Nitrate Removal Efficiency and Net Productivity of Channelized Wastewater-Fed Pleuston Wetlands

Constructed wetlands have been emerging over the last 30 years as a good way to treat bodies of water with undesirable levels of pollutants in general, and of polishing municipal wastewater effluent (MWE) in particular. In this study, the nitrate removal efficiencies of two 37.2 m2 Channelized Aquatic Scrubbers (CAS) in Santa Rosa, CA were compared with respect to vegetation type and harvesting regime. Nitrate removal by a filamentous algae dominated CAS did not differ significantly from a CAS containing a mixture of aquatic vegetation, but biomass production by the algae was roughly half that of the aquatic vegetation mix. Between 13 May 2008 and 1 December 2009 harvested channels of the CAS removed an average of 1050 ± 115 mg N m-2 d-1 (mean ± SE). The average influent and effluent nitrate concentrations of all channels during this time were 14.9 ± 0.4 mg L-1 and 9.9 ± 0.3 mg L-1, respectively (mean ± SE). During this same time period, the average productivity of the vegetation in the CAS was 9.28 ± 1.67 g DW m-2 d-1 (mean ± SE). Denitrification was the predominant mechanism of nitrate removal, and removal was dependent on both temperature and evapotranspiration rates. Species of aquatic vegetation varied seasonally, with harvesting playing an important role in allowing species succession. Based on the results from this study and others like it, CAS are a promising way to meet increasingly stringent regulatory discharge limits on nitrate concentration while generating biomass that can be converted to electricity.