Dissolved Oxygen Influences Nutrient Remediation by Floating Wetlands

Declining water quality and increasing urban demand for water are driving enactment of regulations designed to limit nutrient release into watersheds.  Floating wetlands are potential treatment systems for remediating nutrient-rich water that can be established in existing water basins.  This research examined the nutrient remediation efficacy of floating wetlands at three treatment scales.  These included 1) a two-stage pond system and 2) a replicated trough system from 2008 to 2009 and 3) a replicated mesocosms study in 2009 that evaluated the impact of dissolved oxygen on plant nutrient uptake.  The two-stage pond floating wetland was planted with Agrostis sp., Canna flaccida, Eleocharis montana, and Juncus effusus in 2008.  In 2009, all species except E. montana were used to replant the floating wetland.  The replicated trough floating wetlands were planted with C. flaccida and J. effusus during both growing seasons.  The mesocosm study was planted with J. effusus.  Plant growth parameters (shoot height and root length) were measured on a biweekly basis, and water quality parameters (pH, conductivity, dissolved oxygen, temperature, and nutrient concentrations) were monitored weekly.  Of the plant species examined, C. flaccida and J. effusus nitrogen and phosphorus tissue concentrations were highest when normalized for total plant biomass.  In all studies, increased water column dissolved oxygen enhanced root growth and uptake of nitrogen and phosphorus.  Effluent nutrient concentrations were exceptionally low in the pond and trough treatment systems.  Nitrogen concentrations in effluent averaged 0.12 ± 0.02 and 0.03 ± 0.01 mg/L nitrogen for the ponds and troughs respectively, and phosphorus concentrations in effluent averaged 0.03 ± 0.01 and 0.02 ± 0.01 mg/L total phosphorus for the ponds and troughs respectively.  Nitrogen and phosphorus removal efficiency was low in the mesocosm study.  Juncus effusus should not be used in monoculture for nutrient removal, rather mixed plantings should be employed to facilitate nutrient uptake.  For low-level nutrient loading, floating wetlands appear to be a viable treatment option; however, at higher loading rates, they were not as efficient as other treatment alternatives such as constructed wetlands.  Additional research is required to further evaluate plant species interactions and their influences on nutrient remediation in floating wetland systems. The understanding derived from these studies will allow for optimization and effective application of this promising remediation technology.