Nitrogen removal in constructed wetlands employed to treat domestic wastewater

In many locations nitrogen (N) addition to the environment is of-concern be cause of the existing quality or intended use of groundwater. To address th is concern we evaluated the impact of residence time on N removal in constr ucted wetlands employed to treat domestic wastewater. Constructed subsurfac e flow (SF) wetlands were installed at two locations for this study. Twelve small, II gravel-based, SF wetlands were constructed at the Virginia Tech' s Kentland Research Farm (KRF). Treatments included combinations of two pla nt species [woolgrass (Scirpus cyperinus) and cattail (Typha latifolia)] an d 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 Lank effluent (STE) from a single family residence. Changes in ammonium (NH4+), nitrate (NO3-), and total Kjeldahl N (TKN) were determined by monitoring influent and effluent quality over a 3-year perio d. Plant species had little impact on N concentration or removal. Both NP4 and TKN concentrations in the wetlands decreased exponentially with increa sed residence time. Removal of NH4+ and TKN ranged from 18.1 to 39.0% and f rom 31.3 to 45.8%, respectively, for the KRF site and from 44.4 to 73.4% an d from 46.2 to 67.5%, respectively, at the PRP site. The NO3- concentration s in the influent and effluent at both sites were low and no differences in concentrations with residence time was observed. Temperature dependent rate constants (K-T). developed from NH4+-N and TKN d ata collected from the KRF site, was effective in predicting NP4+-N and TKN concentrations as a function of residence time in the wetlands at the PRP site. (C) 2000 Elsevier Science Ltd. All rights reserved.