Nitrogen and COD Removal from Septic Tank Wastewater in Subsurface Flow Constructed Wetlands: Plants Effects

We evaluated subsurface flow (SSF) constructed wetland treatment performance with respect to organics (COD) and nitrogen (ammonium and nitrate) removal from domestic (septic tank) wastewater as affected by the presence of plants, substrate "rock'' cation exchange capacity (CEC), laboratory versus field conditions and use of synthetic as compared to actual domestic wastewater. This article considers the effects of plants on constructed wetland treatment in the field. Each constructed wetland system was comprised of two beds (2.6 m long by 0.28 m wide and deep filled with similar to 18 mm crushed lava rock) separated by an aeration tank connected in series. The lava rock had a porosity of similar to 47% and a CEC of 4 meq/100 gm. One pair of constructed wetland systems was planted with cattails in May 2008, while an adjacent pair of systems remained unplanted. Collected septic tank or synthesized wastewater was allowed to gravity feed each constructed wetland system and effluent samples were regularly collected and tested for COD and nitrogen species during four time periods spanning November 2008 through June 2009. These effluent concentrations were tested for statistical differences at the 95% level for individual time periods as well as the overall 6-month period. Organics removal from domestic wastewater was 78.8% and 76.1% in the planted and unplanted constructed wetland systems, respectively, while ammonium removal was 94.5% and 90.2%, respectively. Similarly, organics removal from the synthetic wastewater of equivalent strength was 88.8% and 90.1% for planted and unplanted constructed wetland systems, respectively, while ammonium removal was 96.9% and 97.3%, respectively.