidal organic matter and/or clay and trapped into the sticky biofilm of the roots to subsequently
settle on the bottom of the pond. The FTW pond induced a more neutral water column pH and higher
organic release into the water column, likely promoting dissolved phosphorus sorption onto particles.
Surprisingly,the reduced (low redox potential) sediment observed below the FTWdid notinduce P release
probably due to the more neutral pH conditions allowing re-adsorption onto organics and/or clay minerals
(e.g. Al-OH). This resulted in higher P sediment accumulation in the FTW pond. P uptake by plants is not
thought to be a significant removal pathway. Sorption of dissolved P, physical entrapment of PP in roots
and settlement are thought to be the main P removal pathways for ponds equipped with FTWs.
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