Nitrogen removal in constructed wetlands employed to treat domestic wastewater

In many locations nitrogen (N) addition to the environment is of concern because of the existing quality or intended use of groundwater. To address this concern we evaluated the impact of residence time on N removal in constructed wetlands employed to treat domestic wastewater. Constructed subsurface flow (SF) wetlands were installed at two locations for this study. Twelve small, gravel-based, SF wetlands were constructed at the Virginia Tech’s Kentland Research Farm (KRF). Treatments included combinations of two plant species [woolgrass (Scirpus cyperinus) and cattail (Typha latifolia)] and three residence times (2.6, 3.9 and 5.9 days). Twelve larger SF wetlands were constructed at the Powell River Project (PRP) site. Treatments at the PRP site consisted of the same plant species and three residence times (4, 8 and 12 days). The wastewater at both sites was septic tank effluent (STE) from a single family residence. Changes in ammonium (NH+4), nitrate (NO−3), and total Kjeldahl N (TKN) were determined by monitoring influent and effluent quality over a 3-year period. Plant species had little impact on N concentration or removal. Both NH+4 and TKN concentrations in the wetlands decreased exponentially with increased residence time. Removal of NH+4 and TKN ranged from 18.1 to 39.0% and from 31.3 to 45.8%, respectively, for the KRF site and from 44.4 to 73.4% and from 46.2 to 67.5%, respectively, at the PRP site. The NO−3 concentrations in the influent and effluent at both sites were low and no differences in concentrations with residence time was observed. Temperature dependent rate constants (KT), developed from NH+4–N and TKN data collected from the KRF site, was effective in predicting NH+4–N and TKN concentrations as a function of residence time in the wetlands at the PRP site.