CONSTRUCTED WETLAND FOR TERTIARY WASTEWATER TREATMENT: USGS Patuxent Wildlife Research Center

INTRODUCTION

A 6-acre tertiary wastewater treatment facility consisting of seven water storage pools, 14 treatment cells, and a constructed forested wetland was constructed in 1993-94 at the National Wildlife Visitor Center (U.S. Fish and Wildlife Service), Laurel, Maryland.

Photo 1: An aerial photograph of the constructed wetland for the treatment of tertiary wastewater

Photo 1: An aerial photograph of the constructed wetland for the treatment of tertiary wastewater

Wastewater is initially treated in an underground primary and secondary treatment system (SANTEC Model SC-26K) before being pumped to the tertiary system. Eight of the treatment cells were planted with a variety of broad- and narrow-leaved emergent plants, and six of the cells were planted with three species of woody plants. Capacity of the system in 1994 was 6,800 galloons per day and, when plants matured, final capacity was 13,800 gallons per day.

The system is unique in that it is the first known system that uses a combination of emergent and woody plants in treatment cells in combination with a constructed forested wetland to remove nutrients from wastewater. The system was designed to facilitate research on nutrient removal from wastewater by plants. Current studies in the 14 cells are determining the survival and growth of the various herbaceous and woody transplants and the volunteer plants in a nutrient rich environment.

Figure 1: The layout of the emergent and woody cells

Figure 1: The layout of the emergent and woody cells

Photo 2: Planting of emergent plants

Photo 3: Planting of woody plants

Photo 4: Mature plants after one year

Photo 5: Biologist conducting plant survey

Photo 5: Biologist conducting plant survey

Photo 6: Biologist conducting water quality survey

Photo 7: Measuring growth of woody plants

Photo 7: Measuring growth of woody plants

Figure 2: Planted vegetation within the emergent cells

Figure 2: Planted vegetation within the emergent cells

Figure 3: Planted vegetation within the woody cells

Figure 3: Planted vegetation within the woody cells

RESULTS

Based on percent ground cover after four growing seasons, none of the broad-leaved emergents (Cells 1-4) covered more than 5% of the ground and the total cover for all 5 species was only 17%. River bulrush (Schoenoplectus fluviatilis) formed 24% of the ground cover of the narrow-leaved emergent plants (Cells 5-8) and total ground cover was 61%. Although narrow-leaved emergents were overall more successful then the broad-leaved emergent plants, non-planted species (volunteers), such as Juncus effusus, Juncus tenuis, and Scirpus cyperinus, constituted over 60% of the total ground cover in the cells.

Tables 1 and 2: Percent cover for the broad and narrow leaved emergent plants

Tables 1 and 2: Percent cover for the broad and narrow leaved emergent plants

Based on percent survival, buttonbush (Cephalanthus occidentalis) was the most successful among the woody plants in the 6 cells, although other species of woody plants, red maple (Acer rubrum) and winterberry (Ilex verticillata), had high mortality and slow growth. Woody plants in the forested wetland also had high mortality (87%), although bald cypress (Taxodium distichum) was one species that was successful.

Graphs 1 and 2: Mortality & average height of trees at visitor center

Nutrients (nitrate, ammonia, and phosphate) in water samples declined rapidly as water moved through the system from the upper to the lower end. The open system has the added advantage of being a haven for a variety of wildlife species that utilize the plants and invertebrates thriving in the area. Visitors to the Visitor Center frequently use the area for observation of plants and wildlife and for environmental education.