Introduction

Constructed wetlands are designed and engineered to control hydrology, configuration, substrate and vegetation to utilize the same treatment mechanisms that exist in natural wetlands. The hydraulics and loading rates are managed to maximize naturally occurring physical, chemical and biological processes to treat wastewater.

There are three basic types of constructed wetlands for wastewater treatment (CWWT) that utilize wetland emergent plant species growing in media such as soil, gravel or sand (Watson,1992). These types are surface flow, subsurface flow and vertical flow. Surface flow systems are most like natural marshes with standing water on the surface usually 0.5 to 1 ft. deep. In subsurface now systems, wastewater flows beneath the surface in the root zone of marsh vegetation. The substrate is usually coarse gravel that is 1 to 3 feet deep. In vertical flow systems, water is applied to the surface in timed intervals and flows down through the plant roots, which are growing in porous media. This paper will be limited to a discussion of the surface flow system (also called free water surface, FWS) which is the type usually used for nonpoint source runoff. Surface flow systems have lower installation costs, hydraulics are simple, and the clogging problems sometimes experienced by subsurface flow systems are avoided.

Advantages of CWWT systems include low cost, simplicity of operation and flexibility in rate of pollutant loading. Disadvantages are large land requirements and lack of proven design and operation criteria.

Constructed wetland systems are capable of removing to some degree most of the wastewater constituents that are considered pollutants. Suspended solids are removed by sedimentation and filtration; organic matter (BOD) by microbial degradation; nitrogen by volatilization of ammonia, nitrification, denitrification and plant uptake; phosphorus by precipitation, adsorption on soil colloids and plant uptake; trace elements by precipitation and adsorption; and microorganisms through die-off sedimentation, radiation, desiccation and adsorption.

 

Selecting plant species for CWWT systems

Reddy et al. (1987) listed the following characteristics as being desirable for plants used in wetland treatment systems:

  • Adaptability to the local climate
  • High photosynthetic rates
  • Oxygen transport capability
  • Tolerance to high concentrations of nutrients and pollutants
  • Ability to assimilate pollutants
  • Tolerance to adverse climatic conditions
  • Resistance to pests and diseases
  • Ease of management

Other desirable characteristics of CWWT plants are:

  • Readily available supply of propagules
  • Plants with economic or aesthetic value
  • Large surface area of roots, stems and leaves
  • Not likely to become a weed
  • Deep penetration of roots
  • Plants that secrete microbiocidal exudates

According to Reed (1990) selection of plant species may not be critical for surface flow systems. The rationale for this is that microbes associated with stems and roots are the major factor in treatment processes. Complete cover may be more important than species composition. Theoretically, diversity of plant species would provide protection from and resilience to perturbations such as disease, insects or herbivores.

In most cases, FWS constructed wetlands will develop species composition similar to natural wetlands in the area as adapted plant species invade. Large FWS wastewater treatment systems may also provide important ancillary benefits such as wildlife and fishery habitat, recreation and aesthetics. An example of this type is the Arcata, California system (Gearheart, l 992). Gearheart recommends planting to develop a fully vegetated wetland as quickly as possible in order to reach the functional density of wetland plants. The emergent plants generally used in the Northern California area are hard stem bulrush (Scirpus acutus) and cattail (Typha latifolia). Gearheart prefers hard stem bulrush because its morphological characteristics provide a greater surface area in the water column than cattails to support attached micro flora. Also, hard stem bulrush does not contribute as much detritus to the water column during the dormant period.

Other examples of extensive FWS treatment systems are those developed by the cities of Lakeland and Orlando, Florida. Lakeland's system utilized clay settling ponds abandoned by the phosphate industry (650 ha) and Orlando's 494-ha system was built on pasture land (Jackson, 1989).

At the Orlando site, three major vegetative communities were established: wet prairie, mixed marsh and hardwood swamp. The wet prairie (170 ha) was planted with cattail and bulrush (Scirpus californicus). In the mixed marsh, 10 species were planted and approximately 60 species volunteered. The planted species included maidencane (Panicum hemitomen) , soft rush (Juncus effusus) , arrowhead (Sagitaria lancifolia) and bulrush (Scirpus validus and S. americanus). The third community is a 152- ha hardwood swamp that includes a 49-ha lake. Tree species seedlings planted in the swamp were bald cypress (Taxodium disticum), red maple (acer rubrum), loblolly bay (Gordonia lasianthus), water oak (Quercus nigra) and dahoon holly (Ilex cassine).

These examples indicate that FWS wetland treatment systems may contain a number of plant species that should be adapted to the local climate and physical environment of the CWWT. Planting is usually necessary to provide a quick and uniform cover of a desirable species to prevent colonization by less desirable opportunistic species such as Phragmites. Planting also accelerates development of treatment processes.

Several species of plants have been successfully used in wetland treatment systems in North Carolina. Our experience has been with treatment systems for swine lagoon effluent, domestic sewage effluent and nonpoint source runoff at the Lake Wheeler site. The plants that we recommend at this time are: Scirpus validus , Juncus effusus , Scirpus cyperinus , Sagittaria latifolia , Peltandra virginica and Pontedaria cordata. A description of these plants and some of their charateristics attached

References

Gearhart, R. A. 1992. Use of constructed wetlands to treat domestic wastewater, City of Arcata, California. Water Science Technology. 26: 162S-1627.

Humenik, F.J., J. Zublena, J. Barker and T.M. Disy. 1993. Constructed Wetlands for Animal Wastewater treatment. N.C. Cooperative Extension Service bulletin AG-473-13.

Hunt, P.G., W.O. Thom, A.A. Szogi and F.J. Humenik. 1995. State of the art for animal wastewater treatment in constructed wetlands. pp 53-65. In Proceedings ofthe seventh international symposium on agricultural and food processing wastes. American Society of Agricultural Engineers. Chicago, Illinois.

Jackson, JoAnn. 1989. Man-made wetlands for wastewater treatment: Two case studies. In Hammer, D. A. (ed.). Constructed Wetlands for Wastewater Treatment, Lewis Publishers, Inc., Chelsea, MI. pp. 574-580.

McCaskey, T.A., S.N. Britt, T.C. Harmah, J.T. Eason, V.W.E. Payne end D.A. Hammer. 1994. pp. 23-33. In Proceedings: Constructed wetlands for animal waste management, Lafayette, Indiana.

Reed, S.C. 1990. Natural systems for wastewater treatment. Manual of practice FD-16 WPCF, Alexandria, Virginia.

Reddy, K.R. and R.A. DeBusk. 1987. State-of-the-art utilization of aquatic plants in water pollution control. Water Science Technology. 19:61-79.

Watson, J.T. 1992. Constructed wetlands for municipal wastewater treatment: state of the art. Symposium epuriation des eaux usees par les plants: Perspectives D'Aveniv au Quebec. Montreal Quebec.

 

PLANT SOURCES

NRCS Plant Vendors

 

Compiled By:
Stephen W. Broome
Department of Soil Science ,
North Carolina State University
Campus Box 7619 Raleigh,
North Carolina 27695-7619.