Nutrient Removal of a Constructed Treatment Wetland in the Subtropical Rio Grande Valley, Texas

Constructed treatment wetlands are designed to improve water quality due to nutrient removal by submerged and emergent vegetation. Removal of nutrients especially nitrogen from wastewater is important because nutrients when added to receiving waters can lead to excessive growth of algae and submerged aquatic vegetation. There is no information on nutrient removal efficiency of treatment wetlands in the subtropical Rio Grande Valley of Texas. This information is critical to the Rio Grande Valley cities that construct these facilities. A year-long study (Nov 2010-Oct 2011) was performed to examine nutrient removal efficiency of the City of San Juan treatment wetland.  This wetland consists of six cells or ponds in two series (ponds 1, 2b, 3b, 4b and ponds 1, 2a and 3a). During monthly collecting trips, field measurements (temperature, dissolved oxygen, pH) were taken at the outflow of each pond, at the sewage treatment plant and at treatment wetland outflow. Water samples were collected at outflows for analysis of nitrate-nitrogen, ammonia-nitrogen and dissolved phosphate. The removal efficiency (% decrease in nutrient between inflow and outflow) averaged 30% for nitrate, 50% for ammonia and near 0% for dissolved phosphate. Not all ponds behaved the same. Some ponds were net exporters of ammonia while others retained ammonia. The same was true for nitrate nitrogen also. Although nitrate was removed most efficiently, nitrate levels in the outflow were still extremely high (15400 µM or 215 mg N/L) indicating that a larger wetland is needed or that flow to the wetland should be reduced.